Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!utgpu!water!watmath!clyde!rutgers!husc6!cmcl2!philabs!micomvax!musocs!mcgill-vision!mouse
From: mouse@mcgill-vision.UUCP
Newsgroups: comp.unix.wizards
Subject: Re: File system placement
Message-ID: <783@mcgill-vision.UUCP>
Date: Sun, 31-May-87 02:29:16 EDT
Article-I.D.: mcgill-v.783
Posted: Sun May 31 02:29:16 1987
Date-Received: Wed, 10-Jun-87 05:01:48 EDT
References: <1294@qfdts.OZ> <755@scgvaxd.UUCP>
Organization: McGill University, Montreal
Lines: 58
Keywords: location of key file systems for optimal access

In article <755@scgvaxd.UUCP>, brian@scgvaxd.UUCP (Brian Zill) writes:
> In article <1294@qfdts.OZ> phil@qfdts.OZ (Phil Chadwick) writes:
>> On the day I first booted UNIX from release tape, root, swap, tmp
>> and usr were placed on the disk starting at cylinder 0.
[>> about how they should be in the middle of the disk]
>> I am giving serious consideration to rearranging the larger disks
>> (say those > 300Mb) with root, tmp, swap and usr (not including
>> users' files) fair bang in the middle.

> Well, actually, some people have shifted the 'a' and 'b' partitions
> to the middle of the disk for precisely this reason.

> If you want to change the partition sizes yourself on whatever system
> you have, it's not too difficult if you have source.  Just modify the
> tables in the driver and in /etc/disktab [...]

All of this is ignoring an issue.  How do you boot from such a disk?
Clearly the boot block must still be at sector zero, where the boot
ROMs expect it to be.  The level-zero boot *can* be anywhere, but the
gain of relocating it is probably not worth the pain.  But you will
have to change the partition tables in the standalone drivers for the
level-zero boot if /boot is to reside on a partition beginning not at
sector zero or with a size not equal to the usual partition-at-sector-0
size; and you will have to change the partition tables in /boot if the
root partition is on such a partition.  This is probably why the
distribution extracts itself this way.

I for one am glad it does it the way it does.  We have a MicroVAX here
with a disk that claims (to the driver) to be an RA81.  Unfortunately,
it is not an RA81; it is much smaller than a real RA81.  As a result,
if the distribution driver had tried to put root and swap in the middle
of the disk, it would have had no hope of succeeding.  As it was, the
miniroot ran fine, then on the real root I used adb to patch the
partition tables in the vmunix there, so I could mkfs and extract the
real /usr, and then of course I just changed the driver and rebuilt.

No need to change the level-0 boot or /boot because the root partition
is still the one beginning at offset 0.

Actually, if you can spare a half-megabyte or more of core, you may
want to consider putting /tmp on a silicon disk.  If you are running
4.3BSD on a VAX, or Sun UNIX 3.0 (probably later versions as well) on a
Sun-3 (probably on a Sun-2 as well), I have a silicon disk driver for
you.  (Everyone else is welcome to it as well, but I have no reason to
believe it'll be useful to you.)  Think this over before doing it, or
watch performance carefully before-and-after, because the silicon disk
driver will be taking memory you are likely to find you need worse for
other things.

I also have another pseudo-disk driver.  This one takes a normal file
and makes it appear to be a disk partition, mapping requests for block
N of the parition into filesystem access to block N of the file.  It
runs under 4.2 and 4.3 BSD, but not under Sun UNIX and only very
unreliably under 4.3+NFS (the mtXinu version).

					der Mouse

				(mouse@mcgill-vision.uucp)