*BSD News Article 45515


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From: dillon@best.com (Matt Dillon)
Newsgroups: comp.unix.bsd.freebsd.misc
Subject: Re: Can FreeBSD execute programs in the disk cache?
Date: 17 Jun 1995 13:26:41 -0700
Organization: Best Internet Communications, Inc. (info@best.com)
Lines: 57
Message-ID: <3rvdqi$iaq@shell1.best.com>
References: <3rpu4v$28q@park.uvsc.edu> <3rqa0t$c6h@marina.cinenet.net> <3rr4bl$ho2@shell1.best.com> <3rstuu$2pi@news.cc.utah.edu>
NNTP-Posting-Host: shell1.best.com

:In article <3rstuu$2pi@news.cc.utah.edu>,
:Terry Lambert <terry@cs.weber.edu> wrote:
:>dillon@best.com (Matt Dillon) wrote:
:>]     The answer to the original question, without making too many
:>]     wording jokes about it, is simply:
:>] 
:>] 	YES
:>] 
:>]     The more sophisticated answer is:  YES, but the amount of caching
:>]     depends on how heavily you are using the machine's memory (to run
:>]     other programs, data file accesses, etc...)
:>
:>Despite your dislike for my "wording jokes", the answer is that
:>the question is at best a non-sequitor and at worst too non-specific
:>to answer.
:>
:>"Is it shorter to New York or by bus?  A simple yes or no will do".
:>
:>
:>Let us assume that the proper term was used, and it wasn't just
:>as incorrectly constructed as the lack of elaboration for the
:>question.  So on to providing an answer to each of the possible

    I don't think the question was *that* badly phrased.

:>]     Sillyness is right!  In general, the more on-disk and in-controller
:>]     caching you have, the slower your disk accesses are.
:>
:>Exception: (probably why you chose the term "generally") write
:>caching drives significantly speed write performance.
:>
:>]     Most SCSI disks have some caching.  Most IDE disks have very little.
:>]     Most disk controllers have none and, in fact, you don't want them
:>]     to have any for the reasons mentioned above.  Direct DMA is the best
:>]     way to go, especially on a Pentium/PCI-bus controller.
:>
:>Another exception: track caching.  Most modern disks will by default
:>reverse the sector ordering numbers on the disk and start reading as
:>soon as they hit the track and stop after they have read the desired
:>block.
:>
:>The point in doing this is to have sequential blocks after the
:>requested block in cache and to prevent the read from being a
:>necessrily physical one for sequential access.  Said cache is
:>typically a track buffer on the disk itself.

    These aren't really exceptions, just good cache management, and
    it doesn't take much cache to do it.  I never advocated NO cache,
    just that huge caches have been found not to work as well as 
    people have theorized.

						-Matt

-- 
    Matthew Dillon   VP Engineering, BEST Internet Communications, Inc.
		    <dillon@best.com>, <dillon@apollo.west.oic.com>
    [always include a portion of the original email in any response!]