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

[Stefan Hajnoczi]

On Wed, Feb 22, 2012 at 5:13 PM, Federico Simoncelli
<address@hidden> wrote:
> Preparation
> ===========
>
> $ mkdir /tmp/{src/dst}
> $ qemu-img create -f qcow2 /tmp/src/hd0base.qcow2 20G
> Formatting '/tmp/src/hd0base.qcow2', fmt=qcow2 size=21474836480
> encryption=off cluster_size=65536
>
> Step 1 - Initital Scenario
> ==========================
> VM1 is running on the src/hd0base. (Where "<=" stands for "uses")
>
> [src/hd0base] <= VM1(read-write)
>
> $ qemu-system-x86_64 -hda /tmp/src/hd0base.qcow2 -monitor stdio
> QEMU 1.0.50 monitor - type 'help' for more information
> (qemu)
>
> 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

At this stage /tmp/dst/hd0base.qcow2 does not exist yet.  The qemu-img
output you pasted shows /tmp/src/hd0base.qcow2 was actually used.
Typo?

> (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)
>
> $ cp -a /tmp/src/hd0base.qcow2 /tmp/dst/hd0base.qcow2

Are we missing a fixup step that changes backing_file in
dst/hd0snap1.qcow2 to point at dst/hd0base.qcow2?

> Step 5 - Final Switch to Destination
> ====================================
> VM1 is now able to switch to the destination for both read and write
> operations.
>
> [src/hd0base] <- [src/hd0snap1] <= VM1(read-write)
>
> (qemu) snapshot_blkdev -n ide0-hd0 /tmp/dst/hd0snap1.qcow2
>
> --
> Federico
>
>