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SSDs – Buyer Beware?

The latest wave of product releases is revealing some interesting aspects of SSDs. You have to read beyond the headlines to find the nuggets. Let’s look at one nugget derived from various articles:

Seagate ‘Pulsar’ is spec’ing 4K random reads of 30,000 IOPs while sustained 4K random reads range from 10,500 IOPs down to 2,600 IOPs as a function of drive capacity. The I/O queue depth is listed as Q=32. The SSD controller and firmware are 3rd party and can only be inferred from Seagate’s investment strategy.

OCZ’s Vertex-2 Pro is based on SandForce’s SF-1500 SSD processor. At Benchmark Reviews OCZ preliminary specification for 4K random write is 19,500 IOPs, higher than Seagate’s Pulsar but interestingly the I/O queue depth is listed as Q=8, not Q=32.

Forgoing a SSD technology ‘deep-dive’, keep in mind that storage I/O ‘blocks’ are not the same as flash blocks. Micron provides several NAND Flash Technical Notes that can be found here. The Small-Block vs. Large-Block NAND Flash Devices note describes the array organization of large-block NAND devices. A NAND flash ‘page’ is 2Kbytes+64bytes. A ‘block’ is 64 pages or 128Kbytes+4Kbytes. Read and Program operations can be performed on a Page but Erase operations must be performed on a Block level. Once a file system workload consumes the SSD drive with a specific workload that generates Program cycles the ‘dirty’ flash blocks must be Erased and returned to free pool for future Page level programming. Managing the logical-to-physical translation, erase-before-write requirements and the many endurance requirements is the responsibility of several integrated and co-dependent algorithms, commonly packaged and referred to as a Flash Translation Layer (FTL).

The preliminary SSD specifications reveal that I/O queue depth may be an important factor in SSD write performance. Asymmetric R/W throughput would be consistent with the level of FTL work required to translate the logical 4Kbyte writes into physical flash block program-erase cycles, but not immediately obvious to early adopters of this technology. Planning your SSD adoption strategy will require a thorough understanding of the storage hierarchy workload characteristics.

In summary, buyers beware. Your SSD plan may look good on paper at 30,000 IOPs per drive but in production sustain performance of less than 5000 IOPs per drive under heavy write queuing. Artificially limiting queue depths may have other deleterious effects. Does $30/GB for SSDs still make sense at 5000 IOPs? What other factor flying below the headlines come into play? In-depth real-world Capacity, Performance and Cost analysis are the only way to Right-Size a solution for success.

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