When laying out file systems with bsdlabel(8) or sysinstall(8), remember that hard drives transfer data faster from the outer tracks to the inner. Thus smaller and heavier-accessed file systems should be closer to the outside of the drive, while larger partitions like /usr should be placed toward the inner parts of the disk. It is a good idea to create partitions in an order similar to: root, swap, /var, /usr.
The size of the /var partition reflects the intended machine usage. The /var file system is used to hold mailboxes, log files, and printer spools. Mailboxes and log files can grow to unexpected sizes depending on how many users exist and how long log files are kept. Most users will rarely need more than about a gigabyte of free disk space in /var.
Note: There are a few times that a lot of disk space is required in /var/tmp. When new software is installed with pkg_add(1) the packaging tools extract a temporary copy of the packages under /var/tmp. Large software packages, like Firefox, OpenOffice or LibreOffice may be tricky to install if there is not enough disk space under /var/tmp.
The /usr partition holds many of the files required to support the system, including the ports(7) collection (recommended) and the source code (optional). Both the ports and the sources of the base system are optional at install time, but we recommend at least 2 gigabytes for this partition.
When selecting partition sizes, keep the space requirements in mind. Running out of space in one partition while barely using another can be a hassle.
Note: Some users have found that sysinstall(8)'s Auto-defaults partition sizer will sometimes select smaller than adequate /var and / partitions. Partition wisely and generously.
As a rule of thumb, the swap partition should be about double the size of system memory (RAM). For example, if the machine has 128 megabytes of memory, the swap file should be 256 megabytes. Systems with less memory may perform better with more swap. Less than 256 megabytes of swap is not recommended and memory expansion should be considered. 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 and might create issues later if more memory is added.
On larger systems with multiple SCSI disks (or multiple IDE disks operating on different controllers), it is recommend that a swap is configured on each drive (up to four drives). The swap partitions 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 disks. Large swap sizes are fine, even if swap is not used much. It might be easier to recover from a runaway program before being forced to reboot.
Several users think a single large partition will be fine, but there are several reasons why this is a bad idea. First, each partition has different operational characteristics and separating them allows the file system to tune accordingly. For example, the root and /usr partitions are read-mostly, without much writing. While a lot of reading and writing could occur in /var and /var/tmp.
By properly partitioning a system, fragmentation introduced in the smaller write heavy partitions will not bleed over into the mostly-read partitions. Keeping the write-loaded partitions closer to the disk's edge, will increase I/O performance in the partitions where it occurs the most. Now while I/O performance in the larger partitions may be needed, shifting them more toward the edge of the disk will not lead to a significant performance improvement over moving /var to the edge. Finally, there are safety concerns. A smaller, neater root partition which is mostly read-only has a greater chance of surviving a bad crash.