Re: [Qemu-devel] Live Block Migration using Mirroring

[Stefan Hajnoczi]

On Wed, Feb 22, 2012 at 5:13 PM, Federico Simoncelli
<address@hidden> 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