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 formatted,
and may be changed later with the disklabel 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 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 system
updates the in-core copy of the label if it contains
incomplete information 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
containing 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 modification 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
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