EMC has announced FAST Cache, a flash cache implementation, on the EMC CLARiiON® and EMC Celerra® storage systems. An amount of flash disk storage is configured to be an extension to the cache on the array controller. This allows the array caching algorithms to take advantage of an additional caching layer. This layer is lower performance than the DRAM memory in the array controller but is lower cost and persistent and does not need a battery backup system to retain write integrity. This approach is similar to approaches taken by NetApp with PAM (data is written to PCI-E attached flash modules), and FalconStor Flash SAN accelerator (using flash modules from Violin).
Flash Cache is a simpler way of providing additional cache and is particularly suitable for random reads and writes. The “copy of record” of the data still remains on the drive and is not moved to the cache. Any writes to the flash cache are written out to a drive as if were normal write cache.
Flash cache is an alternative configuration to using either FAST (where the specific volumes need to be identified and placed on the flash drives) or FAST 2 (where sub-LUN extents can be identified as part of a batch process as “hot” and moved to a flash drive). The major advantage is that it is automatic, and the caching algorithms are effective in identifying the optimum use of cache on a dynamic basis. FAST 1 will the method of choice where specific volumes are known to be bottlenecks for very important applications.
The interesting comparison is between FAST 2 and fast cache. For applications that have a consistent requirement for random read performance, FAST could be the optimum choice. CLARiiON and Celerra do not have a large amount of write cache, and FAST Cache would be expected to be particularly useful for workloads with updates to databases that come in bursts. If there is a lack of knowledge or wide variety of applications, FAST cache could be the first first choice for speeding up I/O.
Action Item: The introduction of flash cache on CLARiiON and Celerra is an important extension to the use of flash. Unless there are known bottlenecks on specific LUNs, it is reasonable to expect that the array cache algorithms can theoretically make the best use of the DRAM and Flash resources on a second by second basis. Wikibon would recommend that FAST cache be the first flash implementation to try, especially for database applications (scale-out or scale-up) with high write locking rates.
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