performance with LSI SAS 1064

[Scott Long]

Lutieri G. wrote:
> 2007/8/30, Eric Anderson <anderson at freebsd.org>:
>> I'm confused - you said in your first post you were getting 3MB/s, where
>>   above you show something like 55MB/s.
> Sorry! using blogbench i got 3MB/s and 100% busy. Once is 100% busy i
> thinked that 3MB/s is the maximum speed. But i was wrong...

%busy is a completely useless number for a anything but untagged,
uncached disk subsystems.  It's only an indirect measure of latency, and
there are better tools for measuring latency (gstat).

>> You didn't say what kind of disks, or how many, the configuration, etc -
>> so it's hard to answer much.  The 55MB/s seems pretty decent for many
>> hard drives in a sequential use state (which is what dd tests really).
>>
> SAS disks. Seagate, i don't know what is the right model of disks.
> 
> Ok. If 55Mb/s is a decent speed i'm happy. I'm getting problems with
> squid cache and maybe should be a problem related with disks. But...
> i'm investigating and discharging problems.
> 
> 
>> Your errors before were probably caused because your queue depth is set
>> to 255 (or 256?) and the adapter can't do that many.  You should use
>> camcontrol to reduce it, to maybe 32.  See the camcontrol man page for
>> the right usage.  It's something that needs setting on every boot, so a
>> startup file is a good place for it maybe.
>>
> Is there any way of get the right number to reduce?!
> 

If you're seeing erratic performance in production _AND_ you're seeing
lots of accompanying messages on the console about tag depth jumping
around, you can use camcontrol to force the depth to a lower number of
you're choosing.  This kind of problem is pretty rare, though.

Scott