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
DIOCWLABELioctl(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
DIOCGDINFOioctl;
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
DIOCSDINFOioctl.
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
DIOCWDINFOioctl
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
DIOCGPDINFOioctl
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
DIOCRLDINFOioctl
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-AerieBSD
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.
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.