Tuesday, September 13, 2011

FlashCache Pros and Cons

Ran across a brilliant article over at The Missing Shade of Blue that brought up something I'd never considered: latency is the real way to measure speed.  Throughput is a vital statistic to be sure, but if your throughput comes at the cost of latency you really have to consider that trade off.

Bit of background for noobs: obviously, fast data storage is more expensive than slow data storage.  Data tiering is pretty much the same idea as storing things in your closet: the stuff you use more often are in the front (fast, expensive storage like Flash memory), the stuff you never take out can be hidden deep in the back (slow, cheap storage like SATA).

Some companies, like Compellant, run algorithms to see what data is being read/written to infrequently and move that data to SATA, while the frequently-used data is moved to your faster SAS drives.  NetApp (and EMC after they saw the success NetApp achieved) short circuit this a bit by just adding a single, super-fast tier of cache.

NetApp FlashCache is 512GB of super-fast flash memory. EMC FAST Cache are actually SSD's. Frequently accessed data is kept here so that the system doesn't have to go all the way to the drives, which increases the amount of data packets per second (IOPS) you are able to write or read.

The point that really struck me is that some Cache products, which create a super high tier for your data, can kill you on write latency.  It turns out that EMC FAST Cache either increases write latency because of how it's implemented or opens the spigot for max IOPS so wide that the rest of the system can't keep up, exposing other bottlenecks.  I'm sure that at some point if you throttle down the IOPS you'll see the write latency settle down.  You'll still get a marked increase in IOPS, without the write latency.

This doesn't by any means settle that Cache products have no place in your SAN (it's still a fantastic performance boost for the money), but it does mean you have to factor in this effect when making the decision.  

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