4. Partitioning requirements

4.3.1. Which file systems need their own partitions?

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.

4.3.2. File lifetimes and backup cycles as partitioning criteria

With ext2, partitioning decisions should be governed by backup considerations and to avoid external fragmentation (Section 7.3) 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 (choose incremental backup and expect to change backup media for the weekly/monthly full dump - no unattended operation possible).

Your backup strategy depends on that decision.

When planning and buying disk space, remember to set aside a sufficient amount of money for backup! Unbackuped data is worthless! Data reproduction costs are much higher than backup costs for virtually everyone!

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.

4.4.1. How large should my swap space be?

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.

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?).

4.4.2. Where should I put my swap space?

• 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 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.

• 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.