Hitchhiker's Guide to Openbsd

14.5.2 - Swapping to a partition
On OpenBSD, the 'b' partition of the boot drive is used by default and automatically for swap. No configuration is 
needed for this to take place. If you do not wish to use swap on the boot disk, do not define a "b" partition. If you 
wish to use swap on other partitions or on other disks, you need to define these partitions in 
/etc/fstab
with 
lines something like: 
/dev/sd3b none swap sw 0 0
/dev/sd3d none swap sw 0 0
14.5.3 - Swapping to a file
(Note: if you are looking to swap to a file because you are getting "virtual memory exhausted" errors, you should 
try raising the per-process limits first with 
csh(1)
's unlimit, or 
sh(1)
's ulimit.) 
Sometimes, your initial guess about how much swap you need proves to be wrong, and you have to add additional 
swap space, occasionally in a hurry (as in, "Geez, at the rate it is burning swap, we'll be wedged in five minutes"). 
If you find yourself in this position, adding swap space as a file on an existing file system can be a quick fix. 
The file must not reside on a filesystem which has SoftUpdates enabled (they are disabled by default). To start 
out, you can see how much swap you currently have and how much you are using with the 
swapctl(8)
utility. You 
can do this by using the command: 
$ swapctl -l
Device 512-blocks Used Avail Capacity Priority
swap_device 65520 8 65512 0% 0
This shows the devices currently being used for swapping and their current statistics. In the above example there 
is only one device named "swap_device". This is the predefined area on disk that is used for swapping. (Shows up 
as partition b when viewing disklabels) As you can also see in the above example, that device isn't getting much 
use at the moment, but for the purposes of this document, we will act as if an extra 32M is needed. 
The first step to setting up a file as a swap device is to create the file. It's best to do this with the 
dd(1)
 utility. 
Here is an example of creating the file /var/swap that is 32M in size. 
$ sudo dd if=/dev/zero of=/var/swap bs=1k count=32768
32768+0 records in
32768+0 records out
33554432 bytes transferred in 20 secs (1677721 bytes/sec)
http://www.openbsd.org/faq/faq14.html (11 of 34)9/4/2011 10:02:25 AM

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14 - Disk Setup
Once this has been done, we can turn on swapping to that device. Use the following command to turn on 
swapping to this device 
$ sudo chmod 600 /var/swap
$ sudo swapctl -a /var/swap
Now we need to check to see if it has been correctly added to the list of our swap devices. 
$ swapctl -l
Device 512-blocks Used Avail Capacity Priority
swap_device 65520 8 65512 0% 0
/var/swap 65536 0 65536 0% 0
Total 131056 8 131048 0%
Now that the file is setup and swapping is being done, you need to add a line to your /etc/fstab file so that this file 
is configured on the next boot time also. If this line is not added, your won't have this swap device configured. 
$ cat /etc/fstab
/dev/wd0a / ffs rw 1 1
/var/swap /var/swap swap sw 0 0