Description

Lziprecover is a data recovery tool and decompressor for files in the lzip
compressed data format (.lz). Lziprecover is able to repair slightly damaged
files, produce a correct file by merging the good parts of two or more
damaged copies, reproduce a missing (zeroed) sector using a reference file,
extract data from damaged files, decompress files, and test integrity of
files.

Lziprecover can remove the damaged members from multimember files, for
example multimember tar.lz archives.

Lziprecover provides random access to the data in multimember files; it only
decompresses the members containing the desired data.

Lziprecover facilitates the management of metadata stored as trailing data
in lzip files.

Lziprecover is not a replacement for regular backups, but a last line of
defense for the case where the backups are also damaged.

The lzip file format is designed for data sharing and long-term archiving,
taking into account both data integrity and decoder availability:

   * The lzip format provides very safe integrity checking and some data
     recovery means. The program lziprecover can repair bit flip errors
     (one of the most common forms of data corruption) in lzip files, and
     provides data recovery capabilities, including error-checked merging
     of damaged copies of a file.

   * The lzip format is as simple as possible (but not simpler). The lzip
     manual provides the source code of a simple decompressor along with a
     detailed explanation of how it works, so that with the only help of the
     lzip manual it would be possible for a digital archaeologist to extract
     the data from a lzip file long after quantum computers eventually
     render LZMA obsolete.

   * Additionally the lzip reference implementation is copylefted, which
     guarantees that it will remain free forever.

A nice feature of the lzip format is that a corrupt byte is easier to repair
the nearer it is from the beginning of the file. Therefore, with the help of
lziprecover, losing an entire archive just because of a corrupt byte near
the beginning is a thing of the past.

Compression may be good for long-term archiving. For compressible data,
multiple compressed copies may provide redundancy in a more useful form and
may have a better chance of surviving intact than one uncompressed copy
using the same amount of storage space. This is specially true if the format
provides recovery capabilities like those of lziprecover, which is able to
find and combine the good parts of several damaged copies.

Lziprecover is able to recover or decompress files produced by any of the
compressors in the lzip family; lzip, plzip, minilzip/lzlib, clzip, and
pdlzip.

If the cause of file corruption is a damaged medium, the combination
GNU ddrescue + lziprecover is the recommended option for recovering data
from damaged lzip files.

If a file is too damaged for lziprecover to repair it, all the recoverable
data in all members of the file can be extracted in one step with the
command 'lziprecover -cd -i file.lz > file'.

When recovering data, lziprecover takes as arguments the names of the
damaged files and writes zero or more recovered files depending on the
operation selected and whether the recovery succeeded or not. The damaged
files themselves are kept unchanged.

When decompressing or testing file integrity, lziprecover behaves like lzip
or lunzip.

To give you an idea of its possibilities, when merging two copies, each of
them with one damaged area affecting 1 percent of the copy, the probability
of obtaining a correct file is about 98 percent. With three such copies the
probability rises to 99.97 percent. For large files (a few MB) with small
errors (one sector damaged per copy), the probability approaches 100 percent
even with only two copies. (Supposing that the errors are randomly located
inside each copy).

The lziprecover package also includes unzcrash, a program written to test
robustness to decompression of corrupted data, inspired by unzcrash.c from
Julian Seward's bzip2. Type 'make unzcrash' in the lziprecover source
directory to build it. Then try 'unzcrash --help'.


Copyright (C) 2009-2021 Antonio Diaz Diaz.

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

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