topic Re: Buffer cache overflow in Operating System - HP-UX

Buffer cache overflow

ManojK_1 — Sat, 18 Sep 2010 06:32:44 GMT

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

Re: Buffer cache overflow

Patrick Wallek — Sat, 18 Sep 2010 13:09:47 GMT

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.

Re: Buffer cache overflow

Emil Velez — Sun, 19 Sep 2010 00:11:38 GMT

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.

Re: Buffer cache overflow

ManojK_1 — Sun, 19 Sep 2010 07:02:33 GMT

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

Re: Buffer cache overflow

Dennis Handly — Sun, 19 Sep 2010 17:09:48 GMT

>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?