I do not agree with the advice to dedicate one disc to every table.
Imagine 10 disks, 10 tables, and 10 users accessing the same table. This
would mean 1 disk really busy and 9 idling all the way. If you use a
RAID array, the random access is hopefully split to all disks, giving a
much better performance. There are suggestions about direct disk access
(using O_DIRECT file flags), versus using the OS' caching mechanisms.
This is quite the same, in hardware. Today's hardware is designed to
make the best of it, give it a chance!
PostgreSQL's transaction logs are probably being written sequentially.
This situation is different because the access pattern is predictable,
dedicated disks might be useful since head movement is reduced to near
zero, but if there's no high write volume it's wasted performance.
If your system is so small that web and database are running on the same
machine, you can consider page access being quite like table access, so
I'd put it on the same big array. Your favorite *.html or *.php will be
cached either.
Swap space may be on a dedicated disk, but it's better for a server if
swap is never used. Put enough RAM into that machine! Swapping is quite
a desaster on a server. So you could put swap just where you like it: if
your server is sane, it's never accessed under load.
Same about OS files: is there really heavy traffic on them?
There's a white paper at www.microsoft.com about tuning MSSQL 7.0. If
read carefully, some advice will be applicable to PostgreSQL too.
So as my general rule, valid for >99 % of users: use as much disks as
possible in one big RAID array. Let the hardware do the data scattering,
you'll be better off. For a production system, you will need disk
redundancy either (my experience says one failed disk per year for 20 in
use). Until your system is really heavily loaded, and you're using >10
disks, don't think about dedicated disks. If you want extra performance
for no cost, you can put the most accessed partition on the outer
cylinders of the disk array (probably corresponding to the outer
cylinders of the disk) since throughput is highest there.
Regards,
Andreas