Hitchhiker's Guide to Openbsd

Integrity: If one filesystem is corrupted for some reason then your other filesystems are most likely still OK. 
●
Size: Many machines have limits on the area of a disk where the boot ROM can load the kernel from. In some 
cases, this limit may be very small (504M for an older 486), in other cases, a much larger limit (for example, 2G, 
8G, or 128G on i386 systems). As the kernel can end up anywhere within the root partition, the entire root 
partition should be within this area. For more details, see 
this section
. A good guideline might be to keep your / 
partition completely below 2G, unless you know your platform (and particular machine) can handle more (or less) 
than that. 
●
Read-only: You can mount partitions that you never or rarely need to write to as "Read-only" most of the time, 
which will eliminate the need for fsck(8) after a crash or power interruption, and may help prevent unintended 
data alteration. 
●
fsck(8): Very large partitions require more RAM to fsck(8), and on small-memory systems, you can end up 
having to use swap, resulting in very long fsck times. 
Given sufficient disk space, OpenBSD's installer will default to the following partitions: 
●
/ - root: In addition to being where the other file systems are mounted, the root file system holds all the files 
needed to boot OpenBSD. This includes the kernel, plus the basic utilities in 
/sbin
and 
/bin
, the configuration 
files in 
/etc
, and the device directory, 
/dev
which are all used to bring up the system. The root file system can 
be as small as 60M, though 100M to 200M is easier for a machine that will last through many upgrade cycles. The 
'a' partition of your boot drive becomes your root partition automatically. SOME platforms place restrictions on 
the physical location on the disk (i.e., must be at start of disk) in order to boot. 
●
Swap: In addition to swap, this partition is also used for storing core dumps after system crashes, so it is 
suggested that the swap space (if set up at all) be bigger than the largest amount of RAM you are likely to ever 
install on the machine. Read more about this in 
FAQ 14, Swap
. 
●
/tmp: This is a world-writeable directory used for (as the name implies!) temporary storage. Most systems can get 
by with very modest amounts of storage here, 50M is usually many times what you should ever need, though there 
are a few applications which can use much, much more. While this directory is world-writable, when it is a 
separate partition, OpenBSD defaults to mounting it nodev and nosuid, which minimizes how it can be used to 
abuse your system Note that this directory is cleared on reboot, and files left untouched over 24 hours are cleared 
nightly. 
●
/var: This directory and mount point is used for a LOT of things, and depending on your uses, may be a prime 
candidate to subdivide into more partitions. Some of the things that end up here (and potential additional mount 
points): 
❍
/var/log
: System logs. 
❍
/var/mail
: Incoming mail boxes. 
❍
/var/spool
: Outgoing mail (and other things) 
❍
/var/www
: OpenBSD's 
web server
 lives here. 
❍
/var/tmp
: This is a "persistent" temporary file directory, as files placed here are NOT purged on reboot. 
http://www.openbsd.org/faq/faq4.html (30 of 43)9/4/2011 10:01:58 AM

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4 - OpenBSD 4.9 Installation Guide
For example, vi(1) uses this directory for temporary storage, so if your computer crashes or is rebooted 
while you are editing a file, the files here can be used to recover your editing session. Files left here over 
24 hours though will be purged by the nightly cleanup scripts, 
daily(8)
. 
❍
/var/crash
: If the system panics, it will attempt to save a core dump in the swap partition before 
rebooting. This core dump will then be saved to 
/var/crash
upon reboot, so 
/var
will need at least as 
much free space as the system has RAM for this to work automatically. 
●
/usr: This is where most of OpenBSD resides. Program binaries, libraries, documentation, manual pages, etc. are 
all located in the 
/usr
directory. The files in this mount point are relatively unchanging -- in many cases, you 
could easily mount the 
/usr
partition read-only with no other system changes until your next upgrade or update. 
●