Unix dump/restore tape format

From Computer History Wiki
Revision as of 12:07, 7 September 2022 by Larsbrinkhoff (talk | contribs) (TAPE HEADER: Int is 16 or 32 bits.)
Jump to: navigation, search

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 UFS 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