Re: [Qemu-devel] live migration which includes previos snapshot

[Kuniyasu Suzaki]

Hello Eric,

From: Eric Blake <address@hidden>
Subject: Re: [Qemu-devel] live migration which includes previos snapshot
Date: Fri, 02 Nov 2012 07:12:21 -0600

> On 11/02/2012 04:30 AM, Stefan Hajnoczi wrote:
> 
> > II. If you want to use internal snapshots in a single qcow2 file, you
> > will need to modify QEMU code more:
> > 1. Implement BlockDriverState snapshot slave support so a qcow2
> > snapshot can be read-only accessed as a BlockDriverState while the
> > master BlockDriverState for the image still writes into the image
> > file.  This is mainly qcow2 refactoring and block.c glue code.
> 
> You are not the first to request this - libvirt would also like the
> ability to have read-only access into the contents of an internal
> snapshot while the rest of qemu continues to write into the image.

Do you mean that libvirt can change the access mode of internal
harddisk from read-write to read-only?
Please tell me how to change the mode by libvirt.

Does the qemu which has read-only access only, use another COW file?
Nested COWs sound interested, but the inter COW must be read-only, I think.

> > 2. Use Paolo's runtime NBD server to export the snapshot slave when
> > the VM is forked:
> 
> An NBD server on top of the read-only state is an additional step that
> will make access easier.

Does an NBD work as COW? It looks convenient.

Thank you.
------
suzaki