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This month I am going to take a look at SSD vs. HDD pricing. In my opinion, the claims by some vendors are over the top; their assertions about SSD pricing and density and HDD pricing and density simply do not match the market realities. It is time expose the real data.
I understand that SSDs do offer superior price per IOPS than HDDs (up to ~110,000 with SAS SSDs for reads but only ~40,000 for SAS SSDs for writes), but this is only part of the discussion, as people want to use SSDs for bandwidth applications.
The suggestion for combining NAND with HDDs to optimize performance is good. However, the market forecasts presented by many prognosticators that enterprise SSD prices will soon be on par with HDD prices are built on several faulty assumptions.
Also see an in-depth analysis of SSD vs. HDD Reliability.
Idealistic SSD Claims
While the idea of using flash, NAND or Solid State Drives for an intermediate step between computing nodes and disk storage is not necessarily incorrect for improving performance (Seagate is already doing this with their SSHD hybrid drives), it is built on several of flawed assumptions regarding flash storage.
First, some assume that the price of MLC NAND flash will continue to decrease at a rapid and predictable rate that will make it competitive with HDDs for bandwidth, and nearly for capacity, by 2014 or 2015. This downward trend, it is assumed, will make flash a viable alternative for large storage and to act as a memory or “buffer” to improve performance.
Second, there is a general assumption that prices for bandwidth ($/GB/s) for SSDs is much lower than for HDDs, and that enterprises will measure costs in these terms instead of capacity.
Third, there is no distinction made between flash in general, such as consumer SSDs, and enterprise storage SSDs. It is assumed that MLC NAND will not only reduce in price ($/GB) but also that it will increase in density and larger capacity drives will be developed.
Fourth, it is assumed that the quality of MLC NAND will either remain constant or increase as prices decrease and densities increase, allowing it to improve not only performance, but also reliability and power consumption of the systems it is used in.
Fifth, it is assumed that power consumption for SSDs is, or will shortly be, significantly lower than that of HDDs overall, on a per GB basis and on a per GB/s basis.
Sixth, they assume disk performance will grow at a constant rate of about 20 percent per generation and not improve.
Seventh, they assume file system data layout will not improve to allow better disk utilization.
Most of these assumptions were made in early 2012. So far they have turned out to be partially true at best and wrong at worst.
Reality: The Real Price Trends
While it is true that the cost of NAND and SSDs have been going down over the past few years (and will continue to do so in general into the future), there is no proof that SSDs will match HDDs in price any time soon, especially when comparing high capacity storage options. At a $/GB measure, SSDs have dropped from upwards of $3/GB back in 2005 and 2006 down to as low as $0.67/GB for specific models in 2012, but they are still far above the cost of HDDs, which can be lower than $0.09/GB for some Western Digital and Seagate models. Even if it is assumed that SSDs continue to fall at their historical rates and HDDs fall at their historical rates, SSDs will still be about $0.15/GB in 2020 ($0.06 more than HDDs are today), while the HDDs vendors are promising may be as low as $0.03/GB.
However, several past events and the current market for SSDs means that this trend may not continue into the future or make SSDs competitive on price with HDDs for some time. First, the pricing has not always gone at a steady and predictable downward pace as current SSD prices (ranging from $1.10/GB to as low as $0.78/GB depending on vendor make and model) are actually higher than they were at the end of 2012. This is not the first time this has happened either. In 2009, SSD prices not only failed to follow the predicted 7 percent per month decreases, but actually increased as much as 7.26 percent per month for nearly four months. In fact, prices did not return to their pre-jump figures in May 2009 until a year later in May of 2010.
The current pricing trend and the hike in 2009 demonstrate that the widely discussed and frequently referenced forecast that SSDs will drop at a steady clip to catch up to HDDs in the near term is flawed.
A key component of many people’s models is the idea that disks and SSDs will not be purchased for capacity alone, but also for bandwidth. They assume that SSDs' cost for bandwidth (represented here as $/GB/s) is significantly lower than that for disk. This is not necessarily the case, especially when considering the costs for enterprise disk and enterprise SSDs.