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.

TAPE HEADER

This data is common to all headers, but some of the fields may be valid or ignored depending on the header type.

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

The checksum is a plain two's complement sum of all ints in the 1024-byte heder block.

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 all tapes, from 1
Inumber	Inode number	int	File system inode number
Magic	Format identifier	int	60011	60012
Checksum	Checksum	int	Filled in to make sum 84446 (modulo int)
Inode	Inode data	64 or 128 bytes	As stored on disk
Count	Number of addr bytes or data blocks	int
Addr	Data present	Array of bytes	0 or 1

The inode data looks like this:

Name	Old Size	New Size
Mode	16 bits	16 bits
Nlink	16 bits	16 bits
Uid	16 bits	16 bits
Gid	16 bits	16 bits
Size	32 bits	64 bits
Addr	40 bytes	Not present
Atime	32 bits	32 bits
?	Not present	32 bits
Mtime	32 bits	32 bits
?	Not present	32 bits
Ctime	32 bits	32 bits
?	Not present	92 bytes

TS_TAPE

Header type 1. This header must start all tapes. The Date field must be same on all tapes. The Volume field identifies the individual tape in the dump.

TS_BITS and TS_CLRI

Header types 3 and 6. Stores bit arrays from the file system. The Count field specifies the number of data blocks following the header block.

TS_INODE and TS_ADDR

Header types 2 and 4. TS_INODE stores the inode data from disk. After the header block comes data blocks, as per the Addr field. A one signals a data block is present on tape. A zero indicates a hole in the file.

If the TS_INODE Addr field can't accommodate the file, several TS_ADDR may follow. TS_ADDR stores the same inode data, but following data block are appended to the file.

TS_END

Header type 5. This header signals the end of the dump on the last tape. Several TS_END blocks may be written.