Unix dump/restore tape format

The Unix dump command makes a full or incremental backup of a single disk partition to magnetic tapes. The corresponding restore command recovers data from such tapes. The tape format is vaguely documented in various man pages, but not to a detail necessary to fully understand it.

A dump is logically a series of 1024-byte blocks, grouped ten at a time into tape records. A block can be a header, or data. Header blocks hold metadata and inode information.

Dumps will normally span several tapes. All tapes begin with a TS_TAPE block. The last tape ends with one or several TS_END blocks. The first tape will have TS_BITS and TS_CLRI blocks after TS_TAPE. The rest of the tape blocks are of type TS_INODE, storing raw inodes as stored on disk, followed by data blocks. In some cases a TS_INODE header can not store all information needed, in which case it can be extended with TS_ADDR blocks. Normally the first inodes written to tape is the entire directory structure of the whole file system, and file inodes make up the rest of the tapes.

The tape format comes in two versions. The first version is for the older UNIX file system, and the second version is for Berkeley's FFS file system. On top of that, data can be 16 or 32 bits wide, and big or little endian.

Details to follow. It's a bit of a mess.

TAPE HEADER

An int is 16 or 32 bits depending on the host.

Name	Description	Size	Value (old, new format)
Type	Header type	int	TS_TAPE, TS_BITS, TS_CLRI, TS_INODE, TS_ADDR, TS_END
Date	Date of dump	32 bits	Timestamp
DDate	Dump from	32 bits	Timestamp
Volume	Tape number	int	Tape in dump, from 1
Tapea	Block number	32 bits	Block in dump across tapes, from 1
Inode	Inode number	int	File system inode number
Magic	Format identifier	int	60011	60012
Inode	As stored on disk	64 or 128 bytes
Count	Number of addr bytes	int
Addr		array of 8 bits	0 or 1 depending on whether data block is present