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.
