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

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

Both.

Degradation is inherent to the concept of shared storage. Either you trust the 
fact that you're alone on a disk and use diskcaching or you're not and you need 
to use some other mechanism to sync the changes amongst the participants. This 
can be via I/O or via the network, so you always have delay and bandwith usage.

So, if you would compare ext3 on san disk to a gfs or clvm volume, you would 
get a performance hit. But, would you say, those are things you can't compare 
as gfs en clvm does things an ext3 on a san disk can't. True. But do we _need_ 
those things ? No. In fact, we do _not_want_ the ability to start the same domu 
several times on different machines. And by letting the domu use the san disk 
as a dedicated disk, it can use its diskcache without taking any other machines 
in to account. Even live migration is not a problem, as the memory ( and thus 
the disk cache ) just get migrated too ...

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 complex 
? Complex setups have complex problems in my experience. We solved the problem 
with a very simple script, checking all cluster members to see if a specific 
domu was running or not. Does this weigh up to having to learn,administer,tune 
en possibly debug a clustered file system ?

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.

Peter.
Ps: nice line-up of acronyms, btw 8-)

On Monday 08 September 2008 17:30:25 Javier Guerra wrote:
> On Mon, Sep 8, 2008 at 10:23 AM, Peter Van Biesen
> <peter.vanbiesen@xxxxxxx> wrote:
> > We tested that too. It is overkill and degrades performance.
> 
> do you mean GFS/OCFS, or CLVM/EVMS?
> 
> the former have obviously higher overhead, the later shouldn't....
> 
> it's important because if you don't have netapp-level storage
> subsystems with great administration, then you can do iSCSI/AoE/gnbd
> and CLVM, and get great $/GB results with still really good
> administrability.
> 



-- 
Peter Van Biesen
Sysadmin VAPH

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e-mail: peter.vanbiesen@xxxxxxx
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