RE: About O/S file system cache in any Linux OS: RHEL/OEL

  • From: William Muriithi <william.muriithi@xxxxxxxxxxxxxxxxxxx>
  • To: "exriscer@xxxxxxxxx" <exriscer@xxxxxxxxx>, "sundarmahadevan82@xxxxxxxxx" <sundarmahadevan82@xxxxxxxxx>
  • Date: Sun, 1 Nov 2009 20:59:45 -0600

sundar,


By default you dont have to do any special configuration to use Linux file 
system buffer cache, Linux tries to cache everything.

Correct, by default, caching is enabled. However if you still want to change 
it, look at this two parameters in the file /etc/sysctl.conf. You can see all 
the parameters by typing /sbin/sysctl -a on the terminal

vm.overcommit_ratio = 50
vm.overcommit_memory = 0

Worth looking at this article
http://www.redhat.com/magazine/001nov04/features/vm/

But filesystem buffer read is not very good idea, if you are using Linux 32 bit 
it is ok but if you are using 64 bit you should give more memory to 
db_cache_size and cache in Oracle's own cache and avoid double buffering (file 
system buffer and oracle data block buffer).

On top of double caching as explained above, do not be too aggressive with 
overcommit.  If you are running a program that is misbehaving, it can lead to 
the kernel rudely killing oracle. This would happen after the run away process 
has exhausted all resources and the kernel has to do something for the system 
to be usable again.

If you have less that 4GB RAM,  32 and 64 kernel should just about behave the 
same way. However, with more than 4GB, the oracle can still get killed if you 
run out of the lower memory. It wouldn't matter if there  exist tonnes of 
unused upper memory. The solution is to configure the kernel to use the higher 
memory for caching. Look at the sysctl if that happen to be your case. 

Thanks





On Sun, Nov 1, 2009 at 6:48 AM, sundar mahadevan 
<sundarmahadevan82@xxxxxxxxx<mailto:sundarmahadevan82@xxxxxxxxx>> wrote:
Hi Gurus,
I was reading through metalink to know more about db file scattered
read on Note 223117.1 which further provided link to Note 34558.1

One of the solutions to decreasing wait times was using O/S file
system cache : "It can help to place files which incur frequent
table/index scans on disks which have are buffered by an O/S file
system cache. Often this will allow some of Oracles read requests to
be satisfied from the OS cache rather than from a real disk IO."

I know about the table CACHE feature that can be used to help cache
tables that are frequently scanned within oracle.

But how does one setup O/S file system cache. I googled about O/S file
system cache but not with much help. Any links or thoughts are much
appreciated. Thanks in advance.
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