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diff --git a/doc/lziprecover.texinfo b/doc/lziprecover.texinfo index 142cdae..54834e9 100644 --- a/doc/lziprecover.texinfo +++ b/doc/lziprecover.texinfo @@ -6,8 +6,8 @@ @finalout @c %**end of header -@set UPDATED 31 May 2013 -@set VERSION 1.14 +@set UPDATED 17 June 2013 +@set VERSION 1.15-pre1 @dircategory Data Compression @direntry @@ -35,12 +35,14 @@ This manual is for Lziprecover (version @value{VERSION}, @value{UPDATED}). @menu -* Introduction:: Purpose and features of lziprecover -* Invoking Lziprecover:: Command line interface -* File Format:: Detailed format of the compressed file -* Examples:: A small tutorial with examples -* Problems:: Reporting bugs -* Concept Index:: Index of concepts +* Introduction:: Purpose and features of lziprecover +* Invoking Lziprecover:: Command line interface +* Repairing Files:: Fixing bit-flip and similar errors +* Merging Files:: Fixing several damaged copies +* File Format:: Detailed format of the compressed file +* Examples:: A small tutorial with examples +* Problems:: Reporting bugs +* Concept Index:: Index of concepts @end menu @sp 1 @@ -56,14 +58,15 @@ to copy, distribute and modify it. Lziprecover is a data recovery tool and decompressor for files in the lzip compressed data format (.lz) able to repair slightly damaged files, -recover badly damaged files from two or more copies, extract undamaged -members from multi-member files, decompress files and test integrity of -files. +recover badly damaged files from two or more copies, extract data from +damaged files, decompress files and test integrity of files. 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. This recovery capability contributes to make the lzip format -one of the best options for long-term data archiving. +and pdlzip. It makes lzip files resistant to bit-flip, one of the most +common forms of data corruption, and its recovery capabilities +contribute to make of the lzip format one of the best options for +long-term data archiving. Lziprecover is able to efficiently extract a range of bytes from a multi-member file, because it only decompresses the members containing @@ -80,23 +83,26 @@ damaged files themselves are never modified. When decompressing or testing file integrity, lziprecover behaves like lzip or lunzip. -If the files are too damaged for lziprecover to repair them, data from -damaged members can be partially recovered writing it to stdout as shown -in the following example (the resulting file may contain some garbage -data at the end): +If a file is too damaged for lziprecover to repair it, all the +recoverable data in all members of the file can be extracted with the +following command (the resulting file may contain errors and some +garbage data may be produced at the end of each member): @example -lziprecover -cd rec01file.lz > rec01file +lziprecover -D0 -i -o file -q file.lz @end example If the cause of file corruption is damaged media, the combination @w{GNU ddrescue + lziprecover} is the best option for recovering data from multiple damaged copies. @xref{ddrescue-example}, for an example. +Lziprecover is not a replacement for regular backups, but a last line of +defense for the case where the backups are also damaged. + @node Invoking Lziprecover @chapter Invoking Lziprecover -@cindex invoking lziprecover +@cindex invoking The format for running lziprecover is: @@ -164,18 +170,10 @@ information about the members in the file. @item -m @itemx --merge Try to produce a correct file merging the good parts of two or more -damaged copies. The copies must be single-member files. The merge will -fail if the copies have too many damaged areas or if the same byte is -damaged in all copies. If successful, a repaired copy is written to the -file @samp{@var{file}_fixed.lz}. The exit status is 0 if the file could -be repaired, 2 otherwise. - -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 with -small errors, the probability approaches 100 percent even with only two -copies. +damaged copies. If successful, a repaired copy is written to the file +@samp{@var{file}_fixed.lz}. The exit status is 0 if a correct file could +be produced, 2 otherwise. See the chapter Merging Files (@pxref{Merging +Files}) for a complete description of the merge mode. @item -o @var{file} @itemx --output=@var{file} @@ -192,10 +190,12 @@ Quiet operation. Suppress all messages. @item -R @itemx --repair -Try to repair a small error, affecting only one byte, in a single-member -@var{file}. If successful, a repaired copy is written to the file +Try to repair a file with small errors (up to one byte error per member). +If successful, a repaired copy is written to the file @samp{@var{file}_fixed.lz}. @samp{@var{file}} is not modified at all. The exit status is 0 if the file could be repaired, 2 otherwise. +See the chapter Repairing Files (@pxref{Repairing Files}) for a complete +description of the repair mode. @item -s @itemx --split @@ -252,6 +252,47 @@ invalid input file, 3 for an internal consistency error (eg, bug) which caused lziprecover to panic. +@node Repairing Files +@chapter Repairing Files +@cindex repairing files + +Lziprecover is able to repair files with small errors (up to one byte +error per member). The error may be located anywhere in the file except +in the header (first 6 bytes of each member) or in the @samp{Member +size} field of the trailer (last 8 bytes of each member). This makes +lzip files resistant to bit-flip, one of the most common forms of data +corruption. + +Bit-flip happens when one bit in the file is changed from 0 to 1 or vice +versa. It may be caused by bad RAM or even by natural radiation. I have +seen a case of bit-flip in a file stored in an USB flash drive. + + +@node Merging Files +@chapter Merging Files +@cindex merging files + +If you have several copies of a file but all of them are too damaged to +repair them (@pxref{Repairing Files}), lziprecover can try to produce a +correct file merging the good parts of the damaged copies. + +The merge may succeed even if some copies of the file have all the +headers and trailers damaged, as long as there is at least one copy of +every header and trailer intact, even if they are in different copies of +the file. + +The merge will fail if the damaged areas overlap (at least one byte is +damaged in all copies), or are adjacent and the boundary can't be +determined, or if the copies have too many damaged areas. + +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 with +small errors, the probability approaches 100 percent even with only two +copies. + + @node File Format @chapter File Format @cindex file format @@ -368,9 +409,8 @@ lziprecover -D 10000-15000 file.lz @sp 1 @noindent -Example 5: Repair a one-byte corruption in the single-member file -@samp{file.lz}. (Indented lines are abridged error messages from -lziprecover). +Example 5: Repair small errors in the file @samp{file.lz}. (Indented +lines are abridged diagnostic messages from lziprecover). @example lziprecover -v -R file.lz @@ -422,27 +462,12 @@ Example 8: Recover the first volume of those created with the command @w{@code{lzip -b 32MiB -S 650MB big_db}} from two copies, @samp{big_db1_00001.lz} and @samp{big_db2_00001.lz}, with member 07 damaged in the first copy, member 18 damaged in the second copy, and -member 12 damaged in both copies. Two correct copies are produced and -compared. +member 12 damaged in both copies. The correct file produced is saved in +@samp{big_db_00001.lz}. @example -lziprecover -s big_db1_00001.lz -lziprecover -s big_db2_00001.lz -lziprecover -t rec*big_db1_00001.lz - rec07big_db1_00001.lz: crc mismatch - rec12big_db1_00001.lz: crc mismatch -lziprecover -t rec*big_db2_00001.lz - rec12big_db2_00001.lz: crc mismatch - rec18big_db2_00001.lz: crc mismatch -lziprecover -m -v rec12big_db1_00001.lz rec12big_db2_00001.lz +lziprecover -m -v -o big_db_00001.lz big_db1_00001.lz big_db2_00001.lz Input files merged successfully -cp rec07big_db2_00001.lz rec07big_db1_00001.lz -cp rec12big_db1_00001_fixed.lz rec12big_db1_00001.lz -cp rec12big_db1_00001_fixed.lz rec12big_db2_00001.lz -cp rec18big_db1_00001.lz rec18big_db2_00001.lz -cat rec*big_db1_00001.lz > big_db3_00001.lz -cat rec*big_db2_00001.lz > big_db4_00001.lz -zcmp big_db3_00001.lz big_db4_00001.lz @end example |