megaraid causing high-load

jason andrade jason at
Tue Mar 8 22:11:01 CST 2005

On Tue, 8 Mar 2005, David Lucas wrote:

Hi David,

> I am having some difficulty with the megaraid drivers in a PowerEdge with a 
> CERC ATA100/4ch.  The write speed is notoriously slow and it causes a high 
> load (which jumps up to 5-20) with very little corresponding CPU usage (via 
> top).  The responsivity of the system then becomes very sluggish and I have 
> been having some errors with the filesystem (Reiserfs) which then has caused 
> the system to hang on several occasions.  I am using a single RAID 5 
> partition ... 228Gb in total.

the first things i'd check are making sure you have the latest firmware on
your controller.  in my experience the load average normally rises when you
find the system is queueing requests - i.e it can't read or write fast enough
to keep up or the other side, it can't push the data out fast enough (the
network interface for example is congested).  you can sometimes catch this 
using iostat (though not very reliably :-/) and watching for a while at
the avgrq-sz avgqu-sz await values.

> The load never really drops low ... always hovering around the 0.10-0.40 mark 
> even when the system is barely idle.

heh.  on our fileserver.

  14:03:48  up 70 days, 4 min,  2 users,  load average: 75.99, 85.89, 86.02

0.1-0.4 is busy ? you're kidding me right ? :-)

> I have scoured the mailing lists and the web and have not come up with any 
> real solutions. The system has 1Mb of memory, and I have turned the swap off

i'm assuming 1G here..

> with little to no effect.  The system is barely used, yet suffers such a high

i wouldn't turn off swap myself.. but there may be some tuning that is required.

> load ... If the system usage increases I fear that it will be swamped and I 
> would like to resolve this.
> Anyone with any ideas????

increase the memory to start with - i've found that increased buffering usually
helps significantly with i/o queueing.  i'd go to 2 or even 4G if you could.

you haven't said too much else about what you're doing on the server.. so it's
hard to say much more about tuning it.. and unfortunately most of my tuning
stuff is all for 2.4 :-/



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