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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!]