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  5. Cache or Tier? Flash By Any Other Name. . .: Page 2 of 2
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Cache or Tier? Flash By Any Other Name. . .: Page 2 of 2

Today's flash SSDs are more than just the latest in a long series of storage systems that provide performance at any cost. RAMdisks, head per track disk, short-stroked drives and the like all boosted performance for datasets small enough to fit in the limited space they provided. Vendors are promising that we can use the performance of SSDs for more mainstream applications by moving the "hottest" data into SSDs and leaving the rest behind on capacity oriented drives. Now we just have to agre
Howard Marks
March 30, 2010

Avere calls their FXT a "tiered NAS appliance," and tells me I shouldn't call it a cache. NetApp CEO Tom Georgens says tiering is dying to be replaced by caching, so there's a bit of name confusion. As Juliet might say, if she were a storage geek "What's in a name? That which we call a cache; by any other name would still be wicked fast."  

The cache approach, as an appliance or in-array as NetApp's PAM or ZFS's readzilla/logzilla combo, seems to have a couple of advantages to me. First, caches can react faster to changes in I/O demand when not having to wait for the policy engine to run. When blocks are promoted, a tiering scheme would release the low speed disk space and later copy the data back from the higher tier as it "cools off," and if the blocks were promoted because of frequent reads, as would be true of some indexes, they would have to be copied back down as they cooled. In a cache system, those I/Os could be avoided.

I can't wait for Flash and Trash, but the technology is still young.  I'm sure we'll end up with technologies to merge the low cost/IOP of solid state memory and the low cost/GB of capacity oriented disks that will have aspects of caching and tiering. I'll beat them all into submission in the lab till the best tech wins.

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