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Re: [Qemu-devel] [PATCH v4 0/6] Efficient VM backup for qemu


From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] [PATCH v4 0/6] Efficient VM backup for qemu
Date: Thu, 21 Feb 2013 14:08:27 +0100
User-agent: Mutt/1.5.21 (2010-09-15)

On Wed, Feb 20, 2013 at 04:04:49PM +0000, Dietmar Maurer wrote:
> > Interesting series, the backup block job makes sense to me.  Regarding
> > efficiency, I think incremental backup is a must, otherwise regular backups
> > using this approach won't really be a win over backing files.
> 
> Incremental backup is something different, not touched by this code.
> You can add that later if needed.
>  
> > The backup writer abstraction is a special case interface somewhere
> > between an image format and live migration.  I'd prefer it if the backup
> > block job stuck to BlockDriverState as the destination for backup data.
> > Then you could save a point-in-time copy of a disk to a raw or even
> > qcow2 image.
> 
> backup needs to work on non-seekable pipes.
> 
> > The vmstate feature looks like vmsave duplication.  The problem with
> > vmsave is that it doesn't compose nicely with block jobs or the QMP
> > transaction command, which would allow you to snapshot the disks and
> > then capture the VM state.
> >
> > How would VMA fit in if the backup block job took a BlockDriverState
> > destination and vmsave worked atomically with backup block jobs?  You
> > could have QEMU write to NBD server ports that a VMA writer process has
> > open.  VMA would happen outside the QEMU process.
> 
> We want to write to pipes, so what you suggest simply does not work?

Writing to pipes still works, the data is just passed first to a process
on the host which writes the archive to the pipe.  The archive write
process writes to the pipe, not QEMU.

Stefan



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