[Beowulf] SSD caching for parallel filesystems
diep at xs4all.nl
Sun Feb 10 01:41:27 PST 2013
SSD's are not about bandwidth, they're about latency.
With a raid array of cheapo disks we can also get 3GB/s bandwidth,
more than most 2 socket nodes effectively can handle.
Only theoretically a higher bandwidth will be possible (benchmarks huh).
However getting 20 bytes from a SSD is in the few dozens of
microseconds, versus several milliseconds for the cheapskate disks.
That factor of 50-100 difference roughly in latency difference is the
reason SSD's exist.
Any bandwidth test of a SSD is total nonsense.
On Feb 9, 2013, at 10:32 PM, Bill Broadley wrote:
> On 02/09/2013 01:22 PM, Vincent Diepeveen wrote:
>> SATA is very bad protocol for SSD's.
>> SSD's allows perfectly parallel stores and writes, SATA doesn't.
>> So SATA really limits the SSD's true performance.
> SSDs and controllers often support NCQ which allows multiple
> requests. Not sure if that fits the definition of "perfectly
> but it does allow for outstanding requests to be answered in the order
> of the devices choosing.
> In fact the higher IOP rates would be impossible without it. Note how
> the latency for a single IOP is much higher than peak 1 second / (peak
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