Create a new OpenBSD partition from unused space

The default OpenBSD partition layout uses a pre-defined template. If you have a disk more than 356 GB you will have unused space with the default layout (346 GB before 6.4).

It’s possible to create a new partition to use that space if you did not modify the default layout at installation. You only need to start disklabel with flag -E* and type a to add a partition, default will use all remaining space for the partition.

# disklabel -E sd0
Label editor (enter '?' for help at any prompt)
> a
partition: [m]
offset: [741349952]
size: [258863586]
FS type: [4.2BSD]
> w
> q
No label changes.

The new partition here is m. We can format it with:

# newfs /dev/rsd0m

Then, you should add it to your /etc/fstab, for that, use the same uuid as for other partitions, it would look something like 52fdd1ce48744600

52fdd1ce48744600.e /data ffs rw,nodev,nosuid 1 2

It will be auto mounted at boot, you only need to create the folder /data. Now you can do

# mkdir /data
# mount /data

and /data is usable right now.

You can read disklabel(8) and newfs for more informations.