OpenE2K/qemu-e2k
13
4
Fork 0
Code Issues 4 Pull Requests Projects 2 Releases Activity

qemu-img: Use BDRV_REQ_NO_FALLBACK for pre-zeroing

Browse Source 
If qemu-img convert sees that the target image isn't zero-initialised
yet, it tries to do an efficient zero write for the whole image first
to save the overhead of repeated explicit zero writes during the
conversion. Obviously, this provides only an advantage if the
pre-zeroing is actually efficient. Otherwise, we can end up writing
zeroes slowly while zeroing out the whole image, and then overwrite the
same blocks again with real data, potentially doubling the written data.

Pass BDRV_REQ_NO_FALLBACK to blk_make_zero() to avoid this case. If we
can't efficiently zero out, we'll instead write explicit zeroes only if
there is no data to be written to a block.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Acked-by: Eric Blake <eblake@redhat.com>
This commit is contained in:
Kevin Wolf 2019-03-22 13:49:28 +01:00
parent 738301e117
commit c9fdcf202f
1 changed files with 1 additions and 1 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
qemu-img.c
View File

@ -1932,7 +1932,7 @@ static int convert_do_copy(ImgConvertState *s)
if (!s->has_zero_init && !s->target_has_backing && if (!s->has_zero_init && !s->target_has_backing &&
bdrv_can_write_zeroes_with_unmap(blk_bs(s->target))) bdrv_can_write_zeroes_with_unmap(blk_bs(s->target)))
{ {
ret = blk_make_zero(s->target, BDRV_REQ_MAY_UNMAP); ret = blk_make_zero(s->target, BDRV_REQ_MAY_UNMAP | BDRV_REQ_NO_FALLBACK);
if (ret == 0) { if (ret == 0) {
s->has_zero_init = true; s->has_zero_init = true;
} }