Re: [Qemu-devel] [RFC]VM live snapshot proposal

[Kevin Wolf]

Am 03.03.2014 um 14:47 hat Paolo Bonzini geschrieben:
> Il 03/03/2014 14:30, Kevin Wolf ha scritto:
> >> > So why don't we simply reuse the existing migration code?
> >> I think this is different in the same way that block-backup and
> >> block-mirror are different.  Huangpeng's proposal would let you make
> >> a consistent snapshot of disks and RAM.
> >Right. Though the point isn't about consistency (doing the disk snapshot
> >when memory has converged would be consistent as well), but about
> >having the snapshot semantically right at the time when the monitor
> >command is issued instead of only starting it then and being consistent
> >at the point of completion.
> 
> Right---though it's not entirely true that migration only affects
> the point in time where you have consistency.  For example, with
> migration you cannot use the guest agent for freeze/thaw and, even
> if we changed the code to allow that, the pause would be much longer
> than for live snapshots or block-backup.
> 
> >This is indeed like pre/post-copy live migration, and probably both
> >options have their uses. I would suggest starting with the easy one, and
> >adding the post-copy feature on top.
> 
> The feature matrix for migration and snapshot
> 
>                           disk       RAM        internal snapshot
> non-live                  yes (0)    yes (0)    yes
> live, disk only           yes (1)    N/A        yes (2)
> live, pre-copy            yes (3)    yes        no
> live, post-copy           yes (4)    no         no
> live, point-in-time       yes (5)    no         no
> 
>     (0) just stop VM while doing normal pre-copy migration
>     (1) blockdev-snapshot-sync
>     (2) blockdev-snapshot-internal-sync
>     (3) block-stream
>     (4) drive-mirror
>     (5) drive-backup
> 
> By "the easy one" you mean live savevm with snapshot at the end of
> RAM migration, I guess.  But the functionality is already available
> using migration, while point-in-time snapshots actually add new
> functionality.  I'm not sure what's the status of the kernel
> infrastructure for post-copy.  Andrea?

Yes, it's available, but not with internal snapshots, but only with
RAM snapshots stored in an external file.

An incremental next step would be to avoid writing dirtied memory to two
places, because internal snapshots aren't a streaming, but a random
access interface, so you can overwrite the original place instead of
appending the new copy. That would already be a small advantage.

Once you have this infrastructure, it's probably also a bit easier to
plug in any post-copy/point-in-time features that the migration code can
(be improved to) provide.

Kevin