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Re: [PATCH v3 2/2] qemu-img: Make unallocated part of backing chain obvi

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [PATCH v3 2/2] qemu-img: Make unallocated part of backing chain obvious in map
Date: Sat, 3 Jul 2021 10:25:28 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0

01.07.2021 22:06, Eric Blake wrote:
The recently-added NBD context qemu:allocation-depth is able to
distinguish between locally-present data (even when 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]; and the libnbd
project was recently patched to give the human-readable name "absent"
to an allocation-depth of 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, especially 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":

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' add an
additional "present":false designation to any cluster lacking an
allocation anywhere in the chain, without any change to the "depth"
parameter to avoid breaking existing clients.  The iotests have
several examples where this distinction demonstrates the additional

Signed-off-by: Eric Blake <eblake@redhat.com>
  docs/tools/qemu-img.rst                       |   3 +
  qapi/block-core.json                          |   7 +-
  qemu-img.c                                    |   7 +-
  tests/qemu-iotests/122.out                    |  84 ++++----
  tests/qemu-iotests/154.out                    | 190 +++++++++---------
  tests/qemu-iotests/179.out                    | 133 ++++++++----
  tests/qemu-iotests/223.out                    |  56 +++---
  tests/qemu-iotests/244.out                    |  23 ++-
  tests/qemu-iotests/252.out                    |  10 +-
  tests/qemu-iotests/274.out                    |  48 ++---
  tests/qemu-iotests/291.out                    |  24 +--
  .../tests/nbd-qemu-allocation.out             |  16 +-
  12 files changed, 330 insertions(+), 271 deletions(-)

diff --git a/docs/tools/qemu-img.rst b/docs/tools/qemu-img.rst
index cfe11478791f..d6300f7ee03d 100644
--- a/docs/tools/qemu-img.rst
+++ b/docs/tools/qemu-img.rst
@@ -597,6 +597,9 @@ Command description:
      if false, the sectors are either unallocated or stored as optimized
      all-zero clusters);
    - whether the data is known to read as zero (boolean field ``zero``);
+  - whether the data is actually present (boolean field ``present``);
+    if false, rebasing the backing chain onto a deeper file would pick
+    up data from the deeper file;

Preexisting, but rather strange style of documentation, when described option 
doesn't go first in the paragraph..

    - 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*.
diff --git a/qapi/block-core.json b/qapi/block-core.json
index a54f37dbef06..912a7d9265e5 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -261,6 +261,9 @@
  #         images in the chain)) before reaching one for which the
  #         range is allocated
+# @present: true if this layer provides the data, false if adding a backing
+#           layer could impact this region (since 6.1)
  # @offset: if present, the image file stores the data for this range
  #          in raw format at the given (host) offset
@@ -271,8 +274,8 @@
  { 'struct': 'MapEntry',
    'data': {'start': 'int', 'length': 'int', 'data': 'bool',
-           'zero': 'bool', 'depth': 'int', '*offset': 'int',
-           '*filename': 'str' } }
+           'zero': 'bool', 'depth': 'int', 'present': 'bool',
+           '*offset': 'int', '*filename': 'str' } }

  # @BlockdevCacheInfo:
diff --git a/qemu-img.c b/qemu-img.c
index 7956a8996512..8090facc5087 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -2980,8 +2980,9 @@ static int dump_map_entry(OutputFormat output_format, 
MapEntry *e,
      case OFORMAT_JSON:
          printf("{ \"start\": %"PRId64", \"length\": %"PRId64","
-               " \"depth\": %"PRId64", \"zero\": %s, \"data\": %s",
-               e->start, e->length, e->depth,
+               " \"depth\": %"PRId64", \"present\": %s, \"zero\": %s,"
+               "\"data\": %s", e->start, e->length, e->depth,
+               e->present ? "true" : "false",

Didn't you want to put present at the end? Still, this shouldn't be 
significant. And it make sense to keep present, zero and data together.

You missied a whitespace after '"zero": %s,', which is obvious from further 
test diff hunks.

With it fixed:
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

                 e->zero ? "true" : "false",
                 e->data ? "true" : "false");
          if (e->has_offset) {
@@ -3047,6 +3048,7 @@ static int get_block_status(BlockDriverState *bs, int64_t 
          .offset = map,
          .has_offset = has_offset,
          .depth = depth,
+        .present = !!(ret & BDRV_BLOCK_ALLOCATED),
          .has_filename = filename,
          .filename = filename,
@@ -3062,6 +3064,7 @@ static inline bool entry_mergeable(const MapEntry *curr, 
const MapEntry *next)
      if (curr->zero != next->zero ||
          curr->data != next->data ||
          curr->depth != next->depth ||
+        curr->present != next->present ||
          curr->has_filename != next->has_filename ||
          curr->has_offset != next->has_offset) {
          return false;
diff --git a/tests/qemu-iotests/122.out b/tests/qemu-iotests/122.out
index 3a3e121d579d..83dfcf75281f 100644
--- a/tests/qemu-iotests/122.out
+++ b/tests/qemu-iotests/122.out
@@ -67,12 +67,12 @@ read 65536/65536 bytes at offset 4194304


Best regards,

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