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

[Jeff Sturm <jeff.sturm@xxxxxxxxxx>]

> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx [mailto:xen-users-
> bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Angel L. Mateo
> Sent: Friday, June 19, 2009 2:29 AM
> To: xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: RE: AW: [Xen-users] Best way to use iSCSI in domU
> 
> El jue, 18-06-2009 a las 13:43 -0400, Jeff Sturm escribiÃ:
> > Exporting block devices from DomU instead was much more successful.  The
> > performance bottleneck seemed to vanish, we could tune the network as we
> > see fit (e.g. multipath / jumbo frames, good for storage), and the phy:
> > backend allows for asynchronous writes (although the latter prevented us
> > from doing this on shared GFS filesystems).
> >
>       OK. But supppose I need two domUs (in the same dom0) needing the same
> iSCSI resource. Could they share it if I configure it in the dom0?

Yes.  If you export it read/write however, you need to override Xen's sanity 
check
that would normally prevent you from doing so.

Here's an example:

node01: 'tap:sync:/dev/vg/wwwdocs,xvdg,w!'
node02:         'tap:sync:/dev/vg/wwwdocs,xvdg,w!'

In this example, /dev/vg/wwwdocs contains a clustered filesystem mounted 
read/write
on several domU instances, some of which are on the same dom0, some not.

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.

-Jeff

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