Buffer cache overflow

[ManojK_1]

Hi,

What is the performance impact if buffer cache overflows?

$ sar -b 2 10

HP-UX B.11.31 U ia64 09/18/10

11:28:37 bread/s lread/s %rcache bwrit/s lwrit/s %wcache pread/s pwrit/s
11:28:39 0 178 100 6 15 61 0 0
11:28:41 0 443 100 9 26 66 0 0
11:28:43 0 65 100 1 4 67 0 0
11:28:45 0 60 100 4 8 56 0 0
11:28:47 0 53 100 10 7 0 0 0
11:28:49 0 73 100 4 14 71 0 0
11:28:51 0 52 100 1 2 50 0 0
11:28:53 0 67 100 7 10 30 0 0
11:28:55 0 52 100 5 4 0 0 0
11:28:57 0 58 100 6 3 0 0 0
Average 0 110 100 5 9 44 0 0


There is anything wrong in the above sar -b output? Fine tuning is required?

Manoj K

[Patrick Wallek]

It appears as is 100% of your reads are coming from buffer cache/file cache.

That is a good thing. I don't see anything wrong with that. In fact, with all reads coming from cache, your performance will improve since a read from disk is not required.

[Emil Velez]

if the buffer cache gets to its maximum size then future reads cause the oldest data that has not been accessed to age out of buffer for the new blocks that are accessed. Buffer cache always contains the most recently accessed blocks. Until buffer cache hits its maximum buffer cache just grows.

When you get low on memory vhand starts shrinking buffer cache.

[ManojK_1]

Hi,

Serer is having 64Gb Memory and kerenel parameter filecache_max & filecache_min set as 1%.

If i increase the value what will be the impact?

Manoj K

[Dennis Handly]

>If I increase the value what will be the impact?

You will be able to cache more data.
Are you using a database that can get more benefit from that memory?