[CentOS] Dell Equallogic

Pasi Kärkkäinen pasik at iki.fi
Wed Oct 14 19:04:20 UTC 2009

On Wed, Oct 14, 2009 at 10:36:01AM -0700, James A. Peltier wrote:
> On Tue, 13 Oct 2009, Pasi Kärkkäinen wrote:
> >
> >Hmm.. what kind of problems do you have with snapshots?
> Let's see
> They take up 16MB  of space regardless of the amount of data that changes. 
> A single bit changes and 16MB is taken up.  This is not tunable.

EQL volumes are based on 'pages', that are (afaik) 16 MB in size. 
So the minimum increment is 16 MB if you have changes on the volume.

This is expected.

> Mounting a snapshot requires the generation of a new UUID for the file 
> system in order to be able to use it.  Perhaps this is a XFS limitation, 
> but unlike ZFS they aren't immediately available.  We need to perform this 
> action extensively to backup file systems and allow for quick data 
> recovery.

This is not Equallogic problem, it's a limitation/feature of the

Taking snapshots in the Equallogic storage array takes only seconds, and
the snapshots are immediately available.

> There is no GNU/Linux interface to automating the snapshot process. Sure 
> there is a telnet interface but it changes from version to version.  I 
> know because I've spoken with engineers to discuss this issue.  I wrote 
> software to talk to the EQL using the interface.  There are talks of an 
> SDK for developers but nothing has materialized.

I think Equallogic provides Perl samples how to take snapshots from
Linux/Unix scripts. I remember going through the scripts earlier.

So you can definitely integrate EQL stuff to your Linux environment.

> Don't get me wrong, it's a great device for cheap storage compared to 
> competitors but there are obvious limitations that you need to address in 
> your requirements if you intend to deploy it.

Well.. the stuff you mentioned isn't really Equallogic specific. It's
general stuff you need to sort out with any storage.

-- Pasi

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