I understand that RAM is connected directly to the CPU via a high bandwidth bus, and SSD is a peripheral, but my assumption is that there are also electrical design differences (e.g. memory cell design and layout) that make RAM faster than SSD for reads/writes. If that is true, and with bus bandwidth & latency differences aside, what are those design differences, and how do they affect speed?
closed as too broad by Eugene Sh., Chris Stratton, old_timer, Finbarr, Edgar Brown Dec 20 '18 at 22:01
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The biggest difference is that SSDs are based on NAND flash cells with the serial structure shown below.
Where DRAM accesses all of the bits in a row at the same time, NAND flash serially accesses the row. This significantly slows down read access versus DRAM.
Write access is significantly slowed because flash uses a floating gate which requires orders of magnitude more programming time than the capacitor in a DRAM cell. In most cases, since the write can only set a bit to 0 and erasing is required to set a bit to 1, writes also require erasing before programming. DRAM writes can set either state with no speed penalty.
Their physical structure is totally different. DRAM cells are based on a capacitor holding its voltage.
Flash is based on a floating gate design, where the physical properties of the cell changes with a write.