yet another take on it ... :)

here's my disklabel for /dev/raid0g (a 120G RAID mirror):

a:/	about 100M, for the reasons already stated

b:swap	same size as memory - I tend to think that if you're
	using more than that then you've got problems that more
	swap won't solve ... 

c:	unused
d:	unused

e:/usr	around 300M - big enough for binary sets only

f:/var	varies, but usually 300M also

g:/mnt/raid0g - the remainder of the disk - about 100G

Inside /mnt/raid0g I have directories such as

/data
/home
/usr/pkg
/usr/pkgsrc
/usr/pkgbuild

which are then null mounted over their real location as follows:

/mnt/raid0g/data /data null rw 0 0 
/mnt/raid0g/home /home null rw 0 0 
/mnt/raid0g/usr/pkg /usr/pkg null rw 0 0
/mnt/raid0g/usr/pkgsrc /usr/pkgsrc null rw 0 0
/mnt/raid0g/usr/pkgbuild /usr/pkgbuild null rw 0 0

the nice thing about null mounts is that you can treat them as
individual filesystems for backups, yet keep all your free space
together. I also believe in keeping /, /usr and /var as free from
modifications as possible, allowing them to be blown away easily if
necessary ... null mounting my real data into them helps keep things
seperate.

usual caveats for big filesystems falling over apply, naturally, however
so far it hasn't been an issue and I've been doing this for the last
three years or so ...

I use it a lot at work as well, where we have a big disk partitioned
into 10 ~40GB filesystems (a-l). User accounts are stored with paths
like /home/m but the actual files would be stored in something like
/mnt/sd0j/home/m and null-mounted to where they're meant to be.

I then back up through the /home/m style null mounts. This has two main
benefits - backups never exceed the size of a tape because they're
limited to 40G and if a filesystem is filling, I can copy the data from,
say, /mnt/sd0j/home/m to /mnt/sd0f/home/m, change the null mount point
and then blow away /mnt/sd0j/home/m, knowing that backups will still
work and users won't notice.

-- 
Malcolm Herbert
Computer Support Officer
School of Geosciences
Monash University
ph 9905 4881