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In the BSD computer operating systems (including NetBSD, OpenBSD, and FreeBSD) and in related operating systems such as SunOS, a disklabel is a record stored on a data storage device such as a hard disc that contains information about the location of the partitions on the disc.
Where disklabels are stored
Traditionally, the disklabel was the first sector of the disk. But this system only works when the only operating systems that access the disc are Unix systems that comprehend disklabels. In the world of IBM PC compatibles, discs are usully partitioned with the MBR Partition Table scheme instead, and the BSD partitioning scheme is nested within a single, primary, MBR partition (just as the "extended" partitioning scheme is nested within a single primary partition with Extended Master Boot Records). In BSD parlance, the primary MBR partitions are referred to as "disc slices" and the subdivisions of a primary MBR partition (for the nested BSD partitioning scheme) that are described by its disklabel are called "partitions". The BSD disklabel is contained within the Volume Boot Record of the primary MBR partition.
The MBR partition IDs for primary partitions that are subdivided using BSD disklabels are 0xA5 (386BSD and FreeBSD), 0xA6 (OpenBSD), and 0xA9 (NetBSD).
The contents of disklabels
BSD disklabels contain 16 entries for describing (BSD) partitions. These are, by convention, named with letters of the alphabet, 'a' through to 'p'.
Also by convention, partitions 'a', 'b', and 'c' have fixed meanings:
- Partition 'a' is the "root" partition, the volume from which the operating system is bootstrapped. The boot code in the Volume Boot Record containing the disklabel is thus simplified, as it need only look in one fixed location to find the location of the boot volume.
- Partition 'b' is the "swap" partition.
- Partition 'c' overlaps all of the other partitions and describes the entire BSD-partitioned area of the disc. Its start and length are fixed. As such, BSD disklabels can, in practice, only describe 15 partititions.
References
Further reading
"BSD disklabel"