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%rcache & %wcache

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Yazan Yacoub
Regular Advisor

%rcache & %wcache


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
Exalted Contributor

Re: %rcache & %wcache


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.

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.

Steven E Protter
Owner of ISN Corporation
Honored Contributor

Re: %rcache & %wcache

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:
Windows?, no thanks
Yazan Yacoub
Regular Advisor

Re: %rcache & %wcache

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 ?

Honored Contributor

Re: %rcache & %wcache

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?

a warrior never quits
Honored Contributor

Re: %rcache & %wcache

In oracle you would need to look at all the sga related parameters.
Windows?, no thanks