From Hydrogenaudio Knowledgebase
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| PAR (''parity'') files are created from a source file or files (''recovery set''), and can be used to restore the recovery set if any file becomes corrupted.
| | #REDIRECT [[PAR2]] |
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| =PAR=
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| PAR files were widespreadly used for Usenet newsgroup multi-part archive files. When posting a file a user could also post PAR files which would allow downloaders to recover any part of the archive by downloading an equal number of PAR files. I.e.: if two parts of the archive were missing or corrupt the downloader would just need to download ''any two'' PAR files and they could successfully recreate the archive.
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| PAR files are named as follows:
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| *.PAR
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| *.P01
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| *.P02
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| *.P03
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| *etc.
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| The .PAR file is simply a small (~1KiB) index file for the set, and does not contain any parity data.
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| =PAR2=
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| PAR2 succeeded PAR; its main benefit being that there is no relationship between the size of the source files and the size of the PAR2 files, ''and'' that undamaged blocks in a corrupt PAR2 file can still be used to recover the recovery set. The [http://www.quickpar.org.uk/AboutPAR2.htm QuickPar] website has a good desscription of the main differences between the two formats.
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| PAR2 files are named as follows:
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| '''filename.vol<span style="color:red">nn</span>+<span style="color:purple">mm</span>.PAR2'''
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| *'''<span style="color:red">nn</span>''' is the total number of blocks of all preceding files<br>
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| *'''<span style="color:purple">mm</span>''' is the number of blocks in this file, and therefore the number of blocks it can repair
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| =How Does It Work?=
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| The redundant data in the PAR files is computed using the Reed-Solomon algorithm that allows for recovery of any 'X' real data-blocks for 'X' parity data-blocks present. (data-blocks referring to files ''or'' much smaller virtual slices of files).
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| For more information read [http://parchive.sourceforge.net/docs/specifications/parity-volume-spec/article-spec.html the full PAR 2.0 specification].
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| =Useful Links=
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| *[http://parchive.sourceforge.net/ Parchive: Parity Archive Tool] : Home of PAR
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| *[http://www.quickpar.org.uk/ QuickPar] : A superb PAR/PAR2 GUI
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| *[http://www.dvdisaster.com/ dvdisaster] : Although not exactly PAR, dvdisaster uses the Reed-Solomon algorithm to create a correction file for a DVD image
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Revision as of 15:40, 3 January 2006
