On Wed, Feb 22, 2012 at 5:13 PM, Federico Simoncelli <fsimo...@redhat.com> wrote: > Step 3 - Mirrored Live Snapshot > =============================== > A mirrored live snapshot is issued using src/hd0snap1 and dst/hd0snap1 as > image files. (Where "<-" stands for "has backing file") > > [src/hd0base] <- [src/hd0snap1] <= VM1(read-write) > ... <- [dst/hd0snap1] <= VM1(write-only) > > $ qemu-img create -f qcow2 \ > -b /tmp/src/hd0base.qcow2 /tmp/src/hd0snap1.qcow2 20G > Formatting '/tmp/src/hd0snap1.qcow2', fmt=qcow2 size=21474836480 > backing_file='/tmp/src/hd0base.qcow2' encryption=off cluster_size=65536 > > $ qemu-img create -f qcow2 \ > -b /tmp/dst/hd0base.qcow2 /tmp/dst/hd0snap1.qcow2 20G > Formatting '/tmp/dst/hd0snap1.qcow2', fmt=qcow2 size=21474836480 > backing_file='/tmp/src/hd0base.qcow2' encryption=off cluster_size=65536 > > (qemu) snapshot_blkdev -n ide0-hd0 \ > blkmirror:/tmp/src/hd0snap1.qcow2:/tmp/dst/hd0snap1.qcow2 blkmirror > > Step 4 - Backing File Copy > ========================== > An external manager copies src/hd0base to the destination dst/hd0base. > > [src/hd0base] <- [src/hd0snap1] <= VM1(read-write) > [dst/hd0base] <- [dst/hd0snap1] <= VM1(write-only)
At this stage we have dst/hd0snap1 opened with BDRV_O_NO_BACKING. If it has no backing file and the guest issues a write request that is smaller than a cluster in the image file, the untouched areas of that cluster will be populated with zeroes. Once dst/hd0snap1 is reopened with dst/hd0base in place there will be zeros in clusters where the guest wrote only a few sectors. We will not "see" the backing file data in those clusters. Have you hit this problem or did I miss something? Stefan
