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

[Paolo Bonzini]

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?

Paolo