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qcow2: Add prealloc field to QCowL2Meta

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This field allows us to indicate that the L2 metadata update does not
come from a write request with actual data but from a preallocation
request.

For traditional images this does not make any difference, but for
images with extended L2 entries this means that the clusters are
allocated normally in the L2 table but individual subclusters are
marked as unallocated.

This will allow preallocating images that have a backing file.

There is one special case: when we resize an existing image we can
also request that the new clusters are preallocated. If the image
already had a backing file then we have to hide any possible stale
data and zero out the new clusters (see commit 955c7d6687 for more
details).

In this case the subclusters cannot be left as unallocated so the L2
bitmap must be updated.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <960d4c444a4f5a870e2b47e5da322a73cd9a2f5a.1594396418.git.berto@igalia.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
This commit is contained in:
Alberto Garcia 2020-07-10 18:13:12 +02:00 • committed by Max Reitz
parent 0dd07b298f
commit 40dee94320
3 changed files with 15 additions and 1 deletions
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2
block/qcow2-cluster.c
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@ -1067,7 +1067,7 @@ int qcow2_alloc_cluster_link_l2(BlockDriverState *bs, QCowL2Meta *m)
set_l2_entry(s, l2_slice, l2_index + i, offset | QCOW_OFLAG_COPIED);
/* Update bitmap with the subclusters that were just written */
if (has_subclusters(s)) {
if (has_subclusters(s) && !m->prealloc) {
uint64_t l2_bitmap = get_l2_bitmap(s, l2_slice, l2_index + i);
unsigned written_from = m->cow_start.offset;
unsigned written_to = m->cow_end.offset + m->cow_end.nb_bytes ?: