We return to our periodic discussion of storage system performance, this time on SPECsfs(r)2008 benchmark results for NFS file serving ops per second vs. system reported memory size. For some obscure reason, I was very intrigued with this chart.
We have broken the data out to show those system that only used DRAM in system memory with only hard disk drives and those systems that also included either NAND cache in system memory or SSDs. Current SPECsfs2008 results show 33 systems with DRAM and disk drives and only 6 using SSDs or NAND cache for NFS results.
The horizontal axis shows system memory size for the systems under test and doesn’t include SSD capacity (considered drive capacity by SPECsfs2008) size but does include NAND cache size (considered system memory by SPECsfs2008). The vertical axis shows maximum NFS throughput operations per second attained by the storage. The two lines are Excel generated linear regressions across the two sets of data (DRAM-Disk only systems and SSD or NAND caching systems).
Discussion of results
Given the limited data we probably can’t conclude much from the SSD-NAND linear regression line other than it’s different and somewhat less than what can be gained on average from a system using DRAM and disk only. Also the regression coefficient (R**2) of either linear regression is not that great (~0.62 for DRAM-Disk only and ~0.69 for SSD or NAND use) which might be stretching any real discussion based on statistical normalcy. Nevertheless, one question that emerges is why would SSD or NAND use not generate an relatively equivalent amount of NFS throughput as systems with DRAM only?
Obviously, NAND-SSDs access times are not as fast as DRAM. Thus, if I had 800GB of DRAM, I could potentially access data faster than if I had 800GB of NAND cache or SSDs, all things being equal. However, when talking about access time frames at the sub-msec level one would think it wouldn’t make that much of a difference, but apparently it does.
Also, given the limited data on SSDs vs. NAND cache use, it’s hard to make any distinction between these two types of systems but one can hope that as more data comes in, we can answer this as well. Another question is whether SSD capacity should be considered system memory or drive capacity for benchmark purposes. SPECsfs2008 states that SSD size should be considered drive capacity but that’s open to some debate which more data could help resolve.
Finally, missing from SPECsfs2008 reports is any statement of system cost. As such, it’s impossible to add any measure of relative cost effectiveness factor to this discussion. However, given the current price differential (on $/GB) between DRAM and NAND memory or SSD capacity, one could probably conclude that on a cost effectiveness basis the relative advantages of DRAM only systems might diminish. But without system cost information that’s difficult to quantify.
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As always, we welcome any constructive suggestions on how to improve any of our storage performance analysis.