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qcow2: Forward ZERO_WRITE flag for full preallocation 

The BDRV_REQ_ZERO_WRITE is currently implemented in a way that first the
image is possibly preallocated and then the zero flag is added to all
clusters. This means that a copy-on-write operation may be needed when
writing to these clusters, despite having used preallocation, negating
one of the major benefits of preallocation.

Instead, try to forward the BDRV_REQ_ZERO_WRITE to the protocol driver,
and if the protocol driver can ensure that the new area reads as zeros,
we can skip setting the zero flag in the qcow2 layer.

Unfortunately, the same approach doesn't work for metadata
preallocation, so we'll still set the zero flag there.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200424142701.67053-1-kwolf@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Kevin Wolf 2020-04-24 16:27:01 +02:00
parent bf03dede47
commit eb8a0cf3ba
2 changed files with 21 additions and 5 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

22
block/qcow2.c
View File

@ -4170,9 +4170,25 @@ static int coroutine_fn qcow2_co_truncate(BlockDriverState *bs, int64_t offset,
/* Allocate the data area */
new_file_size = allocation_start +
nb_new_data_clusters * s->cluster_size;
/* Image file grows, so @exact does not matter */
ret = bdrv_co_truncate(bs->file, new_file_size, false, prealloc, 0,
errp);
/*
* Image file grows, so @exact does not matter.
*
* If we need to zero out the new area, try first whether the protocol
* driver can already take care of this.
*/
if (flags & BDRV_REQ_ZERO_WRITE) {
ret = bdrv_co_truncate(bs->file, new_file_size, false, prealloc,
BDRV_REQ_ZERO_WRITE, NULL);
if (ret >= 0) {
flags &= ~BDRV_REQ_ZERO_WRITE;
}
} else {
ret = -1;
}
if (ret < 0) {
ret = bdrv_co_truncate(bs->file, new_file_size, false, prealloc, 0,
errp);
}
if (ret < 0) {
error_prepend(errp, "Failed to resize underlying file: ");
qcow2_free_clusters(bs, allocation_start,

4
tests/qemu-iotests/274.out
View File

@ -187,7 +187,7 @@ read 65536/65536 bytes at offset 9437184
10 MiB (0xa00000) bytes allocated at offset 5 MiB (0x500000)
[{ "start": 0, "length": 5242880, "depth": 1, "zero": true, "data": false},
{ "start": 5242880, "length": 10485760, "depth": 0, "zero": true, "data": false, "offset": 327680}]
{ "start": 5242880, "length": 10485760, "depth": 0, "zero": false, "data": true, "offset": 327680}]
=== preallocation=full ===
Formatting 'TEST_DIR/PID-base', fmt=qcow2 size=16777216 cluster_size=65536 lazy_refcounts=off refcount_bits=16
@ -206,7 +206,7 @@ read 65536/65536 bytes at offset 11534336
4 MiB (0x400000) bytes allocated at offset 8 MiB (0x800000)
[{ "start": 0, "length": 8388608, "depth": 1, "zero": true, "data": false},
{ "start": 8388608, "length": 4194304, "depth": 0, "zero": true, "data": false, "offset": 327680}]
{ "start": 8388608, "length": 4194304, "depth": 0, "zero": false, "data": true, "offset": 327680}]
=== preallocation=off ===
Formatting 'TEST_DIR/PID-base', fmt=qcow2 size=393216 cluster_size=65536 lazy_refcounts=off refcount_bits=16