Re: RE: linux DBA's: O_Direct

Quoting MARK BRINSMEAD <mark.brinsmead_at_shaw.ca>:

> Ah. It seems that my memory *is* failing me. The parameter I was thinking of
> was NOT "BUFFERMEM" (which is no doubt long obsolete) but rather PAGECACHE.
>
> Adding the entry:
> vm.pagecache 5 15 10
> to /etc/sysctl.conf should (if I *now* recall correctly) set the minimum,
> maximum, and "target" filesystem buffers to 5%, 15%, and 10% of RAM
> respectively.
>
> The object /proc/sys/vm/pagecache does, indeed, exist on my RHEL3 servers, so
> I will take that to be a hint that I have got it right this time. It seems
> that I may have delegated sysadmin work to others for too long now, so I
> shall leave it to the "gurus" on the list to confirm or deny this...
>

Hmmm, I'd have a quick look at:
http://www.redhat.com/magazine/001nov04/features/vm/ near the end of the page, just in case:

those values for pagecache don't sound good.

A better choice would be 5 15 25.
Ie, never make the max smaller than what the borrow value is.

In general terms:

the first parameter is the size of cache (relative to memory size) below which the vm will start to reclaim pages exclusively from process cache. the second parameter is the size of cache (relative to memory size) below which the vm will start to reclaim pages from both file system cache and process cache.
the third parameter is the size of cache (relative to memory size) that vm will use for both process page cache and file system cache. in general, cache (both process and file) is reduced as memory is needed for running processes, within the boundaries defined above.

This also does a darn good job of explaining the mechanics of all that (and the whys of the process/file cache): http://surriel.com/lectures/ols2003/

Enjoy.
Nuno Souto
from sunny Sydney

--
http://www.freelists.org/webpage/oracle-l