After years of being touted by startup vendors, and knocked as "the communist" way by the big boys pooled storage has now been embraced by all. Now it is time to make sure everyone understands the downfall. Sure your vendor will tell you all about how it results in an optimal back end disk layout. How it will completely eliminate back end hot spots. These are some of the compelling reasons to utilize pooled storage. When you think about the fact that we are talking about shared resources and shared environments I think it makes perfect sense. Once all the kinks are worked out of the ability to drain, grow, and spread pools on the fly there will only be a single valid arguement against Pooled storage.
The loss of a Raid rank takes out the entire pool. Vendors are taking different approaches to this issue. Some are recommending Raid 6 across the board, some are completly ignorning the issue, and some are arguing that storage arrays are smarter today therefore drives almost never fail hard. In my humble opinion the quickest solution is to get Raid 6 to perform as close to Raid 5 as possible. How do you do this? Calculate both sets of parity somewhere besides the physical drive. This has been accomplished for Raid 5 by all the major players, and I am sure before long Raid 6 will also be similarly optimized.
The only other option today is keeping more than a single copy of data. I would even go as far to say this should be a requirement for any application in which downtime results in a significant loss of $$$$. The question of where and how to keep these additional copies of data will be a topic for another day.... The message today though is that pooled storage is the future of shared storage, but dont jump head first into the pool without know how deep it is...
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