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

[Dietmar Maurer]

> 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?