On Thu, Apr 17, 2025 at 01:39:12PM -0500, Eric Blake wrote: > When doing a sync=full mirroring, QMP drive-mirror requests full > zeroing if it did not just create the destination, and blockdev-mirror > requests full zeroing unconditionally. This is because during a full > sync, we must ensure that the portions of the disk that are not > otherwise touched by the source still read as zero upon completion. > > However, in mirror_dirty_init(), we were blindly assuming that if the > destination allows punching holes, we should pre-zero the entire > image; and if it does not allow punching holes, then treat the entire > source as dirty rather than mirroring just the allocated portions of > the source. Without the ability to punch holes, this results in the > destination file being fully allocated; and even when punching holes > is supported, it causes duplicate I/O to the portions of the > destination corresponding to chunks of the source that are allocated > but read as zero. > > Smarter is to avoid the pre-zeroing pass over the destination if it > can be proved the destination already reads as zero. Note that a > later patch will then further improve things to skip writing to the > destination for parts of the image where the source is zero; but even > with just this patch, it is possible to see a difference for any BDS > that can quickly report that it already reads as zero. > > Signed-off-by: Eric Blake <ebl...@redhat.com> > --- > block/mirror.c | 12 +++++++++++- > 1 file changed, 11 insertions(+), 1 deletion(-)
Reviewed-by: Stefan Hajnoczi <stefa...@redhat.com>
signature.asc
Description: PGP signature
