DISKLABEL(5)              OpenBSD Programmer's Manual             DISKLABEL(5)

NAME
     disklabel - disk pack label

SYNOPSIS
     #include <sys/disklabel.h>

DESCRIPTION
     Each disk or disk pack on a system may contain a disk label which pro-
     vides detailed information about the geometry of the disk and the parti-
     tions into which the disk is divided.  It should be initialized when the
     disk is formatted, and may be changed later with the disklabel(8) pro-
     gram.  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 con-
     tains 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 ge-
     ometry.  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 overwrit-
     ten 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 char-
     acter (``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 la-
     bel was found on the disk, and the driver was able to construct only a
     skeletal label without partition information.  The DIOCWDINFO ioctl oper-
     ation 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 de-
     fault label for a disk.  This simulates the case where there is no physi-
     cal 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 skele-
     tal 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 de-
     fault label so that the disk can be used.  This default label will in-
     clude 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 parti-
     tions 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 en-
     tries you want from the default label into the real one.

SEE ALSO
     disktab(5), disklabel(8)

CAVEATS
     disklabel only supports up to a maximum of 15 partitions, `a' through
     `p', excluding `c'.  The `c' partition is reserved for the entire physi-
     cal 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 4.7                      May 31, 2007                                2
