disklabel - 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 for-
matted, and may be changed later with the disklabel program. This informa-
tion is used by the system disk driver and by the bootstrap program to
determine how to program the drive and where to find the file systems on
the disk partitions. Additional information is used by the file system in
order to use the disk most efficiently and to locate important file system
information. The description of each partition contains an identifier for
the partition type (standard file system, swap area, etc.). The file sys-
tem updates the in-core copy of the label if it contains incomplete infor-
mation about the file system.
The label is located in sector number LABELSECTOR of the drive, usually
sector 0 (zero) where it may be found without any information about the
disk geometry. It is at an offset LABELOFFSET from the beginning of the
sector, to allow room for the initial bootstrap. The disk sector contain-
ing the label is normally made read-only so that it is not accidentally
overwritten by pack-to-pack copies or swap operations; the DIOCWLABEL
ioctl, which is done as needed by the disklabel program, allows modifica-
tion of the label sector.
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, nor 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.
Finally, 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. All of these operations are
normally done using the disklabel program.