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From: gkn@SDS.SDSC.EDU
Newsgroups: comp.os.vms
Subject: file system caches
Message-ID: <870703191345.098@Sds.Sdsc.Edu>
Date: Fri, 3-Jul-87 15:13:44 EDT
Article-I.D.: Sds.870703191345.098
Posted: Fri Jul  3 15:13:44 1987
Date-Received: Sat, 4-Jul-87 19:48:03 EDT
Sender: daemon@ucbvax.BERKELEY.EDU
Distribution: world
Organization: The ARPA Internet
Lines: 38

I've been watching my file system caches lately, and have noticed that the
hit rate on all of them was quite low.  So, I increased the sizes of the
caches and things worked well for the directory index (ACP_DINDXCACHE),
file extent (ACP_EXTCACHE), file ID (ACP_FIDCACHE), file header (ACP_HDRCACHE),
and bitmap (ACP_MAPCACHE) caches.

The hit rates on the directory FCB (ACP_SYSACC), and the directory data
(ACP_DIRCACHE) caches were still low, so I increased the sizes on those
again.

When I did this, the hit rate went *down*;  I mean _through the floor_.
So, I've shrunken the caches back a bit, and I'm back to where I was
before.

Am I misunderstanding something here?  In general, the larger a cache
is the better the chances that you'll find something in it.  This doesn't
seem to be the case with at least some of the XQP caches.

Does anyone else have these kinds of problems (I know of at least one
other site which has similar problems)?  If anyone has any clues as to
why the hit rate goes down after a certain size and how I can improve
the hit rates (ie - optimize the cache size) I'd sure like to hear about
it ...

I'm running a 4-node VAXcluster (two 785s, a 750 (gag) and an 8200, all
of which are way too small for what they're asked to do), and VMS V4.5,
by the way.

gkn
--------------------------------------
Arpa:	GKN@SDS.SDSC.EDU
Bitnet:	GKN@SDSC
Span:	SDSC::GKN (27.1)
USPS:	Gerard K. Newman
	San Diego Supercomputer Center
	P.O. Box 85608
	San Diego, CA 92138
AT&T:	619.534.5076