DISKLABEL(5) OpenBSD Programmer's Manual DISKLABEL(5)NAMEdisklabel - disk pack label
SYNOPSIS
#include <sys/disklabel.h>
DESCRIPTION
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 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.
SEE ALSOdisktab(5), disklabel(8)CAVEATSdisklabel 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 4.9 May 31, 2007 OpenBSD 4.9