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» Enterprise IT Planet » Storage » Storage Features

The Perils of Cache Management

By Henry Newman
October 27, 2010

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Among the challenges of cache management: If you're doing a streaming write from an application (or multiple applications) that do not fit in cache, the performance will be limited to the performance between the cache and the back-end storage. Henry Newman discusses the issue.

More and more vendors are making wild claims about their appliances with Flash cache. Most RAID controller vendors and NAS providers are planning to add Flash to their product designs, which seems like a good idea for all cases as Flash offers a significant amount more cache than is available using standard DRAM. Since the storage stack latency for NAS is often greater than SAN, having cache with a bit more latency is usually does not impact performance.

All of this reminds me of something a late friend of mine, Larry Schermer, used to say back in the 1980’s when he was working on some of the first Cray solid state disks (SSD). He said, “Cache is good if you are reusing data or if it is large enough to handle the data being written to reduce latency. Otherwise, cache management will eat your lunch.”

The point is clear: if your data does not fit in cache, you are not doing small writes that can be coalesced or you are reusing data. In that scenario, cache is not going to help much – it might actually hurt. The issues surrounding caching Web pages as opposed to caching other data are pretty interesting using Larry’s analysis framework. Over the last year, an increasing number of vendors have told me that their cache-based appliances work for all applications and dramatically increase I/O performance. I know that this is just not true for all applications.

Read the rest at Enterprise Storage Forum.

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