tarlz/README

Description

Tarlz is a small and simple implementation of the tar archiver. By default
tarlz creates, lists and extracts archives in a simplified posix pax format
compressed with lzip on a per file basis. Each tar member is compressed in
its own lzip member, as well as the end-of-file blocks. This method is fully
backward compatible with standard tar tools like GNU tar, which treat the
resulting multimember tar.lz archive like any other tar.lz archive. Tarlz
can append files to the end of such compressed archives.

Tarlz can create tar archives with four levels of compression
granularity; per file, per directory, appendable solid, and solid.

Of course, compressing each file (or each directory) individually is
less efficient than compressing the whole tar archive, but it has the
following advantages:

   * The resulting multimember tar.lz archive can be decompressed in
     parallel with plzip, multiplying the decompression speed.

   * New members can be appended to the archive (by removing the EOF
     member) just like to an uncompressed tar archive.

   * It is a safe posix-style backup format. In case of corruption,
     tarlz can extract all the undamaged members from the tar.lz
     archive, skipping over the damaged members, just like the standard
     (uncompressed) tar. Moreover, the option '--keep-damaged' can be
     used to recover as much data as possible from each damaged member,
     and lziprecover can be used to recover some of the damaged members.

   * A multimember tar.lz archive is usually smaller than the
     corresponding solidly compressed tar.gz archive, except when
     individually compressing files smaller than about 32 KiB.

Note that the posix pax format has a serious flaw. The metadata stored
in pax extended records are not protected by any kind of check sequence.
Corruption in a long filename may cause the extraction of the file in the
wrong place without warning. Corruption in a long file size may cause the
truncation of the file or the appending of garbage to the file, both
followed by a spurious warning about a corrupt header far from the place
of the undetected corruption.

Metadata like filename and file size must be always protected in an archive
format because of the adverse effects of undetected corruption in them,
potentially much worse that undetected corruption in the data. Even more so
in the case of pax because the amount of metadata it stores is potentially
large, making undetected corruption more probable.

Because of the above, tarlz protects the extended records with a CRC in
a way compatible with standard tar tools.

Tarlz does not understand other tar formats like gnu, oldgnu, star or v7.

Tarlz is intended as a showcase project for the maintainers of real tar
programs to evaluate the format and perhaps implement it in their tools.

The diagram below shows the correspondence between each tar member
(formed by one or two headers plus optional data) in the tar archive and
each lzip member in the resulting multimember tar.lz archive:

tar
+========+======+=================+===============+========+======+========+
| header | data | extended header | extended data | header | data |   EOF  |
+========+======+=================+===============+========+======+========+

tar.lz
+===============+=================================================+========+
|     member    |                      member                     | member |
+===============+=================================================+========+


Copyright (C) 2013-2018 Antonio Diaz Diaz.

This file is free documentation: you have unlimited permission to copy,
distribute and modify it.

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