Re: [Qemu-devel] Re: KVM call minutes for Sept 14

[Anthony Liguori]

On 09/15/2010 08:30 AM, Kevin Wolf wrote:
> Am 15.09.2010 15:21, schrieb Anthony Liguori:
>    
> > On 09/15/2010 07:38 AM, Kevin Wolf wrote:
> >      
> > > No, we don't really care if the L2 entry is on disk. If the guest want
> > > to have its data safe it needs to issue an explicit flush anyway. The
> > > only thing we want to achieve with bdrv_write_sync is to maintain the
> > > right order between metadata updates to survive a crash without corruption.
> > >
> > >        
> > Ah, yes, this is brand new :-)
> >
> > I was looking at my QED branch which is a few weeks old.
> >      
> Well, the whole bdrv_pwrite_sync thing is new - with your benchmarking
> you probably caught qcow2 at its worst performance in years.

FWIW, we queued a run reverting the sync() stuff entirely as we were 
aware of that.  Should have results this morning.

>   Initially I
> just blindly converted everything to be on the safe side, and now we
> need to optimize to get the performance back. There are probably some
> more syncs that can be removed in less common paths.
>    

Most likely.

Regards,

Anthony Liguori

> Kevin
>