[Gllug] Poor disk performance on IDE raid

[John Hearns]

On Thu, 2005-06-02 at 23:52 +0100, Simon Wilcox wrote:
> Hi folks,
> 
> We have several identical 2.8GHz P4 servers running Debian sarge with
> 2.6.8-1-386 kernel and a 3Ware Escalade 7006-2 IDE raid card, with 2 x
> 160Gb IDE drives in them.
> 
> When reading large files, web server logs for example, top shows the
> processor spending about 80% of its time waiting for disk IO.
> 
> This doesn't seem right to me but it is the same on two machines so I
> don't think there's a hardware problem.
> 
> Could anyone offer advice regarding the best place to start diagnosing and
> fixing this problem ?
> 

Use iostat -x -d /dev/??? 5  to monitor the disk activity.

Use hdparm -t -T do do a very crude speed test.
Use bonnie++ do do a proper test. 
Keep the results, and continue using iostat as you change the parameters
below.

 

You can change the elevator tuning, using elvtune on 2.4 kernels.
I think (?) this is done automagically on 2.6 series kernels,
so is probably not relevant to you.
With elvtune can set how many writes or reads are queued up before being
committed to disk.

You can set /proc/sys/vm/max-readahead and min-readahead on the fly,
and make them permanent using sysctl

3Ware themselves have a technical bulletin on the optimal parameters
they recommend.
Just search their support site.
http://www.3ware.com/KB/kb.asp
Article Q10036 is for 2.4 kernels, but links to one on 2.6 kernels.




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