Re: [Qemu-devel] drive transactions (was Re: [PATCH 2/2 v2] Add the blockdev-reopen and blockdev-migrate commands)

[Paolo Bonzini]

On 02/27/2012 04:24 PM, Anthony Liguori wrote:
> 
> Then you get an error with the block devices still frozen.  You can
> execute another command to reopen back to the old image to roll back the
> transaction.
> 
> Pushing the rollback logic to the client does make the client interface
> a bit more complicated and adds latency to the error path but it's much
> easier than building a complex transaction infrastructure.
> 
> And there are examples of this in the wild too.  LDAP uses a similar
> mechanism.

Actually, have you seen Jeff's atomic snapshot patch?  It really
implements all that is needed to do transactions, and gets 100% ok
error-recovery unlike the existing blockdev_snapshot_sync.  It really
looks like we can do better than client-side error management, and there
is not that much complexity at all.

Jeff could rework his patches to work with transaction begin/commit
commands, and Federico can then add drive-reopen and drive-migrate on top.

Paolo