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Path: utzoo!linus!sid
From: sid@linus.UUCP (Sid Stuart)
Newsgroups: net.unix-wizards
Subject: Memory buffers for a Sun disk server?
Message-ID: <339@linus.UUCP>
Date: Thu, 25-Apr-85 21:49:24 EST
Article-I.D.: linus.339
Posted: Thu Apr 25 21:49:24 1985
Date-Received: Fri, 26-Apr-85 22:45:51 EST
Reply-To: sid@faron.UUCP (Sid Stuart)
Distribution: net
Organization: The MITRE Coporation, Bedford, MA
Lines: 41
Keywords: Sun, disk server


	We have a Sun 170 with one (soon two) Fujitsu Eagles and two
megabytes of memory that is used strictly as a disk server.
I have been thinking that we could get an increase in speed on the diskless
nodes if we increased the number of buffers available
for file I/O and for swapping. The default number of buffers is what
can be contained in about 10% of the memory on the machine.
Not having Sun source code available tonight, I looked at the code for
my 780. On a Vax the number of buffers is calculated in 
/sys/vax/machine.c. To increase the number of buffers available from 10% of
memory to 50% on the Vax, I would just change the line:

	"bufpages = (physmem * NBPG) / 10 / CLBYTES;

to something like:

	"bufpages = (physmem * NBPG) / 2 / CLBYTES; "

I figure the code is similar in the Sun kernal.  I have two questions before
I attempt this though:

First, in the code below where the bufpages is calculated, the available
buffers are split between file I/O buffers and swap buffers. Then the number
of swap buffers is limited to be at most 256. Next to this limit operation
is the comment, "sanity". Does that mean I have to be insane to increase
the number of swap buffers above 256?

Second, I figure the guys at Sun are a lot smarter than I am, why haven't they
put something like this in their code for disk servers? (This one has me
worried.)


					Waiting for wisdom,
					sid

ps.
	Thinking about the buffering between disk servers and diskless
nodes brings up another interesting question: Are swap pages from
a diskless node held in file buffers or swap buffers on the disk server?