[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
filesystem.

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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