Re: [Xen-users] loopback domU write

["Daniel P. Berrange" <berrange@xxxxxxxxxx>]

On Tue, Jun 12, 2007 at 06:57:11PM +0200, Petersson, Mats wrote:
>  
> 
> > -----Original Message-----
> > From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
> > [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
> > Chris Moore
> > Sent: 08 June 2007 20:18
> > To: xen-users@xxxxxxxxxxxxxxxxxxx
> > Subject: [Xen-users] loopback domU write
> > 
> > 
> > I have a architecture-esque question:
> > 
> > When a file-backed(loopback, not blktap) PV domU writes, I 
> > have heard that it does not write directly to disk and is 
> > buffered somewhere.
> > Where then, is it buffered?  Is it in the dom0 loopback 
> > driver(mem), the dom0 filesystem cache(mem) or somewhere else? 
> 
> The dom0 filesystem cache will DEFINITELY be a part of the
> caching/buffering. I don't know if the loopback driver also has some
> caches - it seems silly to cache here (because it is better to have one
> (larger) cache in one place (compared to multiple smaller caches), and
> loopback ends up in a filesystem driver sooner or later), but it's
> obviously POSSIBLE to do. 

loopback does indeed cache data for an arbitrarily long time. This is
the primary reason why you should *never* use loopback under any 
circumstance. It does not honour guest requests to flush data to disk,
so if your Dom0 crashes say goodbye vast amounts of data from your
DomU filesystems.  Always use blktap  for file backed guests if you
care about your data in the slightest. BTW is this only a paravirt 
problem really - HVM guests don't use the loopback driver as QEMU 
handles writing to files/devices natively without needing loopback
devices.

Dan.
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