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

[Anthony Liguori]

On 02/27/2012 08:54 AM, Paolo Bonzini wrote:
> On 02/27/2012 03:46 PM, Anthony Liguori wrote:
> > I think a better way to think of this is as a batch submission.  It
> > would be relatively easy to model in QMP too (just have a batch-command
> > that has a list of commands as it's argument).
> >
> > The difference between batch submission and a transaction is atomic
> > rollback. But I don't think atomic rollback is really needed here.
>
> A transaction enforces atomicity at the block layer level.  It's
> different from batch commands in two ways:
>
> * bdrv_drain_all/bdrv_flush needs to be called at the beginning of the
> commit.  This may not be the case with batch commands.
>
> * with batch commands, atomicity happens by chance because VCPUs cannot
> send I/O while the monitor is holding the global mutex.

If the commands are designed correctly, then this works well.  The problem is 
that the current commands are not designed well.  For instance, multi-snapshot 
could look like:

block-freeze ide0-hd0
block-freeze ide1-hd1
block-reopen ide0-hd0 my-new-file0.qcow2
block-reopen ide1-hd1 my-new-file1.qcow2
block-unfreeze ide1-hd1
block-unfreeze ide1-hd0

This would work regardless of whether the commands were implemented 
asynchronously within QEMU too.

Regards,

Anthony Liguori

> Paolo