disklabel - disk pack label
#include <sys/disklabel.h>
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 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(2),
which is done as needed by the disklabel(8) 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 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. The 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 DIOCRLDINFO 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 disklabel(8).
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 fdisk(8)
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 nonOpenBSD 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.
disktab(5), disklabel(8)
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.
OpenBSD 3.6 August 6, 2001
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