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

[Marcelo Tosatti <mtosatti@redhat.com>]

On Wed, Feb 22, 2012 at 05:13:32PM +0000, Federico Simoncelli wrote:
> 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.

What is the motivation here? What is the limitation with image streaming
that this tries to solve?

> 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