disk pack label
Each disk or disk pack on a system may contain a disk label which provides detailed information about the geometry of the disk and the partitions into which the disk is divided. It should be initialized when the disk is formatted, and may be changed later with the disklabel(8) program. This information is used by the system disk driver and by the bootstrap program to determine how to program the drive and where to find the filesystems on the disk partitions. Additional information is used by the filesystem in order to use the disk most efficiently and to locate important filesystem information. The description of each partition contains an identifier for the partition type (standard filesystem, swap area, etc.). The filesystem updates the in-core copy of the label if it contains incomplete information about the filesystem.
The label is located in sector number
LABELSECTOR of the drive, usually sector 0 where it
may be found without any information about the disk geometry. It is at an
LABELOFFSET from the beginning of the sector,
to allow room for the initial bootstrap.
A copy of the in-core label for a disk can be obtained with the
DIOCGDINFO ioctl; this works with
a file descriptor for a block or character (“raw”) device for
any partition of the disk. The in-core copy of the label is set by the
DIOCSDINFO ioctl. The offset of a
partition cannot generally be changed while it is open, nor can it be made
smaller while it is open. One exception is that any change is allowed if no
label was found on the disk, and the driver was able to construct only a
skeletal label without partition information. The
DIOCWDINFO ioctl operation sets
the in-core label and then updates the on-disk label; there must be an
existing label on the disk for this operation to succeed. Thus, the initial
label for a disk or disk pack must be installed by writing to the raw disk.
DIOCGPDINFO ioctl operation
gets the default label for a disk. This simulates the case where there is no
physical label on the disk itself and can be used to see the label the
kernel would construct in that case. The
ioctl operation causes the kernel to update its copy of
the label based on the physical label on the disk. It can be used when the
on-disk version of the label was changed directly or, if there is no
physical label, to update the kernel's skeletal label if some variable
affecting label generation has changed (e.g. the fdisk partition table). All
of these operations are normally done using
Note that when a disk has no real BSD
disklabel the kernel creates a default label so that the disk can be used.
This default label will include other partitions found on the disk if they
are supported on your architecture. For example, on systems that support
partitions the default label will also include DOS and Linux partitions.
However, these entries are not dynamic, they are fixed at the time
disklabel(8) is run. That means that subsequent changes that affect
non-OpenBSD partitions will not be present in the
default label, though you may update them by hand. To see the default label,
run disklabel(8) with the
-d flag. You can then
run disklabel(8) with the
-e flag and paste any
entries you want from the default label into the real one.
disklabel only supports up to a maximum of
15 partitions, ‘a’ through ‘p’, excluding
‘c’. The ‘c’ partition is reserved for the
entire physical disk. By convention, the ‘a’ partition of the
boot disk is the root partition, and the ‘b’ partition of the
boot disk is the swap partition, but all other letters can be used in any
order for any other partitions as desired.