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. This will ease recovery in case of disaster, but it
is not technically necessary. It must be of type 0x83 "Linux native". If
you are using a version of lilo before
21-3 (ie, from the 1990s), 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 modern version of lilo, or a means other than lilo to load 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 (across drives) 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.
File Systems
Everything in your linux file system can go in the same (single)
partition. However, there are circumstances when you may want to
restrict the growth of certain file systems. For example, if your mail
spool was in the same partition as your root fs and it filled the
remaining space in the partition, your computer would basically
hang.
| /var
| This fs contains spool directories such as those for mail and
printing. In addition, it contains the error log
directory. If your machine is a server and develops a
chronic error, those msgs can fill the partition. Server
computers ought to have /var in a different partition than
/.
|
| /usr
| This is where most executable binaries go. In addition, the
kernel source tree goes here, and much documentation.
|
| /tmp
| Some programs write temporary data files here. Usually, they
are quite small. However, if you run computationally
intensive jobs, like science or engineering applications,
hundreds of megabytes could be required for brief periods of
time. In this case, keep /tmp in a different partition than
/.
|
| /home
| This is where users home directories go. If you do not impose
quotas on your users, this ought to be in its own partition.
|
| /boot
| This is where your kernel images go. See discussion above
for placement on old systems.
|
With ext2, partitioning decisions should be governed by backup
considerations and to avoid external fragmentation from different
file lifetimes. Files have lifetimes. After a file has been created,
it will remain some time on the system and then be removed. File
lifetime varies greatly throughout the system and is partly dependent on
the pathname of the file. For example, files in /bin,
/sbin, /usr/sbin, /usr/bin and
similar directories are likely to have a very long lifetime: many months
and above. Files in /home are likely to have a medium
lifetime: several weeks or so. File in /var are usually
short lived: Almost no file in /var/spool/news will remain
longer than a few days, files in /var/spool/lpd measure
their lifetime in minutes or less.
For backup it is useful if the amount of daily backup is
smaller than the capacity of a single backup medium. A daily
backup can be a complete backup or an incremental backup.
You can decide to keep your partition sizes small enough that
they fit completely onto one backup medium (choose daily full
backups). In any case a partition should be small enough that
its daily delta (all modified files) fits onto one backup
medium.
Your backup strategy depends on that decision.
For performance it is useful to keep files of different
lifetimes on different partitions. This way the short lived
files on the news partition may be fragmented very heavily.
This has no impact on the performance of the / or /home
partition.
Swap Partitions
Conventional wisdom creates swap space equal to the amount of 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, a graphics program 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 the program is not accessed.
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. A user who
never never quits programs once launched would want a lot of
swap space for the same reason.
Servers typically are configured with more swap space than their desktop
counterparts. Even though a given amount of swap is sufficient for its
operations, the server might come under transient heavy loads which
cause it to page out at a high rate. Some administrators prefer this to
the server crashing altogether. In these cases, swap might be several times
the size of ram.
Currently, the maximum size of a swap partition is
architecture-dependent. For i386, m68k, ARM and PowerPC, it is "officially" 2Gb. It is 128Gb on alpha, 1Gb on
sparc, and 3Tb on sparc64. An opteron on the 2.6 kernel can write to a 16
Tb swap partition. 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 (8 max). After 2.4, 32 swap areas are
"officially" possible. See setting up swap for
details.
The short answer is anywhere is fine. However, if you are
interested in extracting as much speed as possible, there are
two basic strategies (other than buying more RAM):
- Split the swap space across multiple drives, or at least on the
drive you write to least.
- Put each swap partition on the outer tracks.
Here are the considerations:
- Choice of drive
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.
Reading data from different heads is fast, since it is purely
electronic. Reading data from different tracks is slow, since it
involves physically moving the head.
It follows then that writing swap on a separate drive will be
faster than moving the head back and forth on a single drive.
- Placement:
Older disks have the same number of sectors on all tracks.
With these 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.
(In general, low-numbered cylinders are associated low
partition numbers. However, see Kristian's more recent
comments on this issue. -Tony)
- Usage:
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 near 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.
- Striping:
Speed can be increased by writing to multiple swap areas simultaneously.
Swap spaces with the same priority will be written to like a RAID. See setting up swap for details.
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.
max swap size: With kernel 2.4, the limit is
64 swap spaces at a maximum of 64Gb each, although this is not reflected
in the man page for mkswap. With the 64 bit opteron on the
2.6 kernel, 128 swap areas are permitted, each a whopping 16 Tb! (thanks
to Peter Chubb for the calculation)
