topic Re: Buffer Cache usage in a cluster in Operating System - HP-UX

Buffer Cache usage in a cluster

Alexander Santamaria Ji — Thu, 03 Apr 2003 19:44:21 GMT

We have a cluster with MC/Serviceguard and 2
rp7410 servers attached to a SAN with 1 x VA7100, if a package of Data Storage Protector is asigned to a server then the
Buffer cache usage changes from 0.5 Gb to 2GB, then if the package is moved to the other node then buffer cache remains using 2GB.

How can i control this usage?

Re: Buffer Cache usage in a cluster

Geoff Wild — Thu, 03 Apr 2003 19:54:53 GMT

Change the dbc_max_pct kernel parameter.

The default is 50% - for SAP / Oracle servers we set it to 10% or 15%.

RGds...Geoff

Re: Buffer Cache usage in a cluster

Patrick Wallek — Thu, 03 Apr 2003 19:57:52 GMT

Once your buffer cache usage increases, it will only decrease if you start running into memory pressure. Otherwise it will keep increasing until it reaches the value governed by dbc_max_pct.

Re: Buffer Cache usage in a cluster

A. Clay Stephenson — Thu, 03 Apr 2003 20:14:18 GMT

That is typical of dynamic buffer cache. I suspect different settings of dbc_max_pct and dbc_min_pct on the two boxes. One option is to use a fixed buffer cache on both boxes by setting bufpages to a non-zero value. Bugpafes is set in units of 4k pages (unless some has altered the ralted paramters - very unlikely).

Re: Buffer Cache usage in a cluster

Bill Hassell — Fri, 04 Apr 2003 02:25:27 GMT

Just a recommendation: 2Gb is way too large for a typical database system. It could be useful for read-mostly situations but tends to increase system overhead for 50:50 read:write ratios as so many of the write records must be written to the disk at each sync cycle. Keep the buffer cache below 500-700 megs max. You can then give the extra RAM to the DBA who quickly use it in shared memory.

Re: Buffer Cache usage in a cluster

melvyn burnard — Fri, 04 Apr 2003 08:44:08 GMT

I agree with Bill's recommendations.
I have seen a system end up doing a SG TOC due to the system being too busy to flush a very large buffer cache, and not being able to service the cmcld daemon.

Re: Buffer Cache usage in a cluster

Alexander Santamaria Ji — Fri, 04 Apr 2003 14:41:16 GMT

Bill, Melvyn;

I agree with you that 2GB is too large because our systems have 5GB of memory each one, i am going to limit the buffer cache size at 750MB max.

Thanks for your recommendations (and also for Geoff, Patrick and Clay)

Best Regards

Alexander Santamaria