performance with LSI SAS 1064

[Eric Anderson]

Scott Long wrote:
> 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, you are far more of a SCSI guru than I, so please correct me if 
this is incorrect.  Can't you get a good estimate, by knowing the queue 
depth of the target(s), and dividing it by the number of initiators?  So 
in his case, he has one initiator, and (let's say) one target.  If the 
queue depth of the target (being the Seagate SAS drive) is 128 (see 
Seagate's paper here: 
http://www.seagate.com/staticfiles/support/disc/manuals/enterprise/savvio/Savvio%2015K.1/SAS/100407739b.pdf 
), then he should have to reduce it down from 25[56] to 128, correct?

With QLogic cards connected to a fabric, I saw queue depth issues under 
heavy load.

Eric