On Oct 20, 2009, at 1:58 PM, Robert Dupuy wrote:
"there is no consistent latency measurement in the industry"
You bring up an important point, as did another poster earlier in
the thread, and certainly its an issue that needs to be addressed.
"I'd be surprised if anyone could answer such a question while
simultaneously being credible."
http://download.intel.com/design/flash/nand/extreme/extreme-sata-ssd-product-brief.pdf
Intel: X-25E read latency 75 microseconds
... but they don't say where it was measured or how big it was...
http://www.sun.com/storage/disk_systems/sss/f5100/specs.xml
Sun: F5100 read latency 410 microseconds
... for 1M transfers... I have no idea what the units are, though...
bytes?
http://www.fusionio.com/PDFs/Data_Sheet_ioDrive_2.pdf
Fusion-IO: read latency less than 50 microseconds
Fusion-IO lists theirs as .05ms
...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 find the latency measures to be useful.
Yes, but since we are seeing benchmarks showing 1.6 MIOPS (mega-IOPS :-)
on a system which claims 410 microseconds of latency, it really isn't
clear to me how to apply the numbers to capacity planning. To wit, there
is some limit to the number of concurrent IOPS that can be processed per
device, so do I need more devices, faster devices, or devices which can
handle more concurrent IOPS?
I know it isn't perfect, and I agree benchmarks can be deceiving,
heck I criticized one vendors benchmarks in this thread already :)
But, I did find, that for me, I just take a very simple, single
thread, read as fast you can approach, and get the # of random
access per second, as one type of measurement, that gives you some
data, on the raw access ability of the drive.
No doubt in some cases, you want to test multithreaded IO too, but
my application is very latency sensitive, so this initial test was
telling.
cool.
As I got into the actual performance of my app, the lower latency
drives, performed better than the higher latency drives...all of
this was on SSD.
Note: the F5100 has SAS expanders which add latency.
-- richard
(I did not test the F5100 personally, I'm talking about the SSD
drives that I did test).
So, yes, SSD and HDD are different, but latency is still important.
--
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