[PATCH] qemu-img: Use "depth":-1 to make backing probes obvious

Eric Blake posted 1 patch 2 days ago
Patches applied successfully (tree, apply log)
git fetch https://github.com/patchew-project/next-importer-push tags/patchew/20210610213906.1313440-1-eblake@redhat.com
Maintainers: Max Reitz <mreitz@redhat.com>, Kevin Wolf <kwolf@redhat.com>
docs/tools/qemu-img.rst    |   7 ++-
qemu-img.c                 |   3 ++
tests/qemu-iotests/122.out |  34 ++++++-------
tests/qemu-iotests/154.out | 100 +++++++++++++++++++------------------
tests/qemu-iotests/179.out |  67 +++++++++++++++++++++----
tests/qemu-iotests/223.out |  24 ++++-----
tests/qemu-iotests/244.out |   7 +--
tests/qemu-iotests/252.out |   4 +-
tests/qemu-iotests/274.out |  16 +++---
tests/qemu-iotests/291.out |   8 +--
tests/qemu-iotests/309.out |   4 +-
11 files changed, 165 insertions(+), 109 deletions(-)

[PATCH] qemu-img: Use "depth":-1 to make backing probes obvious

Posted by Eric Blake 2 days ago
The recently-added NBD context qemu:allocation-depth makes an obvious
case for why it is important to distinguish between locally-present
data (even with that data is sparse) [shown as depth 1 over NBD], and
data that could not be found anywhere in the backing chain [shown as
depth 0].  But qemu-img map --output=json predates that addition, and
has the unfortunate behavior that all portions of the backing chain
that resolve without finding a hit in any backing layer report the
same depth as the final backing layer.  This makes it harder to
reconstruct a qcow2 backing chain using just 'qemu-img map' output
when using "backing":null to artificially limit a backing chain,
because it is impossible to distinguish between a
QCOW2_CLUSTER_UNALLOCATED (which defers to a [missing] backing file)
and a QCOW2_CLUSTER_ZERO_PLAIN cluster (which would override any
backing file), since both types of clusters otherwise show as
"data":false,"zero":true" (but note that we can distinguish a
QCOW2_CLUSTER_ZERO_ALLOCATED, which would also have an "offset":
listing).

The task of reconstructing a qcow2 chain was made harder in commit
0da9856851 (nbd: server: Report holes for raw images), because prior
to that point, it was possible to abuse NBD's block status command to
see which portions of a qcow2 file resulted in BDRV_BLOCK_ALLOCATED
(showing up as NBD_STATE_ZERO in isolation) vs. missing from the chain
(showing up as NBD_STATE_ZERO|NBD_STATE_HOLE); but now qemu reports
more accurate sparseness information over NBD.

An obvious solution is to make 'qemu-img map --output=json' visually
distinguish between clusters that have a local allocation from those
that are found nowhere in the chain, by adding "depth":-1 as the new
witness of data that could not be tied to a specific backing image.
Several iotests are impacted, but glancing through the changes shows
that it is an improvement in that it shows more accurate details.

Note that the documentation is specifically worded to allow qemu-img
to report "depth":-1 both in the scenario where the last file in the
backing chain still defers the cluster (corresponding to
BDRV_BLOCK_ALLOCATED not being set anywhere in the chain), and in the
scenario where qemu is unable to determine which backing chain element
(if any) provides the data.  The latter case does not exist now, but
I'm considering an upcoming patch to add a BDRV_BLOCK_BACKING that
would let a specific driver (such as NBD) inform the block layer that
it is known that a cluster comes from a backing layer, but where there
is insufficient data to determine which layer.

As a quick demonstration:

    # Create a qcow2 image with a raw backing file:
    $ qemu-img create base.raw $((4*64*1024))
    $ qemu-img create -f qcow2 -b base.raw -F raw top.qcow2

    # Write to first 3 clusters of base:
    $ qemu-io -f raw -c "w -P 65 0 64k" -c "w -P 66 64k 64k" \
      -c "w -P 67 128k 64k" base.raw

    # Write to second and third clusters of top, hiding base:
    $ qemu-io -f qcow2 -c "w -P 69 64k 64k" -c "w -z 128k 64k" top.qcow2

    # Examine the full backing chain
    $ qemu-img map --output=json -f qcow2 top.qcow2
    [{ "start": 0, "length": 65536, "depth": 1, "zero": false, "data": true, "offset": 0},
    { "start": 65536, "length": 65536, "depth": 0, "zero": false, "data": true, "offset": 327680},
    { "start": 131072, "length": 65536, "depth": 0, "zero": true, "data": false},
    { "start": 196608, "length": 65536, "depth": 1, "zero": true, "data": false, "offset": 196608}]

    # Repeat, but with the backing chain clamped. Pre-patch:
    $ qemu-img map --output=json -f qcow2 json:'{"driver":"qcow2", \
      "backing":null, "file":{"driver":"file", "filename":"top.qcow2"}}'
    [{ "start": 0, "length": 65536, "depth": 0, "zero": true, "data": false},
    { "start": 65536, "length": 65536, "depth": 0, "zero": false, "data": true, "offset": 327680},
    { "start": 131072, "length": 131072, "depth": 0, "zero": true, "data": false}]

    # Repeat, but post-patch:
    $ qemu-img map --output=json -f qcow2 json:'{"driver":"qcow2", \
      "backing":null, "file":{"driver":"file", "filename":"top.qcow2"}}'
    [{ "start": 0, "length": 65536, "depth": -1, "zero": true, "data": false},
    { "start": 65536, "length": 65536, "depth": 0, "zero": false, "data": true, "offset": 327680},
    { "start": 131072, "length": 65536, "depth": 0, "zero": true, "data": false},
    { "start": 196608, "length": 65536, "depth": -1, "zero": true, "data": false}]

Note that pre-patch, it was impossible to determine which portions of
the qcow2 file override the backing file because the "depth":0 regions
were combined, so even though qemu internally can tell the difference
between sclusters 2 and 3, the command line user could not.  But
post-patch, the "depth":-1 markings match the "depth":1 markings when
the backing chain is intact, and it becomes obvious which clusters are
important.

Signed-off-by: Eric Blake <eblake@redhat.com>
---

If desired, I can send a followup patch that omits "depth":-1 output
from qemu-img output instead (that's another couple lines tweak to the
sources, plus re-touching all of the same affected tests).

 docs/tools/qemu-img.rst    |   7 ++-
 qemu-img.c                 |   3 ++
 tests/qemu-iotests/122.out |  34 ++++++-------
 tests/qemu-iotests/154.out | 100 +++++++++++++++++++------------------
 tests/qemu-iotests/179.out |  67 +++++++++++++++++++++----
 tests/qemu-iotests/223.out |  24 ++++-----
 tests/qemu-iotests/244.out |   7 +--
 tests/qemu-iotests/252.out |   4 +-
 tests/qemu-iotests/274.out |  16 +++---
 tests/qemu-iotests/291.out |   8 +--
 tests/qemu-iotests/309.out |   4 +-
 11 files changed, 165 insertions(+), 109 deletions(-)

diff --git a/docs/tools/qemu-img.rst b/docs/tools/qemu-img.rst
index cfe11478791f..afe64c5ee064 100644
--- a/docs/tools/qemu-img.rst
+++ b/docs/tools/qemu-img.rst
@@ -598,8 +598,11 @@ Command description:
     all-zero clusters);
   - whether the data is known to read as zero (boolean field ``zero``);
   - in order to make the output shorter, the target file is expressed as
-    a ``depth``; for example, a depth of 2 refers to the backing file
-    of the backing file of *FILENAME*.
+    a ``depth``; a depth of 0 means the data comes from the given
+    file, a positive depth means the data comes from a specific file
+    in the backing chain, and a depth of -1 merely implies that the
+    data is not local, but no information was available on which
+    backing file (if any) provides the data.

   In JSON format, the ``offset`` field is optional; it is absent in
   cases where ``human`` format would omit the entry or exit with an error.
diff --git a/qemu-img.c b/qemu-img.c
index a5993682aad4..a6f0c5565865 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -3035,6 +3035,9 @@ static int get_block_status(BlockDriverState *bs, int64_t offset,
         bdrv_refresh_filename(file);
         filename = file->filename;
     }
+    if (!(ret & BDRV_BLOCK_ALLOCATED)) {
+        depth = -1;
+    }

     *e = (MapEntry) {
         .start = offset,
diff --git a/tests/qemu-iotests/122.out b/tests/qemu-iotests/122.out
index 3a3e121d579d..2aca0f066fa8 100644
--- a/tests/qemu-iotests/122.out
+++ b/tests/qemu-iotests/122.out
@@ -68,11 +68,11 @@ read 65536/65536 bytes at offset 4194304
 read 65536/65536 bytes at offset 8388608
 64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 [{ "start": 0, "length": 65536, "depth": 0, "zero": false, "data": true},
-{ "start": 65536, "length": 4128768, "depth": 0, "zero": true, "data": false},
+{ "start": 65536, "length": 4128768, "depth": -1, "zero": true, "data": false},
 { "start": 4194304, "length": 65536, "depth": 0, "zero": false, "data": true},
-{ "start": 4259840, "length": 4128768, "depth": 0, "zero": true, "data": false},
+{ "start": 4259840, "length": 4128768, "depth": -1, "zero": true, "data": false},
 { "start": 8388608, "length": 65536, "depth": 0, "zero": false, "data": true},
-{ "start": 8454144, "length": 4128768, "depth": 0, "zero": true, "data": false}]
+{ "start": 8454144, "length": 4128768, "depth": -1, "zero": true, "data": false}]
 read 65536/65536 bytes at offset 0
 64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 read 65536/65536 bytes at offset 4194304
@@ -95,11 +95,11 @@ wrote 1024/1024 bytes at offset 1046528
 wrote 1024/1024 bytes at offset 0
 1 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 [{ "start": 0, "length": 65536, "depth": 0, "zero": false, "data": true},
-{ "start": 65536, "length": 65536, "depth": 0, "zero": true, "data": false},
+{ "start": 65536, "length": 65536, "depth": -1, "zero": true, "data": false},
 { "start": 131072, "length": 196608, "depth": 0, "zero": false, "data": true},
-{ "start": 327680, "length": 655360, "depth": 0, "zero": true, "data": false},
+{ "start": 327680, "length": 655360, "depth": -1, "zero": true, "data": false},
 { "start": 983040, "length": 65536, "depth": 0, "zero": false, "data": true},
-{ "start": 1048576, "length": 1046528, "depth": 0, "zero": true, "data": false}]
+{ "start": 1048576, "length": 1046528, "depth": -1, "zero": true, "data": false}]
 read 16384/16384 bytes at offset 0
 16 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 read 16384/16384 bytes at offset 16384
@@ -195,31 +195,31 @@ wrote 1024/1024 bytes at offset 17408

 convert -S 4k
 [{ "start": 0, "length": 4096, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
-{ "start": 4096, "length": 4096, "depth": 0, "zero": true, "data": false},
+{ "start": 4096, "length": 4096, "depth": -1, "zero": true, "data": false},
 { "start": 8192, "length": 4096, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
-{ "start": 12288, "length": 4096, "depth": 0, "zero": true, "data": false},
+{ "start": 12288, "length": 4096, "depth": -1, "zero": true, "data": false},
 { "start": 16384, "length": 4096, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
-{ "start": 20480, "length": 67088384, "depth": 0, "zero": true, "data": false}]
+{ "start": 20480, "length": 67088384, "depth": -1, "zero": true, "data": false}]

 convert -c -S 4k
 [{ "start": 0, "length": 1024, "depth": 0, "zero": false, "data": true},
-{ "start": 1024, "length": 7168, "depth": 0, "zero": true, "data": false},
+{ "start": 1024, "length": 7168, "depth": -1, "zero": true, "data": false},
 { "start": 8192, "length": 1024, "depth": 0, "zero": false, "data": true},
-{ "start": 9216, "length": 8192, "depth": 0, "zero": true, "data": false},
+{ "start": 9216, "length": 8192, "depth": -1, "zero": true, "data": false},
 { "start": 17408, "length": 1024, "depth": 0, "zero": false, "data": true},
-{ "start": 18432, "length": 67090432, "depth": 0, "zero": true, "data": false}]
+{ "start": 18432, "length": 67090432, "depth": -1, "zero": true, "data": false}]

 convert -S 8k
 [{ "start": 0, "length": 24576, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
-{ "start": 24576, "length": 67084288, "depth": 0, "zero": true, "data": false}]
+{ "start": 24576, "length": 67084288, "depth": -1, "zero": true, "data": false}]

 convert -c -S 8k
 [{ "start": 0, "length": 1024, "depth": 0, "zero": false, "data": true},
-{ "start": 1024, "length": 7168, "depth": 0, "zero": true, "data": false},
+{ "start": 1024, "length": 7168, "depth": -1, "zero": true, "data": false},
 { "start": 8192, "length": 1024, "depth": 0, "zero": false, "data": true},
-{ "start": 9216, "length": 8192, "depth": 0, "zero": true, "data": false},
+{ "start": 9216, "length": 8192, "depth": -1, "zero": true, "data": false},
 { "start": 17408, "length": 1024, "depth": 0, "zero": false, "data": true},
-{ "start": 18432, "length": 67090432, "depth": 0, "zero": true, "data": false}]
+{ "start": 18432, "length": 67090432, "depth": -1, "zero": true, "data": false}]

 === -n to a non-zero image ===

@@ -235,7 +235,7 @@ Formatting 'TEST_DIR/t.IMGFMT.orig', fmt=IMGFMT size=67108864
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864
 [{ "start": 0, "length": 67108864, "depth": 0, "zero": true, "data": false}]
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864
-[{ "start": 0, "length": 67108864, "depth": 0, "zero": true, "data": false}]
+[{ "start": 0, "length": 67108864, "depth": -1, "zero": true, "data": false}]

 === -n to an empty image with a backing file ===

diff --git a/tests/qemu-iotests/154.out b/tests/qemu-iotests/154.out
index 4863e2483864..03b07c1a5429 100644
--- a/tests/qemu-iotests/154.out
+++ b/tests/qemu-iotests/154.out
@@ -12,13 +12,13 @@ wrote 2048/2048 bytes at offset 17408
 wrote 2048/2048 bytes at offset 27648
 2 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 [{ "start": 0, "length": 4096, "depth": 0, "zero": true, "data": false},
-{ "start": 4096, "length": 4096, "depth": 1, "zero": true, "data": false},
+{ "start": 4096, "length": 4096, "depth": -1, "zero": true, "data": false},
 { "start": 8192, "length": 4096, "depth": 0, "zero": true, "data": false},
-{ "start": 12288, "length": 4096, "depth": 1, "zero": true, "data": false},
+{ "start": 12288, "length": 4096, "depth": -1, "zero": true, "data": false},
 { "start": 16384, "length": 4096, "depth": 0, "zero": true, "data": false},
-{ "start": 20480, "length": 4096, "depth": 1, "zero": true, "data": false},
+{ "start": 20480, "length": 4096, "depth": -1, "zero": true, "data": false},
 { "start": 24576, "length": 8192, "depth": 0, "zero": true, "data": false},
-{ "start": 32768, "length": 134184960, "depth": 1, "zero": true, "data": false}]
+{ "start": 32768, "length": 134184960, "depth": -1, "zero": true, "data": false}]

 == backing file contains non-zero data before write_zeroes ==
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=134217728
@@ -41,11 +41,11 @@ read 1024/1024 bytes at offset 65536
 1 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 read 2048/2048 bytes at offset 67584
 2 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-[{ "start": 0, "length": 32768, "depth": 1, "zero": true, "data": false},
+[{ "start": 0, "length": 32768, "depth": -1, "zero": true, "data": false},
 { "start": 32768, "length": 4096, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
-{ "start": 36864, "length": 28672, "depth": 1, "zero": true, "data": false},
+{ "start": 36864, "length": 28672, "depth": -1, "zero": true, "data": false},
 { "start": 65536, "length": 4096, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
-{ "start": 69632, "length": 134148096, "depth": 1, "zero": true, "data": false}]
+{ "start": 69632, "length": 134148096, "depth": -1, "zero": true, "data": false}]

 == backing file contains non-zero data after write_zeroes ==
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=134217728
@@ -68,11 +68,11 @@ read 1024/1024 bytes at offset 44032
 1 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 read 3072/3072 bytes at offset 40960
 3 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-[{ "start": 0, "length": 32768, "depth": 1, "zero": true, "data": false},
+[{ "start": 0, "length": 32768, "depth": -1, "zero": true, "data": false},
 { "start": 32768, "length": 4096, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
-{ "start": 36864, "length": 4096, "depth": 1, "zero": true, "data": false},
+{ "start": 36864, "length": 4096, "depth": -1, "zero": true, "data": false},
 { "start": 40960, "length": 4096, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
-{ "start": 45056, "length": 134172672, "depth": 1, "zero": true, "data": false}]
+{ "start": 45056, "length": 134172672, "depth": -1, "zero": true, "data": false}]

 == write_zeroes covers non-zero data ==
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=134217728
@@ -101,15 +101,15 @@ wrote 2048/2048 bytes at offset 29696
 2 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 read 4096/4096 bytes at offset 28672
 4 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-[{ "start": 0, "length": 4096, "depth": 1, "zero": true, "data": false},
+[{ "start": 0, "length": 4096, "depth": -1, "zero": true, "data": false},
 { "start": 4096, "length": 4096, "depth": 0, "zero": true, "data": false},
-{ "start": 8192, "length": 4096, "depth": 1, "zero": true, "data": false},
+{ "start": 8192, "length": 4096, "depth": -1, "zero": true, "data": false},
 { "start": 12288, "length": 4096, "depth": 0, "zero": true, "data": false},
-{ "start": 16384, "length": 4096, "depth": 1, "zero": true, "data": false},
+{ "start": 16384, "length": 4096, "depth": -1, "zero": true, "data": false},
 { "start": 20480, "length": 4096, "depth": 0, "zero": true, "data": false},
-{ "start": 24576, "length": 4096, "depth": 1, "zero": true, "data": false},
+{ "start": 24576, "length": 4096, "depth": -1, "zero": true, "data": false},
 { "start": 28672, "length": 4096, "depth": 0, "zero": true, "data": false},
-{ "start": 32768, "length": 134184960, "depth": 1, "zero": true, "data": false}]
+{ "start": 32768, "length": 134184960, "depth": -1, "zero": true, "data": false}]

 == spanning two clusters, non-zero before request ==
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=134217728
@@ -142,16 +142,16 @@ read 1024/1024 bytes at offset 67584
 1 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 read 5120/5120 bytes at offset 68608
 5 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-[{ "start": 0, "length": 32768, "depth": 1, "zero": true, "data": false},
+[{ "start": 0, "length": 32768, "depth": -1, "zero": true, "data": false},
 { "start": 32768, "length": 4096, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
 { "start": 36864, "length": 4096, "depth": 0, "zero": true, "data": false},
-{ "start": 40960, "length": 8192, "depth": 1, "zero": true, "data": false},
+{ "start": 40960, "length": 8192, "depth": -1, "zero": true, "data": false},
 { "start": 49152, "length": 4096, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
 { "start": 53248, "length": 4096, "depth": 0, "zero": true, "data": false},
-{ "start": 57344, "length": 8192, "depth": 1, "zero": true, "data": false},
+{ "start": 57344, "length": 8192, "depth": -1, "zero": true, "data": false},
 { "start": 65536, "length": 4096, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
 { "start": 69632, "length": 4096, "depth": 0, "zero": true, "data": false},
-{ "start": 73728, "length": 134144000, "depth": 1, "zero": true, "data": false}]
+{ "start": 73728, "length": 134144000, "depth": -1, "zero": true, "data": false}]

 == spanning two clusters, non-zero after request ==
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=134217728
@@ -184,16 +184,16 @@ read 7168/7168 bytes at offset 65536
 7 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 read 1024/1024 bytes at offset 72704
 1 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-[{ "start": 0, "length": 32768, "depth": 1, "zero": true, "data": false},
+[{ "start": 0, "length": 32768, "depth": -1, "zero": true, "data": false},
 { "start": 32768, "length": 4096, "depth": 0, "zero": true, "data": false},
 { "start": 36864, "length": 4096, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
-{ "start": 40960, "length": 8192, "depth": 1, "zero": true, "data": false},
+{ "start": 40960, "length": 8192, "depth": -1, "zero": true, "data": false},
 { "start": 49152, "length": 4096, "depth": 0, "zero": true, "data": false},
 { "start": 53248, "length": 4096, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
-{ "start": 57344, "length": 8192, "depth": 1, "zero": true, "data": false},
+{ "start": 57344, "length": 8192, "depth": -1, "zero": true, "data": false},
 { "start": 65536, "length": 4096, "depth": 0, "zero": true, "data": false},
 { "start": 69632, "length": 4096, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
-{ "start": 73728, "length": 134144000, "depth": 1, "zero": true, "data": false}]
+{ "start": 73728, "length": 134144000, "depth": -1, "zero": true, "data": false}]

 == spanning two clusters, partially overwriting backing file ==
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=134217728
@@ -213,7 +213,7 @@ read 1024/1024 bytes at offset 5120
 read 2048/2048 bytes at offset 6144
 2 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 [{ "start": 0, "length": 8192, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
-{ "start": 8192, "length": 134209536, "depth": 1, "zero": true, "data": false}]
+{ "start": 8192, "length": 134209536, "depth": -1, "zero": true, "data": false}]

 == spanning multiple clusters, non-zero in first cluster ==
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=134217728
@@ -226,10 +226,10 @@ read 2048/2048 bytes at offset 65536
 2 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 read 10240/10240 bytes at offset 67584
 10 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-[{ "start": 0, "length": 65536, "depth": 1, "zero": true, "data": false},
+[{ "start": 0, "length": 65536, "depth": -1, "zero": true, "data": false},
 { "start": 65536, "length": 4096, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
 { "start": 69632, "length": 8192, "depth": 0, "zero": true, "data": false},
-{ "start": 77824, "length": 134139904, "depth": 1, "zero": true, "data": false}]
+{ "start": 77824, "length": 134139904, "depth": -1, "zero": true, "data": false}]

 == spanning multiple clusters, non-zero in intermediate cluster ==
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=134217728
@@ -240,9 +240,9 @@ wrote 7168/7168 bytes at offset 67584
 7 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 read 12288/12288 bytes at offset 65536
 12 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-[{ "start": 0, "length": 65536, "depth": 1, "zero": true, "data": false},
+[{ "start": 0, "length": 65536, "depth": -1, "zero": true, "data": false},
 { "start": 65536, "length": 12288, "depth": 0, "zero": true, "data": false},
-{ "start": 77824, "length": 134139904, "depth": 1, "zero": true, "data": false}]
+{ "start": 77824, "length": 134139904, "depth": -1, "zero": true, "data": false}]

 == spanning multiple clusters, non-zero in final cluster ==
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=134217728
@@ -255,10 +255,10 @@ read 10240/10240 bytes at offset 65536
 10 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 read 2048/2048 bytes at offset 75776
 2 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-[{ "start": 0, "length": 65536, "depth": 1, "zero": true, "data": false},
+[{ "start": 0, "length": 65536, "depth": -1, "zero": true, "data": false},
 { "start": 65536, "length": 8192, "depth": 0, "zero": true, "data": false},
 { "start": 73728, "length": 4096, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
-{ "start": 77824, "length": 134139904, "depth": 1, "zero": true, "data": false}]
+{ "start": 77824, "length": 134139904, "depth": -1, "zero": true, "data": false}]

 == spanning multiple clusters, partially overwriting backing file ==
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=134217728
@@ -277,57 +277,61 @@ read 2048/2048 bytes at offset 74752
 2 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 read 1024/1024 bytes at offset 76800
 1 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-[{ "start": 0, "length": 65536, "depth": 1, "zero": true, "data": false},
+[{ "start": 0, "length": 65536, "depth": -1, "zero": true, "data": false},
 { "start": 65536, "length": 4096, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
 { "start": 69632, "length": 4096, "depth": 0, "zero": true, "data": false},
 { "start": 73728, "length": 4096, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
-{ "start": 77824, "length": 134139904, "depth": 1, "zero": true, "data": false}]
+{ "start": 77824, "length": 134139904, "depth": -1, "zero": true, "data": false}]

 == unaligned image tail cluster, no allocation needed ==
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776
 wrote 512/512 bytes at offset 134217728
 512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 2048/2048 bytes allocated at offset 128 MiB
-[{ "start": 0, "length": 134219776, "depth": 0, "zero": true, "data": false}]
+[{ "start": 0, "length": 134217728, "depth": -1, "zero": true, "data": false},
+{ "start": 134217728, "length": 2048, "depth": 0, "zero": true, "data": false}]
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776
 wrote 512/512 bytes at offset 134219264
 512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 2048/2048 bytes allocated at offset 128 MiB
-[{ "start": 0, "length": 134219776, "depth": 0, "zero": true, "data": false}]
+[{ "start": 0, "length": 134217728, "depth": -1, "zero": true, "data": false},
+{ "start": 134217728, "length": 2048, "depth": 0, "zero": true, "data": false}]
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776
 wrote 1024/1024 bytes at offset 134218240
 1 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 2048/2048 bytes allocated at offset 128 MiB
-[{ "start": 0, "length": 134219776, "depth": 0, "zero": true, "data": false}]
+[{ "start": 0, "length": 134217728, "depth": -1, "zero": true, "data": false},
+{ "start": 134217728, "length": 2048, "depth": 0, "zero": true, "data": false}]
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776
 wrote 2048/2048 bytes at offset 134217728
 2 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 2048/2048 bytes allocated at offset 128 MiB
-[{ "start": 0, "length": 134219776, "depth": 0, "zero": true, "data": false}]
+[{ "start": 0, "length": 134217728, "depth": -1, "zero": true, "data": false},
+{ "start": 134217728, "length": 2048, "depth": 0, "zero": true, "data": false}]
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=134218752
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 512/512 bytes at offset 134217728
 512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 2048/2048 bytes allocated at offset 128 MiB
-[{ "start": 0, "length": 134217728, "depth": 1, "zero": true, "data": false},
+[{ "start": 0, "length": 134217728, "depth": -1, "zero": true, "data": false},
 { "start": 134217728, "length": 2048, "depth": 0, "zero": true, "data": false}]
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 512/512 bytes at offset 134219264
 512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 2048/2048 bytes allocated at offset 128 MiB
-[{ "start": 0, "length": 134217728, "depth": 1, "zero": true, "data": false},
+[{ "start": 0, "length": 134217728, "depth": -1, "zero": true, "data": false},
 { "start": 134217728, "length": 2048, "depth": 0, "zero": true, "data": false}]
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 1024/1024 bytes at offset 134218240
 1 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 2048/2048 bytes allocated at offset 128 MiB
-[{ "start": 0, "length": 134217728, "depth": 1, "zero": true, "data": false},
+[{ "start": 0, "length": 134217728, "depth": -1, "zero": true, "data": false},
 { "start": 134217728, "length": 2048, "depth": 0, "zero": true, "data": false}]
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 2048/2048 bytes at offset 134217728
 2 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 2048/2048 bytes allocated at offset 128 MiB
-[{ "start": 0, "length": 134217728, "depth": 1, "zero": true, "data": false},
+[{ "start": 0, "length": 134217728, "depth": -1, "zero": true, "data": false},
 { "start": 134217728, "length": 2048, "depth": 0, "zero": true, "data": false}]
 wrote 512/512 bytes at offset 134217728
 512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
@@ -335,25 +339,25 @@ Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/
 wrote 512/512 bytes at offset 134217728
 512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 2048/2048 bytes allocated at offset 128 MiB
-[{ "start": 0, "length": 134217728, "depth": 1, "zero": true, "data": false},
+[{ "start": 0, "length": 134217728, "depth": -1, "zero": true, "data": false},
 { "start": 134217728, "length": 2048, "depth": 0, "zero": true, "data": false}]
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 512/512 bytes at offset 134219264
 512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 2048/2048 bytes allocated at offset 128 MiB
-[{ "start": 0, "length": 134217728, "depth": 1, "zero": true, "data": false},
+[{ "start": 0, "length": 134217728, "depth": -1, "zero": true, "data": false},
 { "start": 134217728, "length": 2048, "depth": 0, "zero": true, "data": false}]
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 1024/1024 bytes at offset 134218240
 1 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 2048/2048 bytes allocated at offset 128 MiB
-[{ "start": 0, "length": 134217728, "depth": 1, "zero": true, "data": false},
+[{ "start": 0, "length": 134217728, "depth": -1, "zero": true, "data": false},
 { "start": 134217728, "length": 2048, "depth": 0, "zero": true, "data": false}]
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 2048/2048 bytes at offset 134217728
 2 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 2048/2048 bytes allocated at offset 128 MiB
-[{ "start": 0, "length": 134217728, "depth": 1, "zero": true, "data": false},
+[{ "start": 0, "length": 134217728, "depth": -1, "zero": true, "data": false},
 { "start": 134217728, "length": 2048, "depth": 0, "zero": true, "data": false}]
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134218752
 wrote 1024/1024 bytes at offset 134217728
@@ -365,14 +369,14 @@ read 512/512 bytes at offset 134217728
 read 512/512 bytes at offset 134218240
 512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 1024/1024 bytes allocated at offset 128 MiB
-[{ "start": 0, "length": 134217728, "depth": 0, "zero": true, "data": false},
+[{ "start": 0, "length": 134217728, "depth": -1, "zero": true, "data": false},
 { "start": 134217728, "length": 1024, "depth": 0, "zero": false, "data": true, "offset": OFFSET}]
 wrote 1024/1024 bytes at offset 134217728
 1 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 1024/1024 bytes allocated at offset 128 MiB
 read 1024/1024 bytes at offset 134217728
 1 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-[{ "start": 0, "length": 134217728, "depth": 0, "zero": true, "data": false},
+[{ "start": 0, "length": 134217728, "depth": -1, "zero": true, "data": false},
 { "start": 134217728, "length": 1024, "depth": 0, "zero": true, "data": false, "offset": OFFSET}]

 == unaligned image tail cluster, allocation required ==
@@ -386,7 +390,7 @@ read 512/512 bytes at offset 134217728
 512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 read 1536/1536 bytes at offset 134218240
 1.500 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-[{ "start": 0, "length": 134217728, "depth": 1, "zero": true, "data": false},
+[{ "start": 0, "length": 134217728, "depth": -1, "zero": true, "data": false},
 { "start": 134217728, "length": 2048, "depth": 0, "zero": false, "data": true, "offset": OFFSET}]
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=134218752
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
@@ -408,6 +412,6 @@ read 512/512 bytes at offset 134218240
 512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 read 1024/1024 bytes at offset 134218752
 1 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-[{ "start": 0, "length": 134217728, "depth": 1, "zero": true, "data": false},
+[{ "start": 0, "length": 134217728, "depth": -1, "zero": true, "data": false},
 { "start": 134217728, "length": 2048, "depth": 0, "zero": false, "data": true, "offset": OFFSET}]
 *** done
diff --git a/tests/qemu-iotests/179.out b/tests/qemu-iotests/179.out
index 1f7680002ca6..295fe7abdbf4 100644
--- a/tests/qemu-iotests/179.out
+++ b/tests/qemu-iotests/179.out
@@ -13,7 +13,11 @@ wrote 2097152/2097152 bytes at offset 6291456
 2 MiB (0x200000) bytes not allocated at offset 4 MiB (0x400000)
 2 MiB (0x200000) bytes     allocated at offset 6 MiB (0x600000)
 56 MiB (0x3800000) bytes not allocated at offset 8 MiB (0x800000)
-[{ "start": 0, "length": 67108864, "depth": 0, "zero": true, "data": false}]
+[{ "start": 0, "length": 2097152, "depth": -1, "zero": true, "data": false},
+{ "start": 2097152, "length": 2097152, "depth": 0, "zero": true, "data": false},
+{ "start": 4194304, "length": 2097152, "depth": -1, "zero": true, "data": false},
+{ "start": 6291456, "length": 2097152, "depth": 0, "zero": true, "data": false},
+{ "start": 8388608, "length": 58720256, "depth": -1, "zero": true, "data": false}]
 wrote 2097150/2097150 bytes at offset 10485761
 2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 wrote 2097150/2097150 bytes at offset 14680065
@@ -27,7 +31,15 @@ wrote 2097150/2097150 bytes at offset 14680065
 2 MiB (0x200000) bytes not allocated at offset 12 MiB (0xc00000)
 2 MiB (0x200000) bytes     allocated at offset 14 MiB (0xe00000)
 48 MiB (0x3000000) bytes not allocated at offset 16 MiB (0x1000000)
-[{ "start": 0, "length": 67108864, "depth": 0, "zero": true, "data": false}]
+[{ "start": 0, "length": 2097152, "depth": -1, "zero": true, "data": false},
+{ "start": 2097152, "length": 2097152, "depth": 0, "zero": true, "data": false},
+{ "start": 4194304, "length": 2097152, "depth": -1, "zero": true, "data": false},
+{ "start": 6291456, "length": 2097152, "depth": 0, "zero": true, "data": false},
+{ "start": 8388608, "length": 2097152, "depth": -1, "zero": true, "data": false},
+{ "start": 10485760, "length": 2097152, "depth": 0, "zero": true, "data": false},
+{ "start": 12582912, "length": 2097152, "depth": -1, "zero": true, "data": false},
+{ "start": 14680064, "length": 2097152, "depth": 0, "zero": true, "data": false},
+{ "start": 16777216, "length": 50331648, "depth": -1, "zero": true, "data": false}]
 wrote 14680064/14680064 bytes at offset 18874368
 14 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 wrote 2097152/2097152 bytes at offset 20971520
@@ -45,13 +57,21 @@ wrote 6291456/6291456 bytes at offset 25165824
 2 MiB (0x200000) bytes not allocated at offset 16 MiB (0x1000000)
 14 MiB (0xe00000) bytes     allocated at offset 18 MiB (0x1200000)
 32 MiB (0x2000000) bytes not allocated at offset 32 MiB (0x2000000)
-[{ "start": 0, "length": 18874368, "depth": 0, "zero": true, "data": false},
+[{ "start": 0, "length": 2097152, "depth": -1, "zero": true, "data": false},
+{ "start": 2097152, "length": 2097152, "depth": 0, "zero": true, "data": false},
+{ "start": 4194304, "length": 2097152, "depth": -1, "zero": true, "data": false},
+{ "start": 6291456, "length": 2097152, "depth": 0, "zero": true, "data": false},
+{ "start": 8388608, "length": 2097152, "depth": -1, "zero": true, "data": false},
+{ "start": 10485760, "length": 2097152, "depth": 0, "zero": true, "data": false},
+{ "start": 12582912, "length": 2097152, "depth": -1, "zero": true, "data": false},
+{ "start": 14680064, "length": 2097152, "depth": 0, "zero": true, "data": false},
+{ "start": 16777216, "length": 2097152, "depth": -1, "zero": true, "data": false},
 { "start": 18874368, "length": 2097152, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
 { "start": 20971520, "length": 2097152, "depth": 0, "zero": true, "data": false},
 { "start": 23068672, "length": 2097152, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
 { "start": 25165824, "length": 6291456, "depth": 0, "zero": true, "data": false, "offset": OFFSET},
 { "start": 31457280, "length": 2097152, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
-{ "start": 33554432, "length": 33554432, "depth": 0, "zero": true, "data": false}]
+{ "start": 33554432, "length": 33554432, "depth": -1, "zero": true, "data": false}]
 wrote 2097152/2097152 bytes at offset 27262976
 2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 wrote 2097152/2097152 bytes at offset 29360128
@@ -67,7 +87,15 @@ wrote 2097152/2097152 bytes at offset 29360128
 2 MiB (0x200000) bytes not allocated at offset 16 MiB (0x1000000)
 14 MiB (0xe00000) bytes     allocated at offset 18 MiB (0x1200000)
 32 MiB (0x2000000) bytes not allocated at offset 32 MiB (0x2000000)
-[{ "start": 0, "length": 18874368, "depth": 0, "zero": true, "data": false},
+[{ "start": 0, "length": 2097152, "depth": -1, "zero": true, "data": false},
+{ "start": 2097152, "length": 2097152, "depth": 0, "zero": true, "data": false},
+{ "start": 4194304, "length": 2097152, "depth": -1, "zero": true, "data": false},
+{ "start": 6291456, "length": 2097152, "depth": 0, "zero": true, "data": false},
+{ "start": 8388608, "length": 2097152, "depth": -1, "zero": true, "data": false},
+{ "start": 10485760, "length": 2097152, "depth": 0, "zero": true, "data": false},
+{ "start": 12582912, "length": 2097152, "depth": -1, "zero": true, "data": false},
+{ "start": 14680064, "length": 2097152, "depth": 0, "zero": true, "data": false},
+{ "start": 16777216, "length": 2097152, "depth": -1, "zero": true, "data": false},
 { "start": 18874368, "length": 2097152, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
 { "start": 20971520, "length": 2097152, "depth": 0, "zero": true, "data": false},
 { "start": 23068672, "length": 2097152, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
@@ -75,7 +103,7 @@ wrote 2097152/2097152 bytes at offset 29360128
 { "start": 27262976, "length": 2097152, "depth": 0, "zero": true, "data": false},
 { "start": 29360128, "length": 2097152, "depth": 0, "zero": true, "data": false, "offset": OFFSET},
 { "start": 31457280, "length": 2097152, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
-{ "start": 33554432, "length": 33554432, "depth": 0, "zero": true, "data": false}]
+{ "start": 33554432, "length": 33554432, "depth": -1, "zero": true, "data": false}]
 wrote 8388608/8388608 bytes at offset 33554432
 8 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 wrote 2097152/2097152 bytes at offset 35651584
@@ -93,7 +121,15 @@ wrote 2097152/2097152 bytes at offset 37748736
 2 MiB (0x200000) bytes not allocated at offset 16 MiB (0x1000000)
 22 MiB (0x1600000) bytes     allocated at offset 18 MiB (0x1200000)
 24 MiB (0x1800000) bytes not allocated at offset 40 MiB (0x2800000)
-[{ "start": 0, "length": 18874368, "depth": 0, "zero": true, "data": false},
+[{ "start": 0, "length": 2097152, "depth": -1, "zero": true, "data": false},
+{ "start": 2097152, "length": 2097152, "depth": 0, "zero": true, "data": false},
+{ "start": 4194304, "length": 2097152, "depth": -1, "zero": true, "data": false},
+{ "start": 6291456, "length": 2097152, "depth": 0, "zero": true, "data": false},
+{ "start": 8388608, "length": 2097152, "depth": -1, "zero": true, "data": false},
+{ "start": 10485760, "length": 2097152, "depth": 0, "zero": true, "data": false},
+{ "start": 12582912, "length": 2097152, "depth": -1, "zero": true, "data": false},
+{ "start": 14680064, "length": 2097152, "depth": 0, "zero": true, "data": false},
+{ "start": 16777216, "length": 2097152, "depth": -1, "zero": true, "data": false},
 { "start": 18874368, "length": 2097152, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
 { "start": 20971520, "length": 2097152, "depth": 0, "zero": true, "data": false},
 { "start": 23068672, "length": 2097152, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
@@ -101,7 +137,8 @@ wrote 2097152/2097152 bytes at offset 37748736
 { "start": 27262976, "length": 2097152, "depth": 0, "zero": true, "data": false},
 { "start": 29360128, "length": 2097152, "depth": 0, "zero": true, "data": false, "offset": OFFSET},
 { "start": 31457280, "length": 2097152, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
-{ "start": 33554432, "length": 33554432, "depth": 0, "zero": true, "data": false}]
+{ "start": 33554432, "length": 8388608, "depth": 0, "zero": true, "data": false},
+{ "start": 41943040, "length": 25165824, "depth": -1, "zero": true, "data": false}]
 wrote 8388608/8388608 bytes at offset 41943040
 8 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 wrote 8388608/8388608 bytes at offset 50331648
@@ -125,7 +162,15 @@ wrote 2097152/2097152 bytes at offset 62914560
 4 MiB (0x400000) bytes not allocated at offset 54 MiB (0x3600000)
 4 MiB (0x400000) bytes     allocated at offset 58 MiB (0x3a00000)
 2 MiB (0x200000) bytes not allocated at offset 62 MiB (0x3e00000)
-[{ "start": 0, "length": 18874368, "depth": 1, "zero": true, "data": false},
+[{ "start": 0, "length": 2097152, "depth": -1, "zero": true, "data": false},
+{ "start": 2097152, "length": 2097152, "depth": 1, "zero": true, "data": false},
+{ "start": 4194304, "length": 2097152, "depth": -1, "zero": true, "data": false},
+{ "start": 6291456, "length": 2097152, "depth": 1, "zero": true, "data": false},
+{ "start": 8388608, "length": 2097152, "depth": -1, "zero": true, "data": false},
+{ "start": 10485760, "length": 2097152, "depth": 1, "zero": true, "data": false},
+{ "start": 12582912, "length": 2097152, "depth": -1, "zero": true, "data": false},
+{ "start": 14680064, "length": 2097152, "depth": 1, "zero": true, "data": false},
+{ "start": 16777216, "length": 2097152, "depth": -1, "zero": true, "data": false},
 { "start": 18874368, "length": 2097152, "depth": 1, "zero": false, "data": true, "offset": OFFSET},
 { "start": 20971520, "length": 2097152, "depth": 1, "zero": true, "data": false},
 { "start": 23068672, "length": 2097152, "depth": 1, "zero": false, "data": true, "offset": OFFSET},
@@ -139,9 +184,9 @@ wrote 2097152/2097152 bytes at offset 62914560
 { "start": 50331648, "length": 2097152, "depth": 1, "zero": false, "data": true, "offset": OFFSET},
 { "start": 52428800, "length": 4194304, "depth": 0, "zero": true, "data": false},
 { "start": 56623104, "length": 2097152, "depth": 1, "zero": false, "data": true, "offset": OFFSET},
-{ "start": 58720256, "length": 2097152, "depth": 1, "zero": true, "data": false},
+{ "start": 58720256, "length": 2097152, "depth": -1, "zero": true, "data": false},
 { "start": 60817408, "length": 4194304, "depth": 0, "zero": true, "data": false},
-{ "start": 65011712, "length": 2097152, "depth": 1, "zero": true, "data": false}]
+{ "start": 65011712, "length": 2097152, "depth": -1, "zero": true, "data": false}]
 No errors were found on the image.
 No errors were found on the image.

diff --git a/tests/qemu-iotests/223.out b/tests/qemu-iotests/223.out
index 083b62d05386..ec448ac36010 100644
--- a/tests/qemu-iotests/223.out
+++ b/tests/qemu-iotests/223.out
@@ -103,16 +103,16 @@ read 2097152/2097152 bytes at offset 2097152
 [{ "start": 0, "length": 4096, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
 { "start": 4096, "length": 1044480, "depth": 0, "zero": true, "data": false, "offset": OFFSET},
 { "start": 1048576, "length": 3145728, "depth": 0, "zero": false, "data": true, "offset": OFFSET}]
-[{ "start": 0, "length": 65536, "depth": 0, "zero": false, "data": false},
+[{ "start": 0, "length": 65536, "depth": -1, "zero": false, "data": false},
 { "start": 65536, "length": 2031616, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
-{ "start": 2097152, "length": 2097152, "depth": 0, "zero": false, "data": false}]
+{ "start": 2097152, "length": 2097152, "depth": -1, "zero": false, "data": false}]

 === Contrast to small granularity dirty-bitmap ===

 [{ "start": 0, "length": 512, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
-{ "start": 512, "length": 512, "depth": 0, "zero": false, "data": false},
+{ "start": 512, "length": 512, "depth": -1, "zero": false, "data": false},
 { "start": 1024, "length": 2096128, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
-{ "start": 2097152, "length": 2097152, "depth": 0, "zero": false, "data": false}]
+{ "start": 2097152, "length": 2097152, "depth": -1, "zero": false, "data": false}]

 === End qemu NBD server ===

@@ -204,16 +204,16 @@ read 2097152/2097152 bytes at offset 2097152
 [{ "start": 0, "length": 4096, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
 { "start": 4096, "length": 1044480, "depth": 0, "zero": true, "data": false, "offset": OFFSET},
 { "start": 1048576, "length": 3145728, "depth": 0, "zero": false, "data": true, "offset": OFFSET}]
-[{ "start": 0, "length": 65536, "depth": 0, "zero": false, "data": false},
+[{ "start": 0, "length": 65536, "depth": -1, "zero": false, "data": false},
 { "start": 65536, "length": 2031616, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
-{ "start": 2097152, "length": 2097152, "depth": 0, "zero": false, "data": false}]
+{ "start": 2097152, "length": 2097152, "depth": -1, "zero": false, "data": false}]

 === Contrast to small granularity dirty-bitmap ===

 [{ "start": 0, "length": 512, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
-{ "start": 512, "length": 512, "depth": 0, "zero": false, "data": false},
+{ "start": 512, "length": 512, "depth": -1, "zero": false, "data": false},
 { "start": 1024, "length": 2096128, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
-{ "start": 2097152, "length": 2097152, "depth": 0, "zero": false, "data": false}]
+{ "start": 2097152, "length": 2097152, "depth": -1, "zero": false, "data": false}]

 === End qemu NBD server ===

@@ -238,12 +238,12 @@ read 2097152/2097152 bytes at offset 2097152

 === Use qemu-nbd as server ===

-[{ "start": 0, "length": 65536, "depth": 0, "zero": false, "data": false},
+[{ "start": 0, "length": 65536, "depth": -1, "zero": false, "data": false},
 { "start": 65536, "length": 2031616, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
-{ "start": 2097152, "length": 2097152, "depth": 0, "zero": false, "data": false}]
+{ "start": 2097152, "length": 2097152, "depth": -1, "zero": false, "data": false}]
 [{ "start": 0, "length": 512, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
-{ "start": 512, "length": 512, "depth": 0, "zero": false, "data": false},
+{ "start": 512, "length": 512, "depth": -1, "zero": false, "data": false},
 { "start": 1024, "length": 11321, "depth": 0, "zero": false, "data": true, "offset": OFFSET}]
 [{ "start": 12345, "length": 2084807, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
-{ "start": 2097152, "length": 2097152, "depth": 0, "zero": false, "data": false}]
+{ "start": 2097152, "length": 2097152, "depth": -1, "zero": false, "data": false}]
 *** done
diff --git a/tests/qemu-iotests/244.out b/tests/qemu-iotests/244.out
index 99f56ac18c48..b7b417cd599b 100644
--- a/tests/qemu-iotests/244.out
+++ b/tests/qemu-iotests/244.out
@@ -57,11 +57,12 @@ wrote 3145728/3145728 bytes at offset 3145728
 3 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 No errors were found on the image.

-[{ "start": 0, "length": 1048576, "depth": 0, "zero": true, "data": false},
+[{ "start": 0, "length": 1048576, "depth": -1, "zero": true, "data": false},
 { "start": 1048576, "length": 1048576, "depth": 0, "zero": false, "data": true, "offset": 1048576},
 { "start": 2097152, "length": 2097152, "depth": 0, "zero": true, "data": false},
 { "start": 4194304, "length": 1048576, "depth": 0, "zero": true, "data": false, "offset": 4194304},
-{ "start": 5242880, "length": 61865984, "depth": 0, "zero": true, "data": false}]
+{ "start": 5242880, "length": 1048576, "depth": 0, "zero": true, "data": false},
+{ "start": 6291456, "length": 60817408, "depth": -1, "zero": true, "data": false}]

 read 1048576/1048576 bytes at offset 0
 1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
@@ -123,7 +124,7 @@ read 1048576/1048576 bytes at offset 0
 Offset          Length          Mapped to       File
 0               0x100000        0               TEST_DIR/t.qcow2.data
 [{ "start": 0, "length": 1048576, "depth": 0, "zero": false, "data": true, "offset": 0},
-{ "start": 1048576, "length": 66060288, "depth": 0, "zero": true, "data": false}]
+{ "start": 1048576, "length": 66060288, "depth": -1, "zero": true, "data": false}]

 === Copy offloading ===

diff --git a/tests/qemu-iotests/252.out b/tests/qemu-iotests/252.out
index 12dce889f823..5235323c2a4e 100644
--- a/tests/qemu-iotests/252.out
+++ b/tests/qemu-iotests/252.out
@@ -24,7 +24,7 @@ read 131072/131072 bytes at offset 262144
 128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)

 [{ "start": 0, "length": 262144, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
-{ "start": 262144, "length": 131072, "depth": 0, "zero": true, "data": false}]
+{ "start": 262144, "length": 131072, "depth": -1, "zero": true, "data": false}]

 read 131072/131072 bytes at offset 0
 128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
@@ -35,5 +35,5 @@ read 131072/131072 bytes at offset 262144

 [{ "start": 0, "length": 262144, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
 { "start": 262144, "length": 65536, "depth": 0, "zero": true, "data": false},
-{ "start": 327680, "length": 65536, "depth": 1, "zero": true, "data": false}]
+{ "start": 327680, "length": 65536, "depth": -1, "zero": true, "data": false}]
 *** done
diff --git a/tests/qemu-iotests/274.out b/tests/qemu-iotests/274.out
index cfe17a865924..d56dae7cf72c 100644
--- a/tests/qemu-iotests/274.out
+++ b/tests/qemu-iotests/274.out
@@ -37,7 +37,7 @@ Offset          Length          Mapped to       File
 0               0x100000        0x50000         TEST_DIR/PID-base

 [{ "start": 0, "length": 1048576, "depth": 2, "zero": false, "data": true, "offset": 327680},
-{ "start": 1048576, "length": 1048576, "depth": 0, "zero": true, "data": false}]
+{ "start": 1048576, "length": 1048576, "depth": -1, "zero": true, "data": false}]

 Offset          Length          Mapped to       File
 0               0x100000        0x50000         TEST_DIR/PID-base
@@ -220,7 +220,7 @@ read 65536/65536 bytes at offset 5368709120
 1 GiB (0x40000000) bytes not allocated at offset 0 bytes (0x0)
 7 GiB (0x1c0000000) bytes     allocated at offset 1 GiB (0x40000000)

-[{ "start": 0, "length": 1073741824, "depth": 1, "zero": true, "data": false},
+[{ "start": 0, "length": 1073741824, "depth": -1, "zero": true, "data": false},
 { "start": 1073741824, "length": 7516192768, "depth": 0, "zero": true, "data": false}]

 === preallocation=metadata ===
@@ -239,7 +239,7 @@ read 65536/65536 bytes at offset 33285996544
 30 GiB (0x780000000) bytes not allocated at offset 0 bytes (0x0)
 3 GiB (0xc0000000) bytes     allocated at offset 30 GiB (0x780000000)

-[{ "start": 0, "length": 32212254720, "depth": 1, "zero": true, "data": false},
+[{ "start": 0, "length": 32212254720, "depth": -1, "zero": true, "data": false},
 { "start": 32212254720, "length": 536870912, "depth": 0, "zero": true, "data": false, "offset": 327680},
 { "start": 32749125632, "length": 536870912, "depth": 0, "zero": true, "data": false, "offset": 537264128},
 { "start": 33285996544, "length": 536870912, "depth": 0, "zero": true, "data": false, "offset": 1074200576},
@@ -263,7 +263,7 @@ read 65536/65536 bytes at offset 9437184
 5 MiB (0x500000) bytes not allocated at offset 0 bytes (0x0)
 10 MiB (0xa00000) bytes     allocated at offset 5 MiB (0x500000)

-[{ "start": 0, "length": 5242880, "depth": 1, "zero": true, "data": false},
+[{ "start": 0, "length": 5242880, "depth": -1, "zero": true, "data": false},
 { "start": 5242880, "length": 10485760, "depth": 0, "zero": false, "data": true, "offset": 327680}]

 === preallocation=full ===
@@ -282,7 +282,7 @@ read 65536/65536 bytes at offset 11534336
 8 MiB (0x800000) bytes not allocated at offset 0 bytes (0x0)
 4 MiB (0x400000) bytes     allocated at offset 8 MiB (0x800000)

-[{ "start": 0, "length": 8388608, "depth": 1, "zero": true, "data": false},
+[{ "start": 0, "length": 8388608, "depth": -1, "zero": true, "data": false},
 { "start": 8388608, "length": 4194304, "depth": 0, "zero": false, "data": true, "offset": 327680}]

 === preallocation=off ===
@@ -301,7 +301,7 @@ read 65536/65536 bytes at offset 259072
 192 KiB (0x30000) bytes not allocated at offset 0 bytes (0x0)
 320 KiB (0x50000) bytes     allocated at offset 192 KiB (0x30000)

-[{ "start": 0, "length": 196608, "depth": 1, "zero": true, "data": false},
+[{ "start": 0, "length": 196608, "depth": -1, "zero": true, "data": false},
 { "start": 196608, "length": 65536, "depth": 0, "zero": false, "data": true, "offset": 327680},
 { "start": 262144, "length": 262144, "depth": 0, "zero": true, "data": false}]

@@ -321,7 +321,7 @@ read 65536/65536 bytes at offset 344064
 256 KiB (0x40000) bytes not allocated at offset 0 bytes (0x0)
 256 KiB (0x40000) bytes     allocated at offset 256 KiB (0x40000)

-[{ "start": 0, "length": 262144, "depth": 1, "zero": true, "data": false},
+[{ "start": 0, "length": 262144, "depth": -1, "zero": true, "data": false},
 { "start": 262144, "length": 262144, "depth": 0, "zero": true, "data": false}]

 === preallocation=off ===
@@ -340,6 +340,6 @@ read 65536/65536 bytes at offset 446464
 256 KiB (0x40000) bytes not allocated at offset 0 bytes (0x0)
 244 KiB (0x3d000) bytes     allocated at offset 256 KiB (0x40000)

-[{ "start": 0, "length": 262144, "depth": 1, "zero": true, "data": false},
+[{ "start": 0, "length": 262144, "depth": -1, "zero": true, "data": false},
 { "start": 262144, "length": 249856, "depth": 0, "zero": true, "data": false}]

diff --git a/tests/qemu-iotests/291.out b/tests/qemu-iotests/291.out
index 23411c0ff4d9..8ad772486f6d 100644
--- a/tests/qemu-iotests/291.out
+++ b/tests/qemu-iotests/291.out
@@ -104,15 +104,15 @@ Format specific information:
 === Check bitmap contents ===

 [{ "start": 0, "length": 3145728, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
-{ "start": 3145728, "length": 1048576, "depth": 0, "zero": false, "data": false},
+{ "start": 3145728, "length": 1048576, "depth": -1, "zero": false, "data": false},
 { "start": 4194304, "length": 6291456, "depth": 0, "zero": false, "data": true, "offset": OFFSET}]
 [{ "start": 0, "length": 1048576, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
-{ "start": 1048576, "length": 1048576, "depth": 0, "zero": false, "data": false},
+{ "start": 1048576, "length": 1048576, "depth": -1, "zero": false, "data": false},
 { "start": 2097152, "length": 8388608, "depth": 0, "zero": false, "data": true, "offset": OFFSET}]
 [{ "start": 0, "length": 2097152, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
-{ "start": 2097152, "length": 1048576, "depth": 0, "zero": false, "data": false},
+{ "start": 2097152, "length": 1048576, "depth": -1, "zero": false, "data": false},
 { "start": 3145728, "length": 7340032, "depth": 0, "zero": false, "data": true, "offset": OFFSET}]
 [{ "start": 0, "length": 2097152, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
-{ "start": 2097152, "length": 1048576, "depth": 0, "zero": false, "data": false},
+{ "start": 2097152, "length": 1048576, "depth": -1, "zero": false, "data": false},
 { "start": 3145728, "length": 7340032, "depth": 0, "zero": false, "data": true, "offset": OFFSET}]
 *** done
diff --git a/tests/qemu-iotests/309.out b/tests/qemu-iotests/309.out
index db75bb6b0df9..110737e12a5b 100644
--- a/tests/qemu-iotests/309.out
+++ b/tests/qemu-iotests/309.out
@@ -13,10 +13,10 @@ wrote 2097152/2097152 bytes at offset 1048576

 [{ "start": 0, "length": 1048576, "depth": 1, "zero": false, "data": true, "offset": 327680},
 { "start": 1048576, "length": 2097152, "depth": 0, "zero": false, "data": true, "offset": 327680},
-{ "start": 3145728, "length": 1048576, "depth": 1, "zero": true, "data": false}]
+{ "start": 3145728, "length": 1048576, "depth": -1, "zero": true, "data": false}]
 [{ "start": 0, "length": 3145728, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
 { "start": 3145728, "length": 1048576, "depth": 0, "zero": true, "data": false, "offset": OFFSET}]
 [{ "start": 0, "length": 1048576, "depth": 0, "zero": true, "data": true, "offset": OFFSET},
-{ "start": 1048576, "length": 2097152, "depth": 0, "zero": false, "data": false},
+{ "start": 1048576, "length": 2097152, "depth": -1, "zero": false, "data": false},
 { "start": 3145728, "length": 1048576, "depth": 0, "zero": false, "data": true, "offset": OFFSET}]
 *** done
-- 
2.31.1


Re: [PATCH] qemu-img: Use "depth":-1 to make backing probes obvious

Posted by Vladimir Sementsov-Ogievskiy 1 day, 14 hours ago
11.06.2021 00:39, Eric Blake wrote:
> The recently-added NBD context qemu:allocation-depth makes an obvious
> case for why it is important to distinguish between locally-present
> data (even with that data is sparse) [shown as depth 1 over NBD], and
> data that could not be found anywhere in the backing chain [shown as
> depth 0].  But qemu-img map --output=json predates that addition, and
> has the unfortunate behavior that all portions of the backing chain
> that resolve without finding a hit in any backing layer report the
> same depth as the final backing layer.  This makes it harder to
> reconstruct a qcow2 backing chain using just 'qemu-img map' output
> when using "backing":null to artificially limit a backing chain,
> because it is impossible to distinguish between a
> QCOW2_CLUSTER_UNALLOCATED (which defers to a [missing] backing file)
> and a QCOW2_CLUSTER_ZERO_PLAIN cluster (which would override any
> backing file), since both types of clusters otherwise show as
> "data":false,"zero":true" (but note that we can distinguish a
> QCOW2_CLUSTER_ZERO_ALLOCATED, which would also have an "offset":
> listing).
> 
> The task of reconstructing a qcow2 chain was made harder in commit
> 0da9856851 (nbd: server: Report holes for raw images), because prior
> to that point, it was possible to abuse NBD's block status command to
> see which portions of a qcow2 file resulted in BDRV_BLOCK_ALLOCATED
> (showing up as NBD_STATE_ZERO in isolation) vs. missing from the chain
> (showing up as NBD_STATE_ZERO|NBD_STATE_HOLE); but now qemu reports
> more accurate sparseness information over NBD.
> 
> An obvious solution is to make 'qemu-img map --output=json' visually
> distinguish between clusters that have a local allocation from those
> that are found nowhere in the chain, by adding "depth":-1 as the new
> witness of data that could not be tied to a specific backing image.
> Several iotests are impacted, but glancing through the changes shows
> that it is an improvement in that it shows more accurate details.
> 
> Note that the documentation is specifically worded to allow qemu-img
> to report "depth":-1 both in the scenario where the last file in the
> backing chain still defers the cluster (corresponding to
> BDRV_BLOCK_ALLOCATED not being set anywhere in the chain), and in the
> scenario where qemu is unable to determine which backing chain element
> (if any) provides the data.  The latter case does not exist now, but
> I'm considering an upcoming patch to add a BDRV_BLOCK_BACKING that
> would let a specific driver (such as NBD) inform the block layer that
> it is known that a cluster comes from a backing layer, but where there
> is insufficient data to determine which layer.
> 
> As a quick demonstration:
> 
>      # Create a qcow2 image with a raw backing file:
>      $ qemu-img create base.raw $((4*64*1024))
>      $ qemu-img create -f qcow2 -b base.raw -F raw top.qcow2
> 
>      # Write to first 3 clusters of base:
>      $ qemu-io -f raw -c "w -P 65 0 64k" -c "w -P 66 64k 64k" \
>        -c "w -P 67 128k 64k" base.raw
> 
>      # Write to second and third clusters of top, hiding base:
>      $ qemu-io -f qcow2 -c "w -P 69 64k 64k" -c "w -z 128k 64k" top.qcow2
> 
>      # Examine the full backing chain
>      $ qemu-img map --output=json -f qcow2 top.qcow2
>      [{ "start": 0, "length": 65536, "depth": 1, "zero": false, "data": true, "offset": 0},
>      { "start": 65536, "length": 65536, "depth": 0, "zero": false, "data": true, "offset": 327680},
>      { "start": 131072, "length": 65536, "depth": 0, "zero": true, "data": false},
>      { "start": 196608, "length": 65536, "depth": 1, "zero": true, "data": false, "offset": 196608}]
> 
>      # Repeat, but with the backing chain clamped. Pre-patch:
>      $ qemu-img map --output=json -f qcow2 json:'{"driver":"qcow2", \
>        "backing":null, "file":{"driver":"file", "filename":"top.qcow2"}}'
>      [{ "start": 0, "length": 65536, "depth": 0, "zero": true, "data": false},
>      { "start": 65536, "length": 65536, "depth": 0, "zero": false, "data": true, "offset": 327680},
>      { "start": 131072, "length": 131072, "depth": 0, "zero": true, "data": false}]
> 
>      # Repeat, but post-patch:
>      $ qemu-img map --output=json -f qcow2 json:'{"driver":"qcow2", \
>        "backing":null, "file":{"driver":"file", "filename":"top.qcow2"}}'
>      [{ "start": 0, "length": 65536, "depth": -1, "zero": true, "data": false},
>      { "start": 65536, "length": 65536, "depth": 0, "zero": false, "data": true, "offset": 327680},
>      { "start": 131072, "length": 65536, "depth": 0, "zero": true, "data": false},
>      { "start": 196608, "length": 65536, "depth": -1, "zero": true, "data": false}]
> 
> Note that pre-patch, it was impossible to determine which portions of
> the qcow2 file override the backing file because the "depth":0 regions
> were combined, so even though qemu internally can tell the difference
> between sclusters 2 and 3, the command line user could not.  But
> post-patch, the "depth":-1 markings match the "depth":1 markings when
> the backing chain is intact, and it becomes obvious which clusters are
> important.
> 
> Signed-off-by: Eric Blake <eblake@redhat.com>

I'm not sure do we need a kind of new option or deprecation period for such incompatible change..

Personally I'm OK with it and like the idea:
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

> ---
> 
> If desired, I can send a followup patch that omits "depth":-1 output
> from qemu-img output instead (that's another couple lines tweak to the
> sources, plus re-touching all of the same affected tests).

[..]

> +    backing file (if any) provides the data.
> 
>     In JSON format, the ``offset`` field is optional; it is absent in
>     cases where ``human`` format would omit the entry or exit with an error.
> diff --git a/qemu-img.c b/qemu-img.c
> index a5993682aad4..a6f0c5565865 100644
> --- a/qemu-img.c
> +++ b/qemu-img.c
> @@ -3035,6 +3035,9 @@ static int get_block_status(BlockDriverState *bs, int64_t offset,
>           bdrv_refresh_filename(file);
>           filename = file->filename;
>       }
> +    if (!(ret & BDRV_BLOCK_ALLOCATED)) {
> +        depth = -1;
> +    }

may be more obvious would be set depth = -1 in "if (bs == NULL) {" where ret becomes 0 for this case.

> 
>       *e = (MapEntry) {
>           .start = offset,
> diff --git a/tests/qemu-iotests/122.out b/tests/qemu-iotests/122.out
> index 3a3e121d579d..2aca0f066fa8 100644
> --- a/tests/qemu-iotests/122.out
> +++ b/tests/qemu-iotests/122.out
> @@ -68,11 +68,11 @@ read 65536/65536 bytes at offset 4194304
>   read 65536/65536 bytes at offset 8388608
>   64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
>   [{ "start": 0, "length": 65536, "depth": 0, "zero": false, "data": true},
> -{ "start": 65536, "length": 4128768, "depth": 0, "zero": true, "data": false},
> +{ "start": 65536, "length": 4128768, "depth": -1, "zero": true, "data": false},
>   { "start": 4194304, "length": 65536, "depth": 0, "zero": false, "data": true},
> -{ "start": 4259840, "length": 4128768, "depth": 0, "zero": true, "data": false},
> +{ "start": 4259840, "length": 4128768, "depth": -1, "zero": true, "data": false},
>   { "start": 8388608, "length": 65536, "depth": 0, "zero": false, "data": true},
> -{ "start": 8454144, "length": 4128768, "depth": 0, "zero": true, "data": false}]
> +{ "start": 8454144, "length": 4128768, "depth": -1, "zero": true, "data": false}]

[..]


-- 
Best regards,
Vladimir

Re: [PATCH] qemu-img: Use "depth":-1 to make backing probes obvious

Posted by Vladimir Sementsov-Ogievskiy 1 day, 14 hours ago
11.06.2021 11:08, Vladimir Sementsov-Ogievskiy wrote:
> 11.06.2021 00:39, Eric Blake wrote:
>> The recently-added NBD context qemu:allocation-depth makes an obvious
>> case for why it is important to distinguish between locally-present
>> data (even with that data is sparse) [shown as depth 1 over NBD], and
>> data that could not be found anywhere in the backing chain [shown as
>> depth 0].  But qemu-img map --output=json predates that addition, and
>> has the unfortunate behavior that all portions of the backing chain
>> that resolve without finding a hit in any backing layer report the
>> same depth as the final backing layer.  This makes it harder to
>> reconstruct a qcow2 backing chain using just 'qemu-img map' output
>> when using "backing":null to artificially limit a backing chain,
>> because it is impossible to distinguish between a
>> QCOW2_CLUSTER_UNALLOCATED (which defers to a [missing] backing file)
>> and a QCOW2_CLUSTER_ZERO_PLAIN cluster (which would override any
>> backing file), since both types of clusters otherwise show as
>> "data":false,"zero":true" (but note that we can distinguish a
>> QCOW2_CLUSTER_ZERO_ALLOCATED, which would also have an "offset":
>> listing).
>>
>> The task of reconstructing a qcow2 chain was made harder in commit
>> 0da9856851 (nbd: server: Report holes for raw images), because prior
>> to that point, it was possible to abuse NBD's block status command to
>> see which portions of a qcow2 file resulted in BDRV_BLOCK_ALLOCATED
>> (showing up as NBD_STATE_ZERO in isolation) vs. missing from the chain
>> (showing up as NBD_STATE_ZERO|NBD_STATE_HOLE); but now qemu reports
>> more accurate sparseness information over NBD.
>>
>> An obvious solution is to make 'qemu-img map --output=json' visually
>> distinguish between clusters that have a local allocation from those
>> that are found nowhere in the chain, by adding "depth":-1 as the new
>> witness of data that could not be tied to a specific backing image.
>> Several iotests are impacted, but glancing through the changes shows
>> that it is an improvement in that it shows more accurate details.
>>
>> Note that the documentation is specifically worded to allow qemu-img
>> to report "depth":-1 both in the scenario where the last file in the
>> backing chain still defers the cluster (corresponding to
>> BDRV_BLOCK_ALLOCATED not being set anywhere in the chain), and in the
>> scenario where qemu is unable to determine which backing chain element
>> (if any) provides the data.  The latter case does not exist now, but
>> I'm considering an upcoming patch to add a BDRV_BLOCK_BACKING that
>> would let a specific driver (such as NBD) inform the block layer that
>> it is known that a cluster comes from a backing layer, but where there
>> is insufficient data to determine which layer.
>>
>> As a quick demonstration:
>>
>>      # Create a qcow2 image with a raw backing file:
>>      $ qemu-img create base.raw $((4*64*1024))
>>      $ qemu-img create -f qcow2 -b base.raw -F raw top.qcow2
>>
>>      # Write to first 3 clusters of base:
>>      $ qemu-io -f raw -c "w -P 65 0 64k" -c "w -P 66 64k 64k" \
>>        -c "w -P 67 128k 64k" base.raw
>>
>>      # Write to second and third clusters of top, hiding base:
>>      $ qemu-io -f qcow2 -c "w -P 69 64k 64k" -c "w -z 128k 64k" top.qcow2
>>
>>      # Examine the full backing chain
>>      $ qemu-img map --output=json -f qcow2 top.qcow2
>>      [{ "start": 0, "length": 65536, "depth": 1, "zero": false, "data": true, "offset": 0},
>>      { "start": 65536, "length": 65536, "depth": 0, "zero": false, "data": true, "offset": 327680},
>>      { "start": 131072, "length": 65536, "depth": 0, "zero": true, "data": false},
>>      { "start": 196608, "length": 65536, "depth": 1, "zero": true, "data": false, "offset": 196608}]
>>
>>      # Repeat, but with the backing chain clamped. Pre-patch:
>>      $ qemu-img map --output=json -f qcow2 json:'{"driver":"qcow2", \
>>        "backing":null, "file":{"driver":"file", "filename":"top.qcow2"}}'
>>      [{ "start": 0, "length": 65536, "depth": 0, "zero": true, "data": false},
>>      { "start": 65536, "length": 65536, "depth": 0, "zero": false, "data": true, "offset": 327680},
>>      { "start": 131072, "length": 131072, "depth": 0, "zero": true, "data": false}]
>>
>>      # Repeat, but post-patch:
>>      $ qemu-img map --output=json -f qcow2 json:'{"driver":"qcow2", \
>>        "backing":null, "file":{"driver":"file", "filename":"top.qcow2"}}'
>>      [{ "start": 0, "length": 65536, "depth": -1, "zero": true, "data": false},
>>      { "start": 65536, "length": 65536, "depth": 0, "zero": false, "data": true, "offset": 327680},
>>      { "start": 131072, "length": 65536, "depth": 0, "zero": true, "data": false},
>>      { "start": 196608, "length": 65536, "depth": -1, "zero": true, "data": false}]
>>
>> Note that pre-patch, it was impossible to determine which portions of
>> the qcow2 file override the backing file because the "depth":0 regions
>> were combined, so even though qemu internally can tell the difference
>> between sclusters 2 and 3, the command line user could not.  But
>> post-patch, the "depth":-1 markings match the "depth":1 markings when
>> the backing chain is intact, and it becomes obvious which clusters are
>> important.
>>
>> Signed-off-by: Eric Blake <eblake@redhat.com>
> 
> I'm not sure do we need a kind of new option or deprecation period for such incompatible change..

I see Kevin worries about same thing..

So, probably better is just add a new optional boolean field, that shows that data is "nowhere in the chain". Such change is a lot more compatible.

> 
> Personally I'm OK with it and like the idea:
> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>


-- 
Best regards,
Vladimir