On 3/23/19 5:59 PM, Nir Soffer wrote: > With Kevin's "block: Fix slow pre-zeroing in qemu-img convert"[1] > we skip the pre zero step called like this: > > blk_make_zero(s->target, BDRV_REQ_MAY_UNMAP | BDRV_REQ_NO_FALLBACK) > > And we write zeroes later using: > > blk_co_pwrite_zeroes(s->target, > sector_num << BDRV_SECTOR_BITS, > n << BDRV_SECTOR_BITS, 0); > > Since we use flags=0, this is translated to NBD_CMD_WRITE_ZEROES with > NBD_CMD_FLAG_NO_HOLE flag, which cause the NBD server to allocated space > instead of punching a hole. >
> > $ qemu-img info dst.img > virtual size: 50M (52428800 bytes) > disk size: 5.0M Up to here makes sense for the commit message... > > Tested on top of Kevin patches: > http://lists.nongnu.org/archive/html/qemu-block/2019-03/msg00755.html > > I'm not sure this change is correct for all cases, posting for > discussion. ...this part probably belongs... > > [1] http://lists.nongnu.org/archive/html/qemu-block/2019-03/msg00761.html > > Signed-off-by: Nir Soffer <nir...@gmail.com> > --- ...here, after the ---, as it is useful for reviewers but not needed for the permanent git log. > qemu-img.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/qemu-img.c b/qemu-img.c > index 8ee63daeae..ca9deb3758 100644 > --- a/qemu-img.c > +++ b/qemu-img.c > @@ -1752,11 +1752,12 @@ static int coroutine_fn > convert_co_write(ImgConvertState *s, int64_t sector_num, > assert(!s->target_has_backing); > break; > } > ret = blk_co_pwrite_zeroes(s->target, > sector_num << BDRV_SECTOR_BITS, > - n << BDRV_SECTOR_BITS, 0); > + n << BDRV_SECTOR_BITS, > + BDRV_REQ_MAY_UNMAP); The idea makes sense to me. Maybe Kevin will come up with a counterargument why we don't want convert to allow holes by default, but unless we have a strong argument against it, it looks like a performance improvement worth having in 4.0. Reviewed-by: Eric Blake <ebl...@redhat.com> > if (ret < 0) { > return ret; > } > break; > } > -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org
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