UNIX for Dummies Questions & Answers

All,

I have doubts related to free command in Linux.
My environment is RHEL5 - 64 bit OS
We have 2 node RAC database installed

Query output from node 1 is

Free -g

Total used free shared buffers cached
Mem: 32 25 7 0 5 15
-/+ buffers/cache: 4 27
Swap: 30 0 30

Query output from node 2 is

Free -g

Total used free shared buffers cached
Mem: 32 6 17 0 1 1
-/+ buffers/cache: 4 27
Swap: 30 0 30



We have Oracle database hosted on this server. We are in doubt that actual memory used is 25 GB or 4 GB. Performance team is telling that database is over utilizing the memory of 25 GB. But DB team is telling that actual memory utilization is 4 GB and remaining 20+ GB is used by buffers / cached.

I have googled it and noticed that 25 GB is summation of 4+5+15 (close to 25). I referred Linux material and it says that memory in buffers and cached will improve the performance.

1.Can somebody explain whether actual memory used is 4 GB or 25 GB?
2.What is the use of buffers and cached?
3.Sometimes used value is peaking close to 31 GB from 25 GB. Mostly occupied by buffers and cached.
4.At a particular moment, -/+ buffers/cache: will be same. But Mem used will be more in node 1 and less in node 2. After particular hours, the Mem used will be shifting from node 1 to node 2. i.e., in node 1, Mem used will be 6 GB and Mem used in node 2 will be 25 GB approximately. Any body have any idea about this behaviour?

Hi user7509,

1. On node 1 the actual usage is 25 GiB. But only a small part is in use by Oracle and large parts by probably almost useless filesystem caching.
2. buffers and cached is part of free memory that is being used for buffering an caching. It remains however free memory. The moment it is needed, the most of the buffers and file system cache gets discarded. This cache is not the cache that Oracle uses.
3. See 2.
4. Usually the memory use of the oracle instances are largely determined by the SGA, which is used almost entirely for the database cache. This is shared memory allocation. Do you have single instance databases that sometimes run on one node and sometimes on the other? You can check the allocated chunks of shared memory by running.

Code:

ipcs -ma

as root or by checking the oracle init file.. The owner of the chunks is the oradba user that runs the database.
Filesystem cache will be of almost no use on a RAC cluster. You would probably be better off by enlarging the SGA of your databases so the memory is put to practical use, but no more than is useful of course.

S.

Scrutinizer,

Your detailed explanation helps me a lot. Thanks