[Xen-users] RE: AW: Best way to use iSCSI in domU

[Jeff Sturm <jeff.sturm@xxxxxxxxxx>]

> -----Original Message-----
> From: Ferenc Wagner [mailto:wferi@xxxxxxx]
> Sent: Monday, June 22, 2009 8:55 AM
> To: Jeff Sturm
> Cc: xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: Re: AW: Best way to use iSCSI in domU
> 
> Jeff Sturm <jeff.sturm@xxxxxxxxxx> writes:
> 
> > We also use tap:sync: to ensure blocks are flushed to the SAN as
> > they are written.  If you have just one dom0 and could tolerate
> > possible data loss, you're probably fine using phy: or tap:aio:
> > instead.
> 
> Doesn't phy: use direct IO (avoiding buffer cache) as well?

Empirically, my guess is "no", or at least that writes are not performed
synchronously.  Our GFS cluster seems stable using tap:sync: as the
backend, whereas it became corrupted on average once a week using phy:.
We have since had 3 weeks of continuous uptime on the cluster.

Frankly, I don't understand the kernel interfaces well enough to say for
certain.  I asked a question about this a few weeks ago on this list,
and didn't receive a reply.  If anyone knows for certain, it would be
good to have confirmation, because I can't completely rule out some
other cause of our fs corruption.

-Jeff



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