Sar output and tuning buffer cache

[Sean OB_1]

Hello!

I want to tune the buffer cache on a system and wanted some help reading the sar output for 'sar -b'

Here it is:

09:23:36 bread/s lread/s %rcache bwrit/s lwrit/s %wcache pread/s pwrit/s
09:23:37 0 53 100 4 8 50 0 0
09:23:38 0 1 100 1 1 0 0 0
09:23:39 0 118 100 7 6 0 0 0
09:23:40 1 436 100 10 66 85 0 0
09:23:41 0 4 100 1 1 0 0 0
09:23:42 0 31 100 10 14 29 0 0
09:23:43 0 7 100 12 15 20 0 0
09:23:44 0 1 100 0 0 0 0 0
09:23:45 0 0 0 6 3 0 0 0
09:23:46 0 2 100 3 6 50 0 0
09:23:47 0 6 100 1 1 0 0 0
09:23:48 0 3 100 7 4 0 0 0
09:23:49 0 49 100 3 20 85 0 0
09:23:50 0 95 100 0 28 100 0 0
09:23:51 0 51 100 8 19 58 0 0
09:23:52 0 27 100 4 14 71 0 0
09:23:53 4 390 99 2 64 97 0 0
09:23:54 0 762 100 6 72 92 0 0
09:23:55 0 21 100 3 6 50 0 0
09:23:56 0 41 100 0 11 100 0 0

Average 0 105 100 4 18 76 0 0

What I want to know is what are we shooting for in this? Do we want 100% for %rcache and %wcache? How will I know when I've reduced the buffer cache too much?

TIA,

Sean

[Sandip Ghosh · Accepted solution]

The acceptable range of %rcahe from sar -b output is 90 to 95%. But you have not mentioned how much the buffer cache is at present. You can gradually reduce the buffer cache and have a look at the performance.

Sandip

[Rodney Hills]

Much depends on the type of applications being run. Some large database applications accessed in a diverse random nature will show a poor cache hit ratio. The only fix for that is to modify the database application.

As long as you have enough memory for your applications so that page swapping does not occur, then Buffer cache should be a very healthy chunk of memory since it generally will cut down on the number of physical I/Os.

My 2 cents.

-- Rod Hills