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Re: Finding the Bottleneck

On Saturday 09 June 2001 20:04, Jason Lim wrote:
> I'm not exactly sure how the Linux kernel would handle this.
> Right now, the swap is untouched. If the server needed more ram,
> wouldn't it be swapping something... anything? I mean, it currently has
> 0kb in swap, and still has free memory.

That is a really good point.  What you say makes a lot of sense.  However 
the Linux kernel policies on when to free cache and when to swap are 
always being tweaked and are very complex.

I have CC'd this message to Rik.  Rik wrote most of the code in question 
and is the expert in this area.

Rik, as a general rule if a machine has 0 swap in use then can it be 
assumed that the gain from adding more RAM will be minimal or 
non-existant?  Or is my previous assumption correct in that it could 
still be able to productively use more RAM for cache?

As a more specific issue, assuming that there is memory which is not 
being touched (there always is some) will that memory ever be paged out 
to allow for more caching?

I believe that Jason is using kernel 2.2.19, but I would like to have 
this issue clarified for both 2.2.19 and 2.4.5 kernels if possible.

> Anyway... as for the raid solution, is there anything I should look out
> for BEFORE i start implementing it? Like any particular disk or ext2
> settings that would benefit the mail queue in any way? Don't want to
> get everything set up, only to find I missed something critical that
> you already thought of!

There is an option to mke2fs to tune it for RAID-0 or RAID-5.  I'm not 
sure if it provides much benefit though, and it does not help RAID-1 
(which is the RAID level you are most likely to use).

I suggest that you firstly run zcav from my bonnie++ suite on your new 
hard drives.  Then allocate partitions for the most speed critical data 
in the fastest parts of the drives.  Then use RAID-1 on those partitions.

Good luck!

http://www.coker.com.au/bonnie++/     Bonnie++ hard drive benchmark
http://www.coker.com.au/postal/       Postal SMTP/POP benchmark
http://www.coker.com.au/projects.html Projects I am working on
http://www.coker.com.au/~russell/     My home page

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