%rcache & %wcache

[Yazan Yacoub]

Hello,

I have two rx7640 machine with following configuration, both servers is running oracle DB, i was checking sar -b output which I am attaching, and I have notice that %rcache & %wcache have small values, which mean than I am not using buffer cache efficiency.

Server 1 (8 CPUs and 16 RAM)
Kernel values:
dbc_max_pct 10
dbc_min_pct 10

Server 2 is (16 CPUs and 32 GB RAM:)
kERNEL values:
dbc_max_pct 3
dbc_min_pct 2

I wondoring what is the situation for the above systems, do I have real problem ? and how can I change %rcache & %wcache values to get near 100%

Many thanks

[Steven E. Protter]

Shalom,

The value difference you see may be to differences in the way the applications are running or different applications.

To get near 100% on these two values is probably not reasonable. Getting the values in the 90's may be possible depending on the first paragraph.

http://www.hpux.ws/?p=6

To get the best value, you need to make adjustments based on performance data.

Collect data, make changes, collect more data. Make sure the collection period is reasonable and representative.

SEP

[likid0 · Accepted solution]

If you only have Oracle working on the systems, you shouldn't be to woried, because oracle has it's own memory management in the SGA, as a matter if fact in some ocassions to improve performance you just jump the OS chache, For JFS file systems, if the file system is mounted with the mincache=direct option and/or the convosync=direct option, reads and writes to the buffer cache are bypassed. you need OnlineJFS.

If you want a good read about buffer cache:

http://docs.hp.com/en/7779/commonMisconfig.pdf

[Yazan Yacoub]

Nice document Adam,
Its oracle database, but I am not able to find oracle cache kernel parameter in my systems.

and is it bad that I have low %rcache & %wcache values ?

Thanks.

[Jeeshan]

there is no oracle cache parameter in OS. you have to check in Oracle database's parameters, that what pointed earlier reply.

are you using sync or async in database?

[likid0]

In oracle you would need to look at all the sga related parameters.