Friday, April 12, 2013

Random Idea: BBU SSD

Given that RAM doesn't seem to be that expensive anymore (there are plenty of choices for 2GB DDR3 sticks around $20 at NewEgg right now), why not put a bit more onboard an SSD along with a small rechargeable battery?

The extended RAM would be dedicated to a large page cache, which would do its best to hold frequently-written data (to extend the flash life by avoiding writes to it for as long as possible.)  A landing zone the size of this cache would be reserved in the NAND, and in event of sudden power loss, the pages pending in the DRAM would be dumped into the landing zone under battery power.

Presumably, the 400+ MB/s that current SSDs quote for sequential writes involve the overhead of the OS, host interface, and wear-leveling scheme, and represents a lower bound on performance of the panic save.  On a 2A drive writing 2.00 GB, also intended to be conservative numbers, 5.12 seconds of power is required, for consumption of 170 mAh.

(If the manufacturer quoted their transfer speed in SI and I'm actually writing GiB of data, those numbers change to 382 MiB/s yielding 5.37 seconds of transfer time and 179 mAh of power.  No big deal.)

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