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block: drop unallocated_blocks_are_zero

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Currently this field only set by qed and qcow2. But in fact, all
backing-supporting formats (parallels, qcow, qcow2, qed, vmdk) share
these semantics: on unallocated blocks, if there is no backing file they
just memset the buffer with zeroes.

So, document this behavior for .supports_backing and drop
.unallocated_blocks_are_zero

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200528094405.145708-10-vsementsov@virtuozzo.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
This commit is contained in:
Vladimir Sementsov-Ogievskiy 2020-05-28 12:44:04 +03:00 • committed by Max Reitz
parent cdf9ebf18f
commit a2adbbf603
5 changed files with 13 additions and 15 deletions
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12
include/block/block_int.h
View file

@ -123,7 +123,17 @@ struct BlockDriver {
*/
bool bdrv_needs_filename;
/* Set if a driver can support backing files */
/*
* Set if a driver can support backing files. This also implies the
* following semantics:
*
* - Return status 0 of .bdrv_co_block_status means that corresponding
* blocks are not allocated in this layer of backing-chain
* - For such (unallocated) blocks, read will:
* - fill buffer with zeros if there is no backing file
* - read from the backing file otherwise, where the block layer
* takes care of reading zeros beyond EOF if backing file is short
*/
bool supports_backing;
/* For handling image reopen for split or non-split files */