| Message ID | 20200424142701.67053-1-kwolf@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | block: Fix resize (extending) of short overlays | expand |
On 24.04.20 16:27, Kevin Wolf wrote: > 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> > --- > block/qcow2.c | 22 +++++++++++++++++++--- > tests/qemu-iotests/274.out | 4 ++-- > 2 files changed, 21 insertions(+), 5 deletions(-) Thanks for the resend! :) Max
24.04.2020 17:27, Kevin Wolf wrote: > 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> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
diff --git a/block/qcow2.c b/block/qcow2.c index 98065d7808..2ba0b17c39 100644 --- a/block/qcow2.c +++ b/block/qcow2.c @@ -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, diff --git a/tests/qemu-iotests/274.out b/tests/qemu-iotests/274.out index 1a796fd07c..9d6fdeb1f7 100644 --- a/tests/qemu-iotests/274.out +++ b/tests/qemu-iotests/274.out @@ -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