BSD disklabel

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A disklabel is an on-disk map used by BSD-derived systems (such as NetBSD, OpenBSD, FreeBSD, and DragonFly BSD) and some related systems like SunOS. It stores information about where the disk’s partitions are located. Disklabels were introduced with 4.3BSD-Tahoe and are usually edited with the disklabel tool. In FreeBSD, the tool was later renamed to bsdlabel and was deprecated in version 15.0.

Early Unix kept partitions fixed inside device drivers, which could overlap. This worked when only a few drives existed, but as standard disk interfaces and many third‑party drives appeared, administrators often had to recompile the kernel to support different partition layouts—an inconvenient process for both users and vendors.

For 4.3-Tahoe, Berkeley implemented a new on‑disk partitioning scheme managed by the disklabel tool. These on‑disk maps were already used by other systems; Berkeley adopted its own format for the BSD world.

In IBM PC-compatible systems, BSD partitions live inside a primary MBR partition. The MBR partition that holds BSD partitions uses one of these IDs: A5h (386BSD and FreeBSD), A6h (OpenBSD), A9h (NetBSD), or 6Ch (DragonFly BSD). The BSD disklabel itself sits in the volume boot record of that primary MBR partition. A drive can contain both BSD disklabel partitions and MS‑DOS/Windows extended partitions, in separate primary partitions, and many BSD systems can access both types.

Traditionally, a disklabel describes up to eight partitions, labeled a through h; some variants support up to 16 partitions, labeled a through p. Partitions a, b, and c have fixed traditional meanings.