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SpanTape can be used as a replacement for dd(1) to make backup using multiple SCSI tapes, giving the possibility to change tapes on the fly, even using only single tape drive (without tape changer).

spantape should be used exclusively with SCSI tape drives - it uses some st(4) ioctls to determine tape block mode and block size and checks for tape-specific EOF/EOD indicators.

The data structure of spantape backup typically looks like this:

Each tape containing part of spantape backup begins with backup ID tag. Backup ID tag consists of backup ID (alphanumeric string) and successive tape number - using tags helps to keep tapes ordered while restoring.

Backup ID tag is followed by stored data blocks, terminated by either EOD (== two consecutive EOF marks), or EOF followed by End-Of-Backup indication block. End-Of-Backup block consist of the same backup ID that is placed in backup ID tag block, and "EOD" keyword instead of tape number.

If one disables the usage of backup ID and end-of-backup tags, spantape uses only EOF/EOD marks to check for end of tape/backup, but doesn't check if tapes restored are the tapes belonging to same backup, or even if they are restored in right order.

Note about tape block sizes:
tape drive can operate in either fixed or variable blocksize mode. For fixed blocksize mode, the st driver must be configured (e.g. using stinit(8)). spantape can then use set blocksize automatically.

op: \ mode: variable blocksize fixed blocksize

read            must be >= than         read(2) length should
                longest block on        be equal to multiple
                the tape read.          block size
                read(2) with too
                small blocksize
                returns ENOMEM.
                (spantape translates
                this to more verbose
                error message ;))

write           can be of any size,     write(2) size should be multiple
                but better use bigger   length of block size. Spantape
                sizes (like 64k)        pads the last block with zeros.
                - interblock spaces
                are quite large.