On Tue, Oct 20 at 21:54, Bob Friesenhahn wrote:
On Tue, 20 Oct 2009, Richard Elling wrote:
Intel: X-25E read latency 75 microseconds
... but they don't say where it was measured or how big it was...
Probably measured using a logic analyzer and measuring the time from
the last bit of the request going in, to the first bit of the
response coming out. It is not clear if this latency is a minimum,
maximum, median, or average. It is not clear if this latency is
while the device is under some level of load, or if it is in a
quiescent state.
This is one of the skimpiest specification sheets that I have ever
seen for an enterprise product.
It may be "skimpy" compared to what you're used to, but it seems to
answer most of your questions, looking at the public X25-E data sheet
on intel.com:
The latency numbers clearly indicate "typical" which I equate to
average (perhaps incorrectly) and regarding the system load, they're
measured doing 4KB reads or writes with a queue depth of 1 which is
traditionally considered very light loading. Correct, it doesn't
explicitly state whether the data transfer phase via 3Gbit/s SATA is
included or not. At 300MB/s the bus transfer is relatively small,
even compared to the quoted numbers.
The read and write performance IOPS numbers clearly indicate a SATA
queue depth of 32, with write cacheing enabled, and that every LBA on
the device has been written to (device 100% full) prior to
measurement, which answers some (granted not all) of the questions
about device preparations before measurement. The full pack IO cases
should be worst case, since the device has the minimum available spare
area for managing wear leveling, garbage collection, and other
features at that point.
The inverse of latency under load in the IOPS testing should give you
a number that you can multiply by queue depth to get typical
individual command latency under load. (Assuming commands are done in
order or mostly in order) It also gives you the typical bandwidth
under load as well, which is about 13MB/s in full-pack random 4KB
writes and about 140MB/s in full-pack random 4KB reads.
--eric
--
Eric D. Mudama
edmud...@mail.bounceswoosh.org
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