Re: [Xen-users] Sharing space on a SAN?

[Peter Van Biesen <peter.vanbiesen@xxxxxxx>]

On Wednesday 10 September 2008 17:43:09 Javier Guerra wrote:
> On Wed, Sep 10, 2008 at 10:09 AM, Peter Van Biesen
> <peter.vanbiesen@xxxxxxx> wrote:
> > Both.
> >
> > Degradation is inherent to the concept of shared storage. Either you trust 
> > the
> 
> in this case the degradation isn't because the storage is shared; it's
> because of the sync mechanisms to avoid stepping on the other
> machine's toes.  And there's a world of difference between locking to
> access the volume partition (CLVM/EVMS-ha) and locking at file level
> (GFS/OCFS).
Point taken. However, during live migration you still need two machines to be 
able to access the same volume at the same time. If this were possible, it 
would be an additional step you needed to do before migration - thus adding one 
more error you could make.

> > I simply do not see the added value of using a clustered filesystem for a 
> > domu. And in that light, any additional overhead is too much. Why make 
> > things
> 
> totally agree
> 
> but i don't find CLVM overhead any worse than LVM alone.  i asked
> because you seemed to advise against it, and wanted to know if that's
> because of specific experience, or just against cluster filesystems.
I had bad experiences with it, but maybe I haven't studied it long enough. I'm 
a bit reluctant to install things that span multiple machines. You can't 
'reset' it without bringing all of them down.

> > Lastly, I really don't see the $/GB argument. A GB cost the same, although 
> > its a bit slower on a clustered filesystem, that's all.
> 
> several not-so-big boxes with OpenFiler are A LOT cheaper than
> comparable capacity NetApp settings.  the only drawback is that you
> can't join/partition/migrate between boxes without help from the
> block-client boxes, thus using CLVM.
I See. Then I misunderstood. Migration could be a problem indeed. We didn't 
change storage system yet, but i suppose i will have to bring my domu's down 
and dd the disks over to do that. CLVM would have fixed that yes. Maybe I 
should pick that up again. Thinking about it, this would greatly simplify the 
'unused lun' problem, mmmmm ...

Another setup I was thinking about is starting every domu with a kernel en 
initrd for iscsi booting, so not using vbd at all. But maybe this would degrade 
performance far more than a clustered fs. And it adds a single point of 
failure, but facilitates migration if you use LVM on it.

> 
> > Peter.
> > Ps: nice line-up of acronyms, btw 8-)
> 
> yep, OTOH, TANSTAAFL, so a11y and r9y are way down, AFAICT  :-P
kindest regards,

Peter.
-- 
Peter Van Biesen
Sysadmin VAPH

tel: +32 (0) 2 225 85 70
fax: +32 (0) 2 225 85 88
e-mail: peter.vanbiesen@xxxxxxx
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