Partitioning
Partitions are logical subsections of a hard drive. OpenBSD can handle different partitions with their own unique privileges. You might make some partitions read-only so that files on them cannot be added, moved, or changed.
OpenBSD might refuse to run programs on a specified partition, and it knows that device nodes should appear only on certain partitions. User files should not have setuid or setgid permissions, so the operating system won’t recognize those privileges on files on the user data partition. While many operating systems support these sorts of privilege controls, OpenBSD uses them by default.
The most difficult part of installing OpenBSD is partitioning. When you don’t know how partitions work, choosing partitioning can be troublesome.
If you’re familiar with other Unix-like operating systems (such as some distributions of Linux), you might be accustomed to using a single large root partition and putting everything on it. This is a bad idea for several reasons. OpenBSD uses partitions as a security tool. A single large partition eliminates per-partition security and privileges. With your log files safely contained on one partition, a process or user gone amok cannot fill your entire drive. While it could fill a partition, you could still create and edit files on other partitions, giving you the flexibility you need to address the problem.
Unlike many installers that have fancy menus and graphic tools, OpenBSD’s installer expects you to know how to use low-level disk management tools such as disklabel(8). Unlike with those operating systems, however, OpenBSD can be installed in a much wider variety of ways on a wider variety of systems, all with a single installer.
If this is your first OpenBSD installation, use the default partitioning offered by the installer. OpenBSD will provide all its standard partitions, but adjust their sizes based on the size of your disk. The discussion here is based on a standard i386 installation on a fairly small disk.
If you’ve previously installed OpenBSD and you’re installing it on a special-purpose machine, you might want special partitioning. In that case, get a piece of paper and a pencil, and write down the size of your hard disk, each partition you need, and each partition’s desired size. Your special-purpose OpenBSD machine should almost certainly have all the same partitions as a default installation, but their sizes will differ. A web server has very different disk space requirements than a desktop machine, which in turn has different requirements than those of a firewall.
If you have a large disk, leave some space unallocated. Having partitions the size you need accelerates filesystem integrity checks; fsck(8) doesn’t spend cycles integrity-checking unused disk space. On solid-state disks, unused space gives wear-leveling algorithms more cells to play with, increasing the life span of the disk and decreasing the odds of failure. It’s better to have spare disk space you never need than to need disk space you don’t have.
Standard OpenBSD Partitions
The standard OpenBSD partitions are / (root), swap space, /tmp, /var, /usr, /usr/X11R6, /usr/local, /usr/src, /usr/obj, and /home. If you create a custom layout and don’t include one of these partitions, the installer will put files that go into that partition into either your root or /usr partition, quickly filling them. If you want to create a partition after installation, you must find space on your disk for it. Unless you left unallocated space on your disk, you’re better off reinstalling the whole system.
Root Partition
The root partition holds the main OpenBSD configuration files and the most essential software needed to get the computer into single-user mode and on the network. Your system needs fast access to the root filesystem, so if you have multiple disks, put the root partition on the fastest (or smallest) one.
The root partition is the only one whose placement on disk is vitally important. Over the years, i386 systems have been repeatedly expanded to surpass their own limits—they’re based on an architecture that could originally handle only up to 640KB of RAM, after all! All modern operating system kernels work around these limits in a manner mostly transparent to users, but when the system is first booting, you’re trapped within the hardware’s limits.
Many old i386 systems have limits on hard drive size. They only recognize 128GB drives, 2TB drives, or some other number. The hardware BIOS cannot access anything beyond that limit. If you’re using a computer that has a 128GB limit on hard drive size, and you put the kernel somewhere beyond the first 128GB of disk space, the computer will be unable to find the kernel and thus unable to boot the system. Check your hardware manual before you get started. If the manual refers to a disk size limit, your entire root partition must fit within that limit.
If you violate this limit, your system will probably appear to work. The second you change the file /bsd, however, it’s likely that your computer will refuse to boot. Save yourself much pain by putting the root partition first on the disk, and making sure it’s small enough to fit within the hardware’s limits.
Swap Space
Swap space is used for virtual memory. When your computer runs low on RAM, it starts to move information that has been sitting idle in memory into swap space. When the computer needs that information, it’s loaded from virtual memory into real memory. This isn’t necessarily bad for performance. Many programs spend the vast majority of their time executing only a small fraction of their code. OpenBSD is pretty good about figuring out which sections of memory can be moved into swap space and which are used too frequently to be swapped. If things go well, your computer will almost never need swap space.
OpenBSD also uses swap space during system failures. If the kernel panics, the computer writes the contents of system memory to the swap partition. This means that the swap partition must be, at its smallest, slightly larger than the amount of physical RAM in the system.
How much swap space do you need? The short answer is, “It depends on the system.” OpenBSD defaults to allocating twice as much swap space as you have physical RAM. This isn’t a bad rule, as long as you understand it’s very general. A swap space three or four times the size of your physical memory won’t hurt. If your computer uses more swap space than that, it’s overloaded and will perform poorly.
If you find yourself using swap space often, consider increasing your physical memory instead. RAM is cheap.
Also consider future upgrades. If your system has 2GB of RAM when you install OpenBSD, but you intend to increase that to 8GB, assigning 16GB of swap space is a good idea. Adding a swap partition later is difficult, unless you leave unallocated disk space when you install the software. (Note that, while you can swap to a file, OpenBSD can write only crash dumps to an actual swap partition.)
/tmp Directory
The /tmp directory is temporary space for all users on the system. Space requirements for /tmp are generally a matter of opinion—after all, you can always use a chunk of space in your /home directory for scratch space. Automated software installers frequently extract files into /tmp. I usually recommend at least 3GB in /tmp, but I do horrible things to my temp space. Many people use a /tmp directory of 256MB or 512MB and get along just fine.
/var Partition
The /var partition contains frequently changing data, such as logs, databases, mail spools, temporary run files, websites, and so on. OpenBSD allocates about 5GB to /var by default. This should be plenty for an educational installation. If you’re building a web, database, or logging server, however, /var should get the majority of your disk space. If you’re on a really tiny system, you could use as little as 10MB for /var.
/usr Partition
The /usr partition holds the operating system programs, compilers, libraries, and add-on programs. The majority of /usr changes only when you upgrade your system. OpenBSD assigns /usr 2GB by default, which is more than sufficient, even on a desktop system.
/usr/X11R6 Partition
The /usr/X11R6 partition contains the X Window System programs and documentation. OpenBSD does package software linked against the X Window System, and a lot of software you might expect to find on servers (such as ImageMagick) requires X libraries.
If you are not going to install any X software, and plan to build all your own software without X, you don’t need this partition. If you’re in doubt, or if this is your first installation, keep this partition.
/usr/local Partition
The /usr/local partition contains add-on OpenBSD software, usually from packages (see Chapter 13). This can be much larger than the /usr partition containing the core OpenBSD software. OpenBSD allocates 5GB of disk space to /usr/local by default, and I’ve never needed more than that.
/usr/src Partition
The /usr/src partition is dedicated to the OpenBSD source code. On a dedicated-purpose machine that doesn’t have a compiler, such as a firewall or a secure web server, you probably don’t need a local copy of the source code. If you don’t plan to upgrade this machine from source code, and you don’t plan to use the source code as a reference on the local machine, you don’t need this partition. If you’re in doubt, keep it.
/usr/obj Partition
The /usr/obj partition is where OpenBSD builds new versions of the operating system and Xenocara. The files in here are temporary; once you’ve installed a new OpenBSD version, you don’t need these files any longer. Creating a new filesystem is faster than erasing the individual files in this kind of filesystem, so /usr/obj is configured as its own partition.
If you don’t intend to build a new OpenBSD from source code, you don’t need /usr/obj. If you find that you do need this partition later, you can either create it from unused space or mount it via NFS.
/home Partition
The /home partition can be described as “everything else.” User directories go into /home, as well as any random data that’s meant for users. The family MP3 and photo collections should go in /home, as well as your personal source code, email, and anything else you want to keep.
Creating Other Partitions
OpenBSD supports up to 16 partitions per disk. If you want other partitions, you can create them using the installer. Does your company have a policy that all add-on software must go in /opt or /usr/companyname? Fine, create that partition. The OpenBSD standards are not a straitjacket, but rather a starting point. You own the system. Make it behave according to your needs.