Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!microsoft!gregj
From: gregj@microsoft.UUCP (Greg JONES)
Newsgroups: comp.os.os2.misc
Subject: Re: HPFS and cache controllers
Summary: Filesystem vs. disk cache
Keywords: HPFS cache
Message-ID: <57437@microsoft.UUCP>
Date: 14 Sep 90 01:04:52 GMT
References: <1990Sep6.141036.11041@sun1.ruf.uni-freiburg.de>
Distribution: comp
Organization: Microsoft Corp., Redmond WA
Lines: 25

In article <1990Sep6.141036.11041@sun1.ruf.uni-freiburg.de>, seifert@sun1.ruf.uni-freiburg.de (Ulrich Seifert) writes:
> Within HPFS there is already the possibility to configure a cache system.
> Does anybody know how it works exactly? Is it a kind of cache that just
> looks if it has the requested sector in memory or does it also some read-
> look-ahead or a kind of ordered write back to disk...?
> Let us assume we put a cache controller into the system. This may do all 
> those things like buffering the most frequently used sectors, read-look-ahead
> of a few sectors, and optimized write back to disk (I believe these are the
> main features of the DPT). Does it help anything?
> 
HPFS caching cannot be totally disabled, it can only be shrunk.  Almost
all disk I/O done by HPFS (and certainly all I/O for control sectors like
directories and so on) goes through its cache.  The disk format is optimized
to make the best use of how this cache operates.  HPFS will cache frequently
used sectors on an LRU basis;  this gets modified according to the relative
likelihood that the sector will be needed again.  For example, a directory
block is likely to be very popular, but the FNODE for a file that's being
deleted probably won't be.  HPFS also does some read-ahead of file data and
background lazywriting.

In general it's better to let the filesystem do the caching, since it knows
what kind of information is best to cache.

uunet!microsof!gregj
[I just happen to work here.  Sheer coincidence.]