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From: burgess@cynjut.infonet.net (Dave Burgess)
Newsgroups: comp.os.386bsd.questions
Subject: Re: Need HW/Drive purchase advice
Date: 2 Mar 1995 13:25:38 -0600
Organization: Configuration Management Svcs, Inc.
Message-ID: <3j563i$2u6@cynjut.infonet.net>
References: <shamrock-1502952240270001@192.0.2.1> <3iv4ri$624@bonnie.tcd-dresden.de> <3j05a6$fhk@cynjut.infonet.net> <3j3j5m$n70@helena.mt.net>
NNTP-Posting-Host: cynjut.infonet.net
Lines: 75
In article <3j3j5m$n70@helena.mt.net>,
Nate Williams <nate@sneezy.sri.com> wrote:
}In article <3j05a6$fhk@cynjut.infonet.net>,
}Dave Burgess <burgess@cynjut.infonet.net> wrote:
}>In article <3iv4ri$624@bonnie.tcd-dresden.de>,
}>J Wunsch <joerg_wunsch@uriah.heep.sax.de> wrote:
}>}Lucky Green <shamrock@netcom.com> wrote:
}>}
}>}>I would like to run FreeBSD
} ^^^^^^^
}....
}
}>}>How much RAM should I add?
}>}
}>}Depends. If you're going to run X11, start with 16 MB total RAM.
}>}
}>}If you can effort it, $$ spent into RAM are never wrong (more RAM is
}>}often the better choice as opposed to a faster CPU).
}
}>
}>One caveat on this. If you have a machine that is a straight ISA bus
}>(not EISA, VLB, or PCI) and you try and use a bus-mastering SCSI
}>controller, you can run into problems when using more than 16M of
}>memory. It is a known, documented limitation of the ISA bus, and should
}>not be ignored.
}
}However, under FreeBSD this is not a problem. Even though there is a
}penalty for using the upper memory (bounce buffers), that penalty is
}trivial compared to the performance gain you are getting with a
}bus-mastering SCSI controller.
}
}At worst case, it is 15% slower to accesss memory w/bounce buffers.
}But, memory is still a few orders of magnitude faster than disk, so this
}is barely noticeable. It's a 'hack', but a very workable and usable
}hack none-the-less.
}
It isn't even really a hack any more. It is a proven technique that
does allow access to more memory than the ISA bus can directly access.
Since it is really the ONLY way that it can be done, it is really quite
clever and very usable.
On the other hand, if you could have 16Meg of memory or another $400
worth of hard drive, there is something to be said for a 1Gig drive. At
$40 a Meg for Memory, versus $0.40 a Meg for a large SCSI hard drive, is
the memory really 100 times as useful?
}Go for SCSI, as it'll never be a bad decision. You can always sell your
}hard-drives to old workstation folks for swap disks, and your CD-ROMS
}will always work on any machine. :-)
}
}
I agree with Nate 100% (as usual). I just wanted everyone to remember
that anytime you try and use more than 16Meg of memory in a straight ISA
machine, there will have to be additional work done to fully utilize
that additional memory.
Bounce buffers is a perfectly acceptable way of dealing with this
problem. The other alternatives (strictly for the ISA crowd) are also
acceptable. In fact, the 1522 (non-DMA) option seems to be a good
middle ground. The SCSI devices are easy to upgrade later, if you are
(like me) a junk user. I can upgrade my CPU and controller later, and
still use all of the SCSI devices I have. I can also use to 20Meg of
memory that I have, all without any special attention from the system.
A LOT depends on what you are doing and to whom you are doing it.
For more information, see the section of the FAQ that Jordan wrote about
the 16Meg limit and Bus Mastering ISA devices.
--
TSgt Dave Burgess | Dave Burgess
NCOIC, USSTRATCOM/J6844 | *BSD FAQ Maintainer
Offutt AFB, NE | Burgess@s069.infonet.net