Parity archive
| Filename extension | .par, .par2, .p??, (.par3 future) |
|---|---|
| Type of format | Erasure code |
Parchive (a portmanteau of parity archive, and formally known as Parity Volume Set Specification) is an erasure code system that produces par files for checksum verification of data integrity, with the capability to perform data recovery operations that can repair or regenerate corrupted or missing data.
Parchive was originally written to solve the problem of reliable file sharing on Usenet, but it is now commonly used for protecting any kind of data from data corruption, disc rot, bit rot, and accidental or malicious damage. Despite the name, Parchive uses more advanced techniques that do not use simplistic parity methods of error detection and correction.
As of 2014, PAR1 is obsolete, PAR2 is mature for widespread use, and PAR3 is an experimental version being developed by MultiPar author Yutaka Sawada. The original SourceForge Parchive project has been inactive since November 9, 2010.
Parchive was intended to increase the reliability of transferring files via Usenet newsgroups. Usenet was originally designed for informal conversations, and the underlying protocol, NNTP was not designed to transmit arbitrary binary data. Another limitation, which was acceptable for conversations but not for files, was that messages were normally fairly short in length and limited to 7-bit ASCII text.
Various techniques were devised to send files over Usenet, such as uuencoding and Base64. Later Usenet software allowed 8 bit Extended ASCII, which permitted new techniques like yEnc. Large files were broken up to reduce the effect of a corrupted download, but the unreliable nature of Usenet remained.
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