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 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.

Paolo