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9.6 Using Multiple Tapes

Often you might want to write a large archive, one larger than will fit on the actual tape you are using. In such a case, you can run multiple tar commands, but this can be inconvenient, particularly if you are using options like ‘--exclude=pattern’ or dumping entire file systems. Therefore, tar provides a special mode for creating multi-volume archives.

Multi-volume archive is a single tar archive, stored on several media volumes of fixed size. Although in this section we will often call ‘volume’ a tape, there is absolutely no requirement for multi-volume archives to be stored on tapes. Instead, they can use whatever media type the user finds convenient, they can even be located on files.

When creating a multi-volume archive, GNU tar continues to fill current volume until it runs out of space, then it switches to next volume (usually the operator is queried to replace the tape on this point), and continues working on the new volume. This operation continues until all requested files are dumped. If GNU tar detects end of media while dumping a file, such a file is archived in split form. Some very big files can even be split across several volumes.

Each volume is itself a valid GNU tar archive, so it can be read without any special options. Consequently any file member residing entirely on one volume can be extracted or otherwise operated upon without needing the other volume. Sure enough, to extract a split member you would need all volumes its parts reside on.

Multi-volume archives suffer from several limitations. In particular, they cannot be compressed.

GNU tar is able to create multi-volume archives of two formats (see section Controlling the Archive Format): ‘GNU’ and ‘POSIX’.

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