Re: IO caching on xen?

[Samuel Thibault <samuel.thibault@eu.citrix.com>]

Dan Magenheimer, le Tue 15 Jul 2008 12:32:53 -0600, a écrit :
> > marian klein, le Tue 15 Jul 2008 11:22:05 +0000, a écrit :
> > > I am worried that the xen (or any other VM) with zvol 
> > device from ZFS
> > > (or any other file system.) may remove the benefits of ZFS,
> > >  because the guest OS can write something to disk but the xen itself
> > > can do some buffering
> > > or lazy writing and it may delay to write to ZFS backend.
> > 
> > Xen doesn't do any kind of buffering or lazy writing.  It provides
> > barriers to the guest, to ensure any needed coherency.
> 
> I'm not sure if this is related or not...
> 
> I was party to some internal irc where it was believed
> to be the case that when a file-backed vbd using file:'...'
> is used, dom0's page cache may contain buffer pages that
> may be inconsistent with the disk

Ah, indeed, in that case the page cache is used, and the barriers may
not have proper effect.

> whereas a file-backed vbd using tap:aio:'...' does not suffer from the
> same problem.

blktap uses O_DIRECT to avoid buffering completely indeed.

Samuel