Boot Drive:
If you want to boot your operating system from the drive you are about to partition, you will need:
- A primary partition
- One or more swap partitions
- Zero or more primary/logical partitions
Any other drive:
- One or more primary/logical partitions
- Zero or more swap partitions
Discussion:
Boot Partition:
Your boot partition ought to be a primary partition, not a
logical partition. It must be of type 0x83 "Linux native". If you are
using lilo, your
boot partition must be contained within the first 1024 cylinders of
the drive. (Typically, the boot partition need only contain the kernel
image.)
If you have more than one boot partition (from other OSs, for
example,) keep them all in the first 1024 cylinders (All DOS
partitions must be within the first 1024). If you are using a means
other than lilo loading your kernel (for example, a boot disk or the
LOADLIN.EXE MS-DOS based Linux loader), the partition can be
anywhere. See the Large-disk HOWTO for details.
Swap Partition:
Unless you swap to files you will need a dedicated swap partition. It
must be of type 0x82 "Linux swap". It may be positioned anywhere on
the disk (but see notes on placement). Either a primary or logical
partition can be used for swap. More than one swap partition can exist
on a drive. 8 total are permitted. See notes on swap size.
Logical Partition:
A single primary partition must be used as a container (extended
partition) for the logical partitions. The extended partition can go
anywhere on the disk. The logical partitions must be contiguous, but
needn't fill the extended partition.
Notes
If you have decided to use a dedicated swap partition, which is
generally a Good Idea [tm], follow these guidelines for
estimating its size:
- In Linux RAM and swap space add up (This is not true for all
Unices). For example, if you have 8 MB of RAM and 12 MB swap
space, you have a total of about 20 MB virtual memory.
- When sizing your swap space, you should have at least 16 MB
of total virtual memory. So for 4 MB of RAM consider at least
12 MB of swap, for 8 MB of RAM consider at least 8 MB of
swap.
- Currently, the maximum size of a swap partition is
architecture-dependent. For i386 and PowerPC, it is approximately
2Gb. It is 128Gb on alpha, 1Gb on sparc, and 3Tb on sparc64. For linux
kernels 2.1 and earlier, the limit is 128Mb. The partition may be
larger than 128 MB, but excess space is never used. If you want more
than 128 MB of swap for a 2.1 and earlier kernel, you have to create
multiple swap partitions. See the man page for mkswap for details.
- When sizing swap space, keep in mind that too much swap space
may not be useful at all.
Every process has a "working set". This is a set of in-memory
pages which will be referenced by the processor in the very
near future. Linux tries to predict these memory accesses
(assuming that recently used pages will be used again in the
near future) and keeps these pages in RAM if possible. If the
program has a good "locality of reference" this assumption
will be true and prediction algorithm will work.
Holding a working set in main memory does only work if there
is enough main memory. If you have too many processes running
on a machine, the kernel is forced to put pages on disk that
it will reference again in the very near future (forcing a
page-out of a page from another working set and then a
page-in of the page referenced). Usually this results in a
very heavy increase in paging activity and in a sustantial
drop of performance. A machine in this state is said to be
"thrashing."
On a thrashing machine the processes are essentially running
from disk and not from RAM. Expect performance to drop by
approximately the ratio between memory access speed and disk
access speed.
A very old rule of thumb in the days of the PDP and the Vax
was that the size of the working set of a program is about
25% of its virtual size. Thus it is probably useless to
provide more swap than three times your RAM.
But keep in mind that this is just a rule of thumb. It is
easily possible to create scenarios where programs have
extremely large or extremely small working sets. For example,
a simulation program with a large data set that is accessed
in a very random fashion would have almost no noticeable
locality of reference in its data segment, so its working set
would be quite large.
On the other hand, an xv with many simultaneously opened
JPEGs, all but one iconified, would have a very large data
segment. But image transformations are all done on one single
image, most of the memory occupied by xv is never touched.
The same is true for an editor with many editor windows
where only one window is being modified at a time. These
programs have - if they are designed properly - a very high
locality of reference and large parts of them can be kept
swapped out without too severe performance impact.
One could suspect that the 25% number from the age of the
command line is no longer true for modern GUI programs
editing multiple documents, but I know of no newer papers
that try to verify these numbers.
So for a configuration with 16 MB RAM, no swap is needed for a
minimal configuration and more than 48 MB of swap are probably
useless. The exact amount of memory needed depends on the
application mix on the machine (what did you expect?).
- Mechanics are slow, electronics are fast.
Modern hard disks have many heads. Switching between heads of
the same track is fast, since it is purely electronic.
Switching between tracks is slow, since it involves moving
real world matter.
So if you have a disk with many heads and one with less heads
and both are identical in other parameters, the disk with
many heads will be faster.
Splitting swap and putting it on both disks will be even
faster, though.
- Older disks have the same number of sectors on all tracks.
With this disks it will be fastest to put your swap in the
middle of the disks, assuming that your disk head will move
from a random track towards the swap area.
- Newer disks use ZBR (zone bit recording). They have more
sectors on the outer tracks. With a constant number of rpms,
this yields a far greater performance on the outer tracks
than on the inner ones. Put your swap on the fast tracks.
- Of course your disk head will not move randomly. If you have
swap space in the middle of a disk between a constantly busy
home partition and an almost unused archive partition, you
would be better of if your swap were in the middle of the
home partition for even shorter head movements. You would be
even better off, if you had your swap on another otherwise
unused disk, though.
Summary: Put your swap on a fast disk with many heads that is
not busy doing other things. If you have multiple disks: Split
swap and scatter it over all your disks or even different
controllers.
Even better: Buy more RAM.