[Qemu-devel] Live Block Migration using Mirroring

[Federico Simoncelli]

Hi,
 recently I've been working on live block migration combining the live
snapshots and the blkmirror patch sent by Marcelo Tosatti few months ago.

The design is summarized at this url as "Mirrored-Snapshot": 

http://www.ovirt.org/wiki/Features/Design/StorageLiveMigration

The design assumes that the qemu process can reach both the source and
destination storages and no real VM migration between hosts is involved.
The principal problem that it tries to solve is moving a VM to a new
reachable storage (more space, faster) without temporarily disrupting its
services.

The following set of patches are implementing the required changes in
QEMU.

Here it is a quick example of the use case (for consistency with the
design at the url above I will use the same step numbers):

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

(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

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