When laying out your filesystem with disklabel(8) or sysinstall(8), it is important to remember that hard drives can transfer data at a faster rate from the outer tracks than the inner. Knowing this, you should place your smaller, heavily-accessed filesystems, such as root and swap, closer to the outside of the drive, while placing larger partitions, such as /usr, towards the inner. To do so, it is a good idea to create partitions in a similar order: root, swap, /var, /usr.
The size of your /var partition reflects the intended use of your machine. /var is primarily used to hold: mailboxes, print spool and log files. Mail boxes and log files, in particular, can grow to unexpected sizes based upon how many users are on your system and how long your log files are kept. If you intend to run a mail server, a /var partition of over a gigabyte can be suitable. Additionally, /var/tmp must be large enough to contain any packages you may wish to add.
The /usr partition holds the bulk of the files required to support the system and a subdirectory within it called /usr/local holds the bulk of the files installed from the ports(7) hierarchy. If you do not use ports all that much and do not intend to keep system source (/usr/src) on the machine, you can get away with a 1 gigabyte /usr partition. However, if you install a lot of ports (especially window managers and Linux binaries), we recommend at least a two gigabyte /usr and if you also intend to keep system source on the machine, we recommend a three gigabyte /usr. Do not underestimate the amount of space you will need in this partition, it can creep up and surprise you!
When sizing your partitions, keep in mind the space requirements for your system to grow. Running out of space in one partition while having plenty in another can lead to much frustration.
Note: Some users who have used sysinstall(8)'s Auto-defaults partition sizer have found either their root or /var partitions too small later on. Partition wisely and generously.
As a rule of thumb, your swap space should typically be double the amount of main memory. For example, if the machine has 128 megabytes of memory, the swap file should be 256 megabytes. Systems with lesser memory may perform better with a lot more swap. It is not recommended that you configure any less than 256 megabytes of swap on a system and you should keep in mind future memory expansion when sizing the swap partition. The kernel's VM paging algorithms are tuned to perform best when the swap partition is at least two times the size of main memory. Configuring too little swap can lead to inefficiencies in the VM page scanning code as well as create issues later on if you add more memory to your machine.
Finally, on larger systems with multiple SCSI disks (or multiple IDE disks operating on different controllers), it is strongly recommend that you configure swap on each drive (up to four drives). The swap partitions on the drives should be approximately the same size. The kernel can handle arbitrary sizes but internal data structures scale to 4 times the largest swap partition. Keeping the swap partitions near the same size will allow the kernel to optimally stripe swap space across the disks. Do not worry about overdoing it a little, swap space is the saving grace of Unix. Even if you do not normally use much swap, it can give you more time to recover from a runaway program before being forced to reboot.
Why partition at all? Why not create one big root partition and be done with it? Then I do not have to worry about undersizing things!
There are several reasons this is not a good idea. First, each partition has different operational characteristics and separating them allows the filesystem to tune itself to those characteristics. For example, the root and /usr partitions are read-mostly, with very little writing, while a lot of reading and writing could occur in /var and /var/tmp.
By properly partitioning your system, fragmentation introduced in the smaller more heavily write-loaded partitions will not bleed over into the mostly-read partitions. Additionally, keeping the write-loaded partitions closer to the edge of the disk, for example before the really big partition instead of after in the partition table, will increase I/O performance in the partitions where you need it the most. Now it is true that you might also need I/O performance in the larger partitions, but they are so large that shifting them more towards the edge of the disk will not lead to a significant performance improvement whereas moving /var to the edge can have a huge impact. Finally, there are safety concerns. Having a small neat root partition that is essentially read-only gives it a greater chance of surviving a bad crash intact.
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