From owner-freebsd-questions  Fri Aug 27 13:44: 6 1999
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To: chas <panda@skinnyhippo.com>
Cc: freebsd-questions@FreeBSD.ORG
Subject: Re: how can you tell when disk i/o is limiting performance ? 
In-reply-to: Your message of "Sat, 28 Aug 1999 04:56:14 +0900."
             <3.0.5.32.19990828045614.00a07d80@mail.skinnyhippo.com> 
From: David Greenman <dg@root.com>
Reply-To: dg@root.com
Date: Fri, 27 Aug 1999 13:43:33 -0700
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>Is there any method to measure disk I/O, in particular to 
>work out if it is limiting performance ?
>
>Context : 
>1 x 18 GB SCSI HD (10,000 RPM)
>
>Each day :
>200,000    x  CGI processes, each reading 2 or 3 flat files 
>100,000    x  CGI processes, each reading/writing to mysql database
>1,000,000+ x  flat files (images, .html etc)
>
>plus 400 MB+ of apache log files are created each day.
>
>There seems to be ample RAM and CPU (and we're not even going 
>into swap) but the website is really crawling. I realise that 
>this could be due to the poor bandwidth in China (where the 
>server is hosted) but would like to also monitor the disk i/o
>if it's possible since the CGIs open a lot of files and the apache
>log files are being written nonstop.
>
>Hopefully then I can plan/budget for more disks and/or a second 
>server as traffic reaches a threshold. 
>
>So :
>a) can this be measured ?  (i.e how do you know when your disk i/o is 
>   the limiting factor ? )

   Use "systat -iostat". Look at the TPS rates for each disk drive. Modern
disk drives top out at around 80-120 TPS.

>b) would it make more sense to have 3 separate (physical) disks for :
>   - apache log files
>   - mysql database 
>   - operating system, applications, website files/cgi

   Yes, more disks is better.

-DG

David Greenman
Co-founder/Principal Architect, The FreeBSD Project - http://www.freebsd.org
Creator of high-performance Internet servers - http://www.terasolutions.com
Pave the road of life with opportunities.


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