Disclosure - NetApp Employee -

It's a personal pet peeve of mine, but most of the latency rules of thumb are at best only marginally informative, and at worst, downright misleading. I'm not having a go at the author or his sources, but this kind of stuff has been so often repeated that it has become a self supporting "truth".

These "one figure" IOPS figures ignore the effect of the advanced queuing and elevator algorithms that have been present in disk drives for the last 10 odd years, which means a SATA drive has about 10 IOPS at a 13ms latency and 130 IOPS at 100ms latency.

I covered that and the impact of various raid and caching configurations in one of my blog posts here

http://storagewithoutborders.com/2010/07/19/data-storage-for-vdi-part-2-disk-latencies/

Also it should be noted that in enterprise arrays, because of SATA's unchangable 512byte block sizes, the usual checksum mechanisms used on FC disks wont work (which use 520 byte blocks which includes an 8 byte checksum). These checksum mechanisms (e.g. slip mask on EMC, Zoned Checksum on NetApp) all impose some extra level of IO which impacts both reads and writes.

Rules of thumb are all well and good, but they're no substitute for consultation between the people who truly understands the performance characteristics of your workload, and the array architecture you are going to implement

Regards
John Martin