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From: ellis@rata.vuw.ac.nz (Brian Ellis)
Newsgroups: comp.unix.ultrix
Subject: Re: A question about swap
Keywords: swap
Message-ID: <1991Jun18.042752.10818@rata.vuw.ac.nz>
Date: 18 Jun 91 04:27:52 GMT
References: <1991Jun14.184609.21178@mlb.semi.harris.com> <967@lhdsy1.chevron.com>
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Organization: Victoria University of Wellington
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In article <967@lhdsy1.chevron.com>, yzarn@lhdsy1.chevron.com (Philip Yzarn de Louraille) writes:
|> In article <1991Jun14.184609.21178@mlb.semi.harris.com> dcb@dave.mis.semi.harris.com writes:
|> >On one of our 5500s, we have configured over 500M of swap space. Someone
|> >recommended using the 'a' and 'b' partitions over five drives, with the
|> >a/b partitions combined into a larger 'a' partition.
|> ... deleted text...
|> I was told once to *never* mess up with the a partition of a disk.
|> Forget about the 16Meg you will be loosing.

Referring to the guide to Configuration file maintenance...

"Avoid selecting partition a of any disk for use as the swap partition. If
partition table information was defined for a disk and swapping occurs on the
a partition, the information is destroyed and data is lost."

Given that either the "a" or the "c" partition may be the "root" partition
of the disk, I would be *very* wary of using the c partition for the same
reasons. 

I have successfully and painlessly changed the size of the "a" partition on some
of my disks. The trick is not to have the disk in use for *anything* at the
time. The only thing you can't do to the a partition is change it's offset.
(Obviously it has to start at sector 0).

I didn't like the idea of loosing 16 MB either, so in one case, I'm using it
as a backup of my root filesystem. On another disk, I changed the partition
tables such that the b partition was now in the "middle" of the disk with a
filesystem on each side. ie:

/dev/ra1a      187 MB  /usr/local
/dev/ra1b      64 MB   swap space
/dev/ra1f      174 MB  /u1

The system I administer is swapping to 3 spindles. In reverence to advice
I received locally I followed these rules:
	- all swap spaces are the same size (64 MB in my case)
	- each one is larger than physical memory
	- total swap space is about 4 times physical memory

In practice (observed using vmstat(1)), this system is nicely distributing it's
swapping across the 3 partitions available to it.

-- 
Brian Ellis                               Computing Services Centre
Domain: ellis@rata.vuw.ac.nz              Victoria University of Wellington
Bang paths... grrrr!!!!!		  P.O Box 600, New Zealand.
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