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authorEric Blake <eblake@redhat.com>2021-07-01 14:06:55 -0500
committerEric Blake <eblake@redhat.com>2021-07-12 11:10:53 -0500
commit8417e1378cadb8928c24755a95ff267def53922f (patch)
tree59a77e0b51cfc29ea4370dda005390e05dbb9928 /qapi
parent1cfd21ccc7576c03914fa48d414451fdd53fb9a5 (diff)
qemu-img: Make unallocated part of backing chain obvious in map
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": 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' 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 accuracy. Signed-off-by: Eric Blake <eblake@redhat.com> Message-Id: <20210701190655.2131223-3-eblake@redhat.com> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com> [eblake: fix more iotest fallout] Signed-off-by: Eric Blake <eblake@redhat.com>
Diffstat (limited to 'qapi')
-rw-r--r--qapi/block-core.json7
1 files changed, 5 insertions, 2 deletions
diff --git a/qapi/block-core.json b/qapi/block-core.json
index c7a311798a..675d8265eb 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -274,6 +274,9 @@
# 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
#
@@ -284,8 +287,8 @@
##
{ '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: