Speed matters, especially with SSDs
Why would any device try to reach a 90μsec write access time and a 60μsec read access time? With the advent of high-speed stock trading where even distance matters, a lot, latency is becoming a hot topic once again.
Although from my perspective it never really went away (see my Storage throughput vs. IO response time and why it matters post). So access times measured in 10’s of μsec. is just fine by me.
How SSD access time translates into storage system latency or response time is another matter. But one can see some seriously fast storage system latencies (or LRT) in TMS’s latest RAMSAN SPC-1 benchmark results, under ~90μsec measured at the host level! (See my May dispatch on latest SPC performance). On the other hand, how they measure 90μsec host level latencies without a logic analyzer attached is beyond me.
How are they doing this?
How can a OCZ’s SATA SSD deliver such fast access times? NAND is too slow to provide this access time for writes so there must be some magic. For instance, NAND writes (programing) can take on the order of a couple of 100μsecs and that doesn’t include the erase time of more like 1/2msec. So the only way to support a 90μsec write or storage system access time with NAND chips is by buffering write data into an “ondevice” DRAM cache.
NAND reads are quite a bit faster on the order of 25μsec for the first byte and 25nsec for each byte after that. As such, SSD read data could conceivably be coming directly from NAND. However you have to set aside some device latency/access time to perform IO command processing, chip addressing, channel setup, etc. Thus, it wouldn’t surprise me to see them using the DRAM cache for read data as well.
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I never thought I would see sub-1msec storage system response times but that was broken a couple of years ago with IBM’s Turbo 8300. With the advent of DRAM caching for NAND SSDs and the new, purpose built all-SSD storage systems, it seems we are already in the age of sub-100μsec response times.
I fear to get much below this we may need something like the next generation SATA or SAS to come out and even faster processing/memory speeds. But from where I sit sub-10μsec response times don’t seem that far away. By then, distance will matter even more.
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