From: eletanjm@nuscc.nus.sg (TAN JIN MENG)
Subject: Re: NT vs OS/2 features (was Re: Misinformation about os2-dos HW access)
Organization: National University of Singapore
Date: Fri, 4 Sep 1992 01:48:03 GMT

db3l@ans.net (David Bolen) writes:
: In article <1992Sep3.022015.8901@nuscc.nus.sg> eletanjm@nuscc.nus.sg (TAN JIN MENG) writes:
: 
: >dic5340@hertz.njit.edu (David Charlap) writes:
: >: It's better to use a separate caching scheme for each file system, the
: >: way OS/2 does.  This is why you have a /CACHE= parameter in the
: >: HPFS.IFS driver, and a DISKCACHE parameter separately for FAT volumes.
: >
: >That's not the contention in the [NT vs OS/2 features list by Brian
: >Sturgill]. Actually, I thought the HPFS cache is for all the file
: >systems and the FAT cache is meant for floppys. 
: 
: The FAT cache (DISKCACHE= line in CONFIG.SYS) is for anything that uses the
: FAT filesystem support built into the OS/2 kernel.  This definitely includes
: floppies, but also any hard disk partitions formatted as FAT.
: 
: The HPFS cache (/CACHE on IFS line in CONFIG.SYS) is for filesystem
: partitions that are formatted for HPFS.  This requires loading the HPFS IFS.
: 
: >Actually, I believe that it is more fault tolerent to have a single
: >cache scheme. This can be done efficiently by having a single file
: >buffer cache for ALL block devices (as in UNIX) plus individual file
: >system info caching separately for each of the file systems supported.
: 
: I'd buy that a particular caching scheme may be more fault tolerant than
: others (obviously write-through caching is more fault tolerant than lazy
: writing, at a performance expense), but I'm not sure why having multiple
: caching schemes is any less fault tolerant than having a single scheme, if
: each scheme is in and of itself fault tolerant.

Sorry, what I essentially meant was "the more things you have the more
things you have that can be broken". Its a generic truth in systems
design esp. when these things interact.

: 
: I also don't think that any single scheme can get the same performance as
: a caching system that understands the filesystem that it is caching.  Having
: a general buffer cache for all block devices is much less efficient than
: having a cache that understands directory nodes vs. file data, for example.
: Yes, both the filesystems are block oriented, but the usefulness of any
: particular block may vary widely among filesystems.

I did not mean to imply that we don't use info on the specific file
system. What I meant was something like a 2 level caching scheme with
one level the block cache (data) and the other the file system info
(inodes whatever) cache - one per file system type.

: 
: You might find that your directory information was being flushed out of the
: cache whenever you accessed a large amount of data.  In many cases, the data
: is less important to cache than the directory information.
: 
: >I wouldn't be suprised if OS/2 (with exception of floppys - which are
: >vastly different from hard disk devices) did it this way.
: 
: I don't know if I'd be surprised or not, but I'd be disappointed.  I'm fairly
: certain that the HPFS (and I'd hope the FAT) caching system is smart enough
: to have a higher priority of keeping directory structure blocks available
: in the cache than for general data.
: 
: --
: -- David
: --
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jin meng
Newsgroups: comp.os.os2.advocacy