News Group Posts

scsi disk performance tuning wio sar

From: "James R. Sullivan" <jim@tarantella.com>
Newsgroups: comp.unix.sco.misc
Subject: Re: Performance question
Date: Wed, 26 Sep 2001 10:14:46 -0700
Message-ID: <3BB20D06.B88B3542@tarantella.com> 
References: <3BB02070.D66CBF1B@aot.com.au>
<GK7ytp.122y@wjv.com>
<3BB107B7.3807D71@aot.com.au> 

Trimmed and commented:

Adrian wrote:
> 
> Hi,
> 
> Thanks for your prompt response.  I actually thought I had posted the memory
> section of the sar output.  I should just post all of the output.  See below.
> 
> 
> Here is the whole sar output (with my comments/questions again):
> 
> ===========================
> 
> SCO_SV thor 3.2v5.0.5 PentII(D)ISA    09/25/2001
> 
> 10:22:49    %usr    %sys    %wio   %idle (-u)
> Average        9      28      45      18
>
> 10:22:49 bread/s lread/s %rcache bwrit/s lwrit/s %wcache pread/s pwrit/s (-b)
> Average      416    5953      93      83     119      31       0       0
> 
> 10:22:49  device   %busy     avque     r+w/s    blks/s    avwait    avserv (-d)
> Average  Sdsk-0    45.84      1.04     54.55    441.62      0.38      8.40
>          Sdsk-2   100.00      1.05    122.42    554.50      0.54      10.28


Here is a problem, without a doubt.  WaitIO (the wio in the first line), indicates
a condition where a process is ready to run, but is blocked waiting for some IO
event to clear.  In all likelyhood, they are waiting for Sdsk-2 to become free.

Some possibilities, drawn from old memories, would be:

increase buffer cache.  You have spare memory and are not swapping, so increasing
the buffer cache would/could help.  Your % of Read Cache is generally good.  The
write % is low, but you're probably writting to different parts of the disk/database
so there's little you could do about that.  I suspect that the program is writting
with a sync of some sort, which may cause the significant waitio number.

increase SDSKOUT.  This used to be the number of SCSI transactions that the system
would queue.  A higher number wouuld queue more transactions and may inprove the disk 
performance.

Get a better disk subsystem :-)

> 10:22:49 runq-sz %runocc swpq-sz %swpocc (-q)
> Average      1.7     100
> 
> => according to 'man sar' if runq-sz is >2 and %runocc is >90% then the
> CPU is heavily loaded and response time will be degraded.
> These results seem to concur with the CPU utilization above, suggesting
> that CP is the bottleneck.  Again, does this make sense?


Not with your Disk situation.  They're ready to run, but the disk is holding
them back.  Fix that first.

Any time that system is in WaitIO, nothing is happening.  In all my performance
tuning over the years, I've always focused on reducing WaitIO when I see it.

my $0.02, from an old SCO SE.


--
Jim Sullivan
Director, North American System Engineers
Tarantella!  http://www.tarantella.com
831 427 7384 - jim@tarantella.com
 

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