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.
Sun: F5100 read latency 410 microseconds
... for 1M transfers... I have no idea what the units are, though... bytes?
Sun's testing is likely done while attached to a system and done with
some standard loading factor rather than while in a quiescent state.
...at the same time they quote 119,790 IOPS @ 4KB. By my calculator,
that is 8.3 microseconds per IOP, so clearly the latency itself doesn't
have a direct impact on IOPs.
I would be interested to know how many IOPS an OS like Solaris is able
to push through a single device interface. The normal driver stack is
likely limited as to how many IOPS it can sustain for a given LUN
since the driver stack is optimized for high latency devices like disk
drives. If you are creating a driver stack, the design decisions you
make when requests will be satisfied in about 12ms would be much
different than if requests are satisfied in 50us. Limitations of
existing software stacks are likely reasons why Sun is designing
hardware with more device interfaces and more independent devices.
Bob
--
Bob Friesenhahn
bfrie...@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer, http://www.GraphicsMagick.org/
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