Compellent and SSDs

There's been a lot of talk this week about Compellent and their support for solid state drives. See the press release here. So now we have two vendors offering SSD devices in their arrays, Compellent join the club with EMC. Which is best?

At a meeting I had last week, we discussed SSD drives and EMC's implementation in particular. The consensus was that SSDs (or should I be calling them EFDs?) in existing DMX array were more of an "also supports" rather than a mainline feature. The reason for that thinking was that DMX was never engineered specifically to support EFDs, but rather they've been added on as a recent value-add option. What's not clear is whether this bolt-on approach really means you get the best from the drives themselves, something that's important with the price point they sit at. Consider that EFDs sit behind a shared architecture of director ports, memory, front-end ports and queues. Do EFDs get priority access (I know they have to be placed in specific slots in the DMX storage cabinet so presumably they are affected by their position on the back-end directors).

The other problem with the EMC approach is that entire EFD LUNs must be given up to a host. With large databases, how do you predict which parts of the database at any one time are the hot parts? How does a re-org or reload affect the layout of the data? Either you need to put all of your database on EFD or spend a lot more time with the DBAs and Sys Admins creating a design that segments out active areas (and possibly repeating this process often).

If Compellent's technology works as described, then LUNs will be analysed at the block level and the active blocks will remain on the fastest storage with the least active moved to lower tiers of disk (or to other parts of the disk) within the same array.

This should offer a more granular approach to using SSDs for active data. In addition, if data can dynamically move up/down the stack of storage tiers, then as data profiles change over time, no application re-mapping or layout should be necessary. Hopefully this means that SSDs are used as efficiently as possible, justifying their inflated cost.

Just to conclude, I'm not saying Compellent have the perfect solution for using SSDs but it is a step in the right direction for making storage usage as efficient as possible.