Re: [Qemu-devel] qcow2 - safe on kill? safe on power fail?

[Anthony Liguori]

Avi Kivity wrote:
> Anthony Liguori wrote:
> > Sure you can.  If you don't have a battery backed disk cache and are 
> > using write-back (which is usually the default), you can definitely 
> > get corruption of the journal.  Likewise, under the right scenarios, 
> > you will get journal corruption with the default mount options of 
> > ext3 because it doesn't use barriers.
>
> What about SCSI or SATA NCQ?  On these, barriers don't impact 
> performance greatly.

Good question, I don't know the answer.  But ext3 doesn't autodetect 
SCSI/NCQ or anything.  It disabled barriers by default.  Some distros 
have changed this behavior historically (SLES I believe).

> > This is very hard to see happen in practice though because these 
> > windows are very small--just like with QEMU.
>
> The exposure window with qemu is not small.  It's as large as the page 
> cache of the host.

Note I was careful to qualify my statements that cache=off was required.

Regards,

Anthony Liguori