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qcow2: Allow preallocation and backing files if extended_l2 is set

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Traditional qcow2 images don't allow preallocation if a backing file
is set. This is because once a cluster is allocated there is no way to
tell that its data should be read from the backing file.

Extended L2 entries have individual allocation bits for each
subcluster, and therefore it is perfectly possible to have an
allocated cluster with all its subclusters unallocated.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <6d5b0f38e7dc5f2f31d8cab1cb92044e9909aece.1594396418.git.berto@igalia.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
This commit is contained in:
Alberto Garcia 2020-07-10 18:13:14 +02:00 committed by Max Reitz
parent 7be2025258
commit 2118771ddf
2 changed files with 5 additions and 4 deletions
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7
block/qcow2.c
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@ -3449,10 +3449,11 @@ qcow2_co_create(BlockdevCreateOptions *create_options, Error **errp)
qcow2_opts->preallocation = PREALLOC_MODE_OFF;
}
if (qcow2_opts->has_backing_file &&
qcow2_opts->preallocation != PREALLOC_MODE_OFF)
qcow2_opts->preallocation != PREALLOC_MODE_OFF &&
!qcow2_opts->extended_l2)
{
error_setg(errp, "Backing file and preallocation cannot be used at "
"the same time");
error_setg(errp, "Backing file and preallocation can only be used at "
"the same time if extended_l2 is on");
ret = -EINVAL;
goto out;
}

2
tests/qemu-iotests/206.out
View File

@ -203,7 +203,7 @@ Job failed: Different refcount widths than 16 bits require compatibility level 1
=== Invalid backing file options ===
{"execute": "blockdev-create", "arguments": {"job-id": "job0", "options": {"backing-file": "/dev/null", "driver": "qcow2", "file": "node0", "preallocation": "full", "size": 67108864}}}
{"return": {}}
Job failed: Backing file and preallocation cannot be used at the same time
Job failed: Backing file and preallocation can only be used at the same time if extended_l2 is on
{"execute": "job-dismiss", "arguments": {"id": "job0"}}
{"return": {}}