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Price per GB for SSD – why its not always the best yardstick

October 12, 2013 Leave a comment

Price per GB, performance and endurance are the yardsticks used to decide which solid state drive (SSD) to buy for use in a corporate data center or to use in a server or flash based storage array.

Are endurance numbers really comparable? Especially when you consider that one vendor might use consumer grade cMLC NAND while another might use enterprise grade eMLC NAND with vastly different program erase (P/E) cycles?  What was the write amplification factor (WAF) – the ratio of SSD controller writes versus the host writes –  that was used for the calculation?  One vendor might quote endurance in TB or PB written while another might use Drive Writes Per Day (DWPD).

One vendor might state fresh-out-of-the-box (FOB) performance numbers in IOPS on their datasheet while another might display steady state numbers.  One might use a synthetic benchmark tool like IOMETER which focuses on queue depth (number of outstanding I/Os), block size and transfer rates instead of an application based benchmark like SysMark which ignores all these criteria and focuses on testing how a real-world application might drive the SSD.  Even with tools like IOMETER whether IOMETER  2006, 2008 or 2010 is used will cause the results to vary.  To add further complexity, the performance numbers will vary widely depending on whether they were measured with aWoman Eating Fruit Outdoors queue depth (number of outstanding I/Os) of 3, 32, 64, 128 or 256.  To compound it one vendor might be looking at compressible data (Word docs, spreadsheets) while another might be quoting numbers for incompressible data (.zip files or .jpeg), some might be using SandForce (now LSI) controllers which compress data before writing it to NAND while others might not.  So what is an SSD buyer to do?  Get a drive from a vendor you trust and run your own benchmarks whether they are synthetic or application based and derive your own conclusions.

Now why do I find $ per GB as a yardstick amusing?  Consider this analogy – could we convince a Japanese consumer that the cantaloupe we buy from a local store for $2.99 here in California is equivalent to a musk melon purchased in Japan for $16,000 yen?  From a $ per melon point of view, the price differences are difficult for us to fathom but to a buyer of the $16,000 melon it is apparently a premium worth paying for.

Categories: DRAM, SSD