The SSD guy usually argues that NAND flash (as an SSD) fits into the memory/storage hierarchy for two reasons:
- SSDs are cheaper than DRAM but more expensive than an HDD
- SSDs are slower than DRAM but faster than an HDD
An SSD is usually a very good alternative to adding more DRAM to a system, but this is not always the case. Users need to look carefully at the SSD they plan to use to make sure they are getting the most for their system spend.
In the case of SLC flash, it makes more sense these days to add DRAM to the system wherever possible because SLC flash currently costs more than DRAM. This is a recent change that is somewhat surprising. Although SLC NAND flash costs less to manufacture than DRAM, market forces have pushed DRAM prices down while pushing SLC NAND flash prices up.
The accompanying chart helps to illustrate this point. This chart presents DRAM and SLC NAND price per gigabyte from late 2007 to the present. DRAM prices are shown in the black line, with the red line showing SLC NAND prices.
For most of the period SLC prices have remained below – often substantially below – DRAM prices. This summer, however, SLC prices increased while DRAM prices plummeted. The result? SLC is now selling at $5.20/GB while a gigabyte of DRAM sells for as little as $2.40.
This will reverse over time, and the upcoming NAND flash glut should force SLC prices back below those of DRAM, but for the time being it will usually make more sense to add DRAM to a system than it would to add an SLC SSD.
Readers who wish to gain a greater understanding of these price dynamics are directed to the Objective Analysis report: Understanding the NAND Market which can be purchased for immediate download on the Objective Analysis website.