Block live migration without pausing VM

Block live migration without pausing VM

[Felix Leimbach]

Hi all,

I want to implement live-migration of a highly available VM but I cannot 
use shared storage.
The -b option to the migrate command already allows for copying the 
block device (locally stored raw file) and that is exactly what I want.
This worked in my experiments but the VM is unreachable (CPU halted?) 
while the block device is copied, which is unacceptable for my use-case.

Is there a way to copy the block device while the VM is running (copy on 
read or such)?
The LiveBlockMigration page in the qemu wiki [1] mentions that some of 
this is already implemented, but I cannot find any of it in the latest 
qemu-kvm.
Any pointers?

Thanks for your suggestions.

[1] http://wiki.qemu.org/Features/LiveBlockMigration

Re: Block live migration without pausing VM

[Stefan Hajnoczi]

On Sun, Sep 23, 2012 at 5:03 PM, Felix Leimbach
<felix.leimbach@gmail.com> wrote:
> I want to implement live-migration of a highly available VM but I cannot use
> shared storage.
> The -b option to the migrate command already allows for copying the block
> device (locally stored raw file) and that is exactly what I want.
> This worked in my experiments but the VM is unreachable (CPU halted?) while
> the block device is copied, which is unacceptable for my use-case.
>
> Is there a way to copy the block device while the VM is running (copy on
> read or such)?
> The LiveBlockMigration page in the qemu wiki [1] mentions that some of this
> is already implemented, but I cannot find any of it in the latest qemu-kvm.
> Any pointers?

You can live migrate storage - either to move image files to a new
file system on the same host or before/after live migration - using
the QMP block-stream command.  See QEMU's ./qapi-schema.json for
documentation on the block streaming commands.

Libvirt exposes this command as "blockpull".

Copying happens in the background while the VM is running.  Note that
image streaming is orthogonal to guest live migration, you need to do
it either before or after (probably onto an NFS export).

Stefan