Thursday, 18 December 2008

Do You Really Need a SAN - Of Course You Do!

The wacky boys at Forrester have a great new article posted relating to the requirement to have a Storage Area Network. Here's a link to their post. Tony Asaro and Hu Yoshida have both posted on the subject already but I couldn't resist having my 2 cents.

SANs evolved for very good reasons; the need to consolidate storage, the need to provide additional connectivity to storage arrays and the need to remove the requirement to closely couple the storage to the server (remember 25m limits on SCSI cabling). SANs and most notably fibre channel, enable that (as does iSCSI for the record).

Some of the Forrester objections to deploying SAN include;

Low Capacity Utilization - I did some work a couple of weeks ago at a client with purely DAS . They had 30% utilisation on their disks and after excluding boot drives, still had 30% utilisation. I've never seen a SAN array only 30% full, unless it was a new array onto which data was being deployed.

Inability to Prioritize Application Performance - Hmm, this seems a bit odd. DMX has Dynamic Cache Partitioning, Optimiser, USP has I/O prioritisation, Compellent can dynamically move data between tiers of storage; 3PAR has similar features which allow performance to be tweaked dynamically. There's also the same options in the fabric, especially with Cisco equipment. DAS has no such benefit, in fact if you have performance issues on a single server then you're potentially in a world of pain to fix it.

Long Provisioning Times - this is not a technology issue but one of process. I can provision terabytes of storage in minutes, however I have to guarantee that within a shared environment I don't take down another production environment - that's the nature of shared systems. In addition, users think they can just demand more and more storage without any consequences. Storage resources are finite - even more so in a non-scalable DAS solution. With sensible process, SAN storage can be turned around in hours - not the case for DAS unless you intend keeping spare disks onsite (at a price).

Soaring Costs - again, another conundrum. If you focus on pure hardware then SANs are inevitably more expensive, however TCO for storage is very rarely done. Don't forget SAN also includes iSCSI, which can be implemented across any IP hardware - hardly expensive.

So, there are other benefits that SAN easily wins over DAS.

Disaster Recovery. SAN-based replication is essential in large environments where the requirement to manually recover each server would be totally impractical. Imagine trying to recover 100+ database servers where each server requires the DBA to log in and perform forward recovery of shipped logs - all in a 2 hour recovery window.

Storage Tiering. SANs allow easy access to multiple storage tiers, either within the same array or across multiple arrays. Without SAN, tiering would be wasteful, as most servers would not be able to utilise multiple tiers fully.

SANs also provide high scalability and availability, simply not achievable with DAS.

There was a reason we moved to SAN. SAN has delivered, despite what Forrester say. However like all technologies, they need to be managed correctly. With sensible planning, standards and process, SANs knock DAS into a cocked hat.


Tony Asaro said...

Excellent and pragmatic points. I was really interested in what the report had to say and was hoping for some real vision and innovative thought.

Chris M Evans said...

Thanks Tony

Like a lot of IT and storage in particular, process is king. Get that right and with some good standards, you're running a tight ship.