Linux Question

[Thomas Guyot-Sionnest]

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On 22/08/08 10:46 AM, Edwin Zoeller wrote:
> We have recently installed Redhat AS4 throughout our network. I have
> installed various plugins from the Nagios Exchange site for Linux and
> all seems to be well, except for the check_ram command. I am running
> this on various servers with different configurations and all are giving
> the same results. Here is what I have on one of the servers:
>  
> check_ram -n -w 20MB -c 10MB ( the system has 8GB installed), results
> display 30MB free (am I doing this right?)
>  
> Other admins here are disputing the results and claim that Linux buffers
> all the memory and gives what it needs. I don't know for fact it this is
> true. I have also run top, free and ps -eo checking on memory size, all
> give back the same results as the Nagios plugin. Is this plugin with the
> option chosen giving real memory results or bogus results.
>  
> My question, in desperation, can anyone explain in very simple terms how
> Linux memory works? Also how are you monitoring memory, using what
> command and how it is configured etc.

All unused memory gets buffered/cached eventually if your server is
doing I/0. I've seen very stable servers with 32GB get down to only a
few MBs free, but the picture is much better if you account the
buffered/cached memory which for most of it can be freed anytime if needed.

For that reason you should add the buffer/cache memory to the total
(i.e. the "-/+ buffers/cache:" line of the "free" command).

My check_memory script does that. it's written in Perl and uses the
Nagios::Plugin Perl module (available on CPAN).

http://www.nagiosexchange.org/cgi-bin/page.cgi?g=1433.html;d=1

- --
Thomas
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