diff options
Diffstat (limited to 'doc/lziprecover.texi')
| -rw-r--r-- | doc/lziprecover.texi | 1141 |
1 files changed, 780 insertions, 361 deletions
diff --git a/doc/lziprecover.texi b/doc/lziprecover.texi index 731515b..6766403 100644 --- a/doc/lziprecover.texi +++ b/doc/lziprecover.texi @@ -6,8 +6,8 @@ @finalout @c %**end of header -@set UPDATED 4 January 2019 -@set VERSION 1.21 +@set UPDATED 2 January 2021 +@set VERSION 1.22 @dircategory Data Compression @direntry @@ -29,68 +29,71 @@ @contents @end ifnothtml +@ifnottex @node Top @top This manual is for Lziprecover (version @value{VERSION}, @value{UPDATED}). @menu -* Introduction:: Purpose and features of lziprecover -* Invoking lziprecover:: Command line interface -* Data safety:: Protecting data from accidental loss -* Repairing files:: Fixing bit flips and similar errors -* Merging files:: Fixing several damaged copies -* Tarlz:: Options supporting the tar.lz format -* File names:: Names of the files produced by lziprecover -* File format:: Detailed format of the compressed file -* Trailing data:: Extra data appended to the file -* Examples:: A small tutorial with examples -* Unzcrash:: Testing the robustness of decompressors -* Problems:: Reporting bugs -* Concept index:: Index of concepts +* Introduction:: Purpose and features of lziprecover +* Invoking lziprecover:: Command line interface +* Data safety:: Protecting data from accidental loss +* Repairing one byte:: Fixing bit flips and similar errors +* Merging files:: Fixing several damaged copies +* Reproducing one sector:: Fixing a missing (zeroed) sector +* Tarlz:: Options supporting the tar.lz format +* File names:: Names of the files produced by lziprecover +* File format:: Detailed format of the compressed file +* Trailing data:: Extra data appended to the file +* Examples:: A small tutorial with examples +* Unzcrash:: Testing the robustness of decompressors +* Problems:: Reporting bugs +* Concept index:: Index of concepts @end menu @sp 1 -Copyright @copyright{} 2009-2019 Antonio Diaz Diaz. +Copyright @copyright{} 2009-2021 Antonio Diaz Diaz. -This manual is free documentation: you have unlimited permission -to copy, distribute and modify it. +This manual is free documentation: you have unlimited permission to copy, +distribute, and modify it. +@end ifnottex @node Introduction @chapter Introduction @cindex introduction -@uref{http://www.nongnu.org/lzip/lziprecover.html,,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, extract data from damaged files, decompress files and test -integrity of files. +@uref{http://www.nongnu.org/lzip/lziprecover.html,,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 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 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 file format is designed for data sharing and long-term archiving, +taking into account both data integrity and decoder availability: @itemize @bullet @item The lzip format provides very safe integrity checking and some data -recovery means. The lziprecover program 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. @xref{Data safety}. +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. @xref{Data safety}. @item The lzip format is as simple as possible (but not simpler). The lzip @@ -105,43 +108,47 @@ Additionally the lzip reference implementation is copylefted, which guarantees that it will remain free forever. @end itemize -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. +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. -For compressible data, multiple lzip-compressed copies have a better -chance of surviving intact than one uncompressed copy using the same -amount of storage space. +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. +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 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. +If the cause of file corruption is a damaged medium, the combination +@w{GNU ddrescue + lziprecover} is the recommended option for recovering data +from damaged lzip files. @xref{ddrescue-example}, and +@ref{ddrescue-example2}, for examples. -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): +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 -D0 -i -o file -q file.lz +lziprecover -cd -i file.lz > file @end example 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. +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. +When decompressing or testing file integrity, lziprecover behaves like lzip +or lunzip. -LANGUAGE NOTE: Uncompressed = not compressed = plain data; it may never -have been compressed. Decompressed is used to refer to data which have -undergone the process of decompression. +LANGUAGE NOTE: Uncompressed = not compressed = plain data; it may never have +been compressed. Decompressed is used to refer to data which have undergone +the process of decompression. @node Invoking lziprecover @@ -158,11 +165,17 @@ lziprecover [@var{options}] [@var{files}] @end example @noindent -When decompressing or testing, @samp{-} used as a @var{file} argument -means standard input. It can be mixed with other @var{files} and is read -just once, the first time it appears in the command line. - -lziprecover supports the following options: +When decompressing or testing, a hyphen @samp{-} used as a @var{file} +argument means standard input. It can be mixed with other @var{files} and is +read just once, the first time it appears in the command line. If no file +names are specified, lziprecover decompresses from standard input to +standard output. + +lziprecover supports the following +@uref{http://www.nongnu.org/arg-parser/manual/arg_parser_manual.html#Argument-syntax,,options}: +@ifnothtml +@xref{Argument syntax,,,arg_parser}. +@end ifnothtml @table @code @item -h @@ -201,14 +214,16 @@ lzma-alone file as follows: @item -c @itemx --stdout -Write decompressed data to standard output; keep input files unchanged. -This option is needed when reading from a named pipe (fifo) or from a -device. Use it also to recover as much of the decompressed data as -possible when decompressing a corrupt file. +Write decompressed data to standard output; keep input files unchanged. This +option (or @samp{-o}) is needed when reading from a named pipe (fifo) or +from a device. Use it also to recover as much of the decompressed data as +possible when decompressing a corrupt file. @samp{-c} overrides @samp{-o}, +but @samp{-c} has no effect when merging, removing members, repairing, +reproducing, splitting, testing or listing. @item -d @itemx --decompress -Decompress the specified files. If a file does not exist or can't be +Decompress the files specified. If a file does not exist or can't be opened, lziprecover continues decompressing the rest of the files. If a file fails to decompress, or is a terminal, lziprecover exits immediately without decompressing the rest of the files. @@ -216,19 +231,46 @@ decompressing the rest of the files. @item -D @var{range} @itemx --range-decompress=@var{range} Decompress only a range of bytes starting at decompressed byte position -@samp{@var{begin}} and up to byte position @w{@samp{@var{end} - 1}}. -Byte positions start at 0. This option provides random access to the -data in multimember files; it only decompresses the members containing -the desired data. In order to guarantee the correctness of the data -produced, all members containing any part of the desired data are -decompressed and their integrity is verified. +@var{begin} and up to byte position @w{@var{end} - 1}. Byte positions start +at 0. This option provides random access to the data in multimember files; +it only decompresses the members containing the desired data. In order to +guarantee the correctness of the data produced, all members containing any +part of the desired data are decompressed and their integrity is verified. +@anchor{range-format} Four formats of @var{range} are recognized, @samp{@var{begin}}, @samp{@var{begin}-@var{end}}, @samp{@var{begin},@var{size}}, and -@samp{,@var{size}}. If only @var{begin} is specified, @var{end} is taken -as the end of the file. If only @var{size} is specified, @var{begin} is -taken as the beginning of the file. The produced bytes are sent to -standard output unless the @samp{--output} option is used. +@samp{,@var{size}}. If only @var{begin} is specified, @var{end} is taken as +the end of the file. If only @var{size} is specified, @var{begin} is taken +as the beginning of the file. The bytes produced are sent to standard output +unless the option @samp{--output} is used. + +@anchor{--reproduce} +@item -e +@itemx --reproduce +Try to recover a missing (zeroed) sector in @var{file} using a reference +file and the same version of lzip that created @var{file}. If successful, a +repaired copy is written to the file @samp{@var{file}_fixed.lz}. @var{file} +is not modified at all. The exit status is 0 if the member containing the +zeroed sector could be repaired, 2 otherwise. Note that +@samp{@var{file}_fixed.lz} may still contain errors in the members following +the one repaired. @xref{Reproducing one sector}, for a complete description +of the reproduce mode. + +@item --lzip-level=@var{digit}|a|m[@var{length}] +Try only the given compression level or match length limit when reproducing +a zeroed sector. @samp{--lzip-level=a} tries all the compression levels +@w{(0 to 9)}, while @samp{--lzip-level=m} tries all the match length limits +@w{(5 to 273)}. + +@item --lzip-name=@var{name} +Set the name of the lzip executable used by @samp{--reproduce}. If +@samp{--lzip-name} is not specified, @samp{lzip} is used. + +@item --reference-file=@var{file} +Set the reference file used by @samp{--reproduce}. It must contain the +uncompressed data corresponding to the missing compressed data of the zeroed +sector, plus some context data before and after them. @item -f @itemx --force @@ -236,14 +278,21 @@ Force overwrite of output files. @item -i @itemx --ignore-errors -Make @samp{--range-decompress} ignore errors and continue decompressing -the remaining members in the file. For example, -@w{@samp{lziprecover -D0 -i file.lz > file}} decompresses all the -recoverable data in all members of @samp{file.lz} without having to -split it first. - -Make @samp{--list}, @samp{--dump}, @samp{--remove} and @samp{--strip} -ignore format errors. +Make @samp{--decompress}, @samp{--test}, and @samp{--range-decompress} +ignore format and data errors and continue decompressing the remaining +members in the file; keep input files unchanged. For example, the commands +@w{@samp{lziprecover -cd -i file.lz > file}} or +@w{@samp{lziprecover -D0 -i file.lz > file}} decompress all the recoverable +data in all members of @samp{file.lz} without having to split it first. The +@w{@samp{-cd -i}} method resyncs to the next member header after each error, +and is immune to some format errors that make @w{@samp{-D0 -i}} fail. The +range decompressed may be smaller than the range requested, because of the +errors. + +Make @samp{--list}, @samp{--dump}, @samp{--remove}, and @samp{--strip} +ignore format errors. The sizes of the members with errors (specially the +last) may be wrong. The exit status is set to 0 unless other errors are +found (I/O errors, for example). @item -k @itemx --keep @@ -251,21 +300,21 @@ Keep (don't delete) input files during decompression. @item -l @itemx --list -Print the uncompressed size, compressed size and percentage saved of the -specified files. Trailing data are ignored. The values produced are -correct even for multimember files. If more than one file is given, a -final line containing the cumulative sizes is printed. With @samp{-v}, -the dictionary size, the number of members in the file, and the amount -of trailing data (if any) are also printed. With @samp{-vv}, the -positions and sizes of each member in multimember files are also -printed. With @samp{-i}, format errors are ignored, and with -@samp{-ivv}, gaps between members are shown. The member numbers shown -coincide with the file numbers produced by @samp{--split}. +Print the uncompressed size, compressed size, and percentage saved of the +files specified. Trailing data are ignored. The values produced are correct +even for multimember files. If more than one file is given, a final line +containing the cumulative sizes is printed. With @samp{-v}, the dictionary +size, the number of members in the file, and the amount of trailing data (if +any) are also printed. With @samp{-vv}, the positions and sizes of each +member in multimember files are also printed. With @samp{-i}, format errors +are ignored, and with @samp{-ivv}, gaps between members are shown. The +member numbers shown coincide with the file numbers produced by +@samp{--split}. @samp{-lq} can be used to verify quickly (without decompressing) the -structural integrity of the specified files. (Use @samp{--test} to -verify the data integrity). @samp{-alq} additionally verifies that none -of the specified files contain trailing data. +structural integrity of the files specified. (Use @samp{--test} to verify +the data integrity). @samp{-alq} additionally verifies that none of the +files specified contain trailing data. @item -m @itemx --merge @@ -277,51 +326,51 @@ description of the merge mode. @item -o @var{file} @itemx --output=@var{file} -Place the output into @samp{@var{file}} instead of into -@samp{@var{file}_fixed.lz}. If splitting, the names of the files -produced are in the form @samp{rec01@var{file}}, @samp{rec02@var{file}}, -etc. If decompressing from standard input and @samp{--stdout} has not -been specified, use @samp{@var{file}} as the name of the decompressed -file. If converting a lzma-alone file from standard input and -@samp{--stdout} has not been specified, use @samp{@var{file}.lz} as the -name of the converted file. (Or plain @samp{@var{file}} if it already -ends in @samp{.lz} or @samp{.tlz}). +Place the output into @var{file} instead of into @samp{@var{file}_fixed.lz}. +If splitting, the names of the files produced are in the form +@samp{rec01@var{file}}, @samp{rec02@var{file}}, etc. + +If decompressing, or converting lzma-alone files, and @samp{-c} has not been +also specified, write the decompressed or converted output to @var{file}; +keep input files unchanged. This option (or @samp{-c}) is needed when +reading from a named pipe (fifo) or from a device. @w{@samp{-o -}} is +equivalent to @samp{-c}. @samp{-o} has no effect when testing or listing. @item -q @itemx --quiet Quiet operation. Suppress all messages. +@anchor{--repair} @item -R @itemx --repair -Try to repair a file with small errors (up to one single-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. -@xref{Repairing files}, for a complete description of the repair mode. +Try to repair a @var{file} with small errors (up to one single-byte error +per member). If successful, a repaired copy is written to the file +@samp{@var{file}_fixed.lz}. @var{file} is not modified at all. The exit +status is 0 if the file could be repaired, 2 otherwise. @xref{Repairing one +byte}, for a complete description of the repair mode. @item -s @itemx --split -Search for members in @samp{@var{file}} and write each member in its own -file. Gaps between members are detected and each gap is saved in its own -file. Trailing data (if any) are saved alone in the last file. You can -then use @samp{lziprecover -t} to test the integrity of the resulting -files, decompress those which are undamaged, and try to repair or -partially decompress those which are damaged. Gaps may contain garbage -or may be members with corrupt headers or trailers. If other lziprecover -functions fail to work on a multimember @var{file} because of damage in -headers or trailers, try to split @var{file} and then work on each -member individually. +Search for members in @var{file} and write each member in its own file. Gaps +between members are detected and each gap is saved in its own file. Trailing +data (if any) are saved alone in the last file. You can then use +@w{@samp{lziprecover -t}} to test the integrity of the resulting files, +decompress those which are undamaged, and try to repair or partially +decompress those which are damaged. Gaps may contain garbage or may be +members with corrupt headers or trailers. If other lziprecover functions +fail to work on a multimember @var{file} because of damage in headers or +trailers, try to split @var{file} and then work on each member individually. The names of the files produced are in the form @samp{rec01@var{file}}, -@samp{rec02@var{file}}, etc, and are designed so that the use of -wildcards in subsequent processing, for example, @w{@samp{lziprecover --cd rec*@var{file} > recovered_data}}, processes the files in the -correct order. The number of digits used in the names varies depending -on the number of members in @samp{@var{file}}. +@samp{rec02@var{file}}, etc, and are designed so that the use of wildcards +in subsequent processing, for example, +@w{@samp{lziprecover -cd rec*@var{file} > recovered_data}}, processes the +files in the correct order. The number of digits used in the names varies +depending on the number of members in @var{file}. @item -t @itemx --test -Check integrity of the specified files, but don't decompress them. This +Check integrity of the files specified, but don't decompress them. This really performs a trial decompression and throws away the result. Use it together with @samp{-v} to see information about the files. If a file fails the test, does not exist, can't be opened, or is a terminal, lziprecover @@ -342,7 +391,7 @@ In other modes, increasing verbosity levels show final status, progress of operations, and extra information (for example, the failed areas). @item --loose-trailing -When decompressing, testing or listing, allow trailing data whose first +When decompressing, testing, or listing, allow trailing data whose first bytes are so similar to the magic bytes of a lzip header that they can be confused with a corrupt header. Use this option if a file triggers a "corrupt header" error and the cause is not indeed a corrupt header. @@ -350,7 +399,7 @@ be confused with a corrupt header. Use this option if a file triggers a @item --dump=[@var{member_list}][:damaged][:tdata] Dump the members listed, the damaged members (if any), or the trailing data (if any) of one or more regular multimember files to standard -output, or to a file if the @samp{--output} option is used. If more than +output, or to a file if the option @samp{--output} is used. If more than one file is given, the elements dumped from all files are concatenated. If a file does not exist, can't be opened, or is not regular, lziprecover continues processing the rest of the files. If the dump @@ -367,12 +416,12 @@ member in the file (r1). Negated versions of both kinds of lists exist (^1,3-6:r^1,3-6) which selects all the members except those in the list. The strings "damaged" and "tdata" select the damaged members and the trailing data respectively. If the same member is selected more than -once, for example by @code{1:r1} in a single-member file, it is dumped +once, for example by @samp{1:r1} in a single-member file, it is dumped just once. See the following examples: @multitable {@code{3,12:damaged:tdata}} {members 3, 12, damaged members, trailing data} @headitem @code{--dump} argument @tab Elements dumped -@item @code{1,3-6} @tab members 1, 3, 4, 5 and 6 +@item @code{1,3-6} @tab members 1, 3, 4, 5, 6 @item @code{r1-3} @tab last 3 members in file @item @code{^13,15} @tab all but 13th and 15th members in file @item @code{r^1} @tab all but last member in file @@ -389,10 +438,9 @@ data (if any) from regular multimember files in place. The date of each file is preserved if possible. If all members in a file are selected to be removed, the file is left unchanged and the exit status is set to 2. If a file does not exist, can't be opened, is not regular, or is left -unchanged, lziprecover continues processing the rest of the files. In -case of I/O error, lziprecover exits immediately without processing the -rest of the files. See @samp{--dump} above for a description of the -argument. +unchanged, lziprecover continues processing the rest of the files. In case +of I/O error, lziprecover exits immediately without processing the rest of +the files. See @samp{--dump} above for a description of the argument. This option may be dangerous even if only the trailing data is being removed because the file may be corrupt or the trailing data may contain @@ -404,7 +452,7 @@ removal of trailing data. @item --strip=[@var{member_list}][:damaged][:tdata] Copy one or more regular multimember files to standard output (or to a -file if the @samp{--output} option is used), stripping the members +file if the option @samp{--output} is used), stripping the members listed, the damaged members (if any), or the trailing data (if any) from each file. If all members in a file are selected to be stripped, the trailing data (if any) are also stripped even if @samp{tdata} is not @@ -418,6 +466,84 @@ above for a description of the argument. @end table +Lziprecover also supports the following debug options (for experts): + +@table @code +@item -E @var{range}[,@var{sector_size}] +@itemx --debug-reproduce=@var{range}[,@var{sector_size}] +Load the compressed @var{file} into memory, set all bytes in the positions +specified by @var{range} to 0, and try to reproduce a correct compressed +file. @xref{--reproduce}. @xref{range-format}, for a description of +@var{range}. If a @var{sector_size} is specified, set each sector to 0 in +sequence and try to reproduce the file, printing to standard output final +statistics of the number of sectors reproduced successfully. Exit with +nonzero status only in case of fatal error. + +@item -M +@itemx --md5sum +Print to standard output the MD5 digests of the input @var{files} one per +line in the same format produced by the @command{md5sum} tool. Lziprecover +uses MD5 digests to verify the result of some operations. This option allows +the verification of lziprecover's implementation of the MD5 algorithm. + +@item -S[@var{value}] +@itemx --nrep-stats[=@var{value}] +Compare the frequency of sequences of N repeated bytes of a given +@var{value} in the compressed LZMA streams of the input @var{files} with the +frequency expected for random data (1 / 2^(8N)). If @var{value} is not +specified, print the frequency of repeated sequences of all possible byte +values. Print cumulative data for all files followed by the name of the +first file with the longest sequence. + +@item -U +@itemx --unzcrash +Test 1-bit errors in the LZMA stream of the input @var{file} like the +command @w{@samp{unzcrash -b1 -p7 -s-20 'lzip -t' @var{file}}} but in +memory, and therefore much faster. @xref{Unzcrash}. This option tests all +the members independently in a multimember file, skipping headers and +trailers. If a decompression succeeds, the decompressed output is compared +with the original decompressed output of @var{file} using MD5 digests. The +compressed @var{file} must not contain errors and must decompress correctly +for the comparisons to work. + +By default @samp{--unzcrash} only prints the interesting cases; CRC +mismatches, size mismatches, unsupported marker codes, unexpected EOFs, +apparently successful decompressions, and decoder errors detected 50_000 or +more bytes beyond the byte being tested. At verbosity level 1 (-v) it also +prints decoder errors detected 10_000 or more bytes beyond the byte being +tested. At verbosity level 2 (-vv) it prints all cases. + +@item -W @var{position},@var{value} +@itemx --debug-decompress=@var{position},@var{value} +Load the compressed @var{file} into memory, set the byte at @var{position} +to @var{value}, and decompress the modified compressed data to standard +output. + +@item -X[@var{position},@var{value}] +@itemx --show-packets[=@var{position},@var{value}] +Load the compressed @var{file} into memory, optionally set the byte at +@var{position} to @var{value}, decompress the modified compressed data +(discarding the output), and print to standard output descriptions of the +LZMA packets being decoded. + +@item -Y @var{range} +@itemx --debug-delay=@var{range} +Load the compressed @var{file} into memory and then repeatedly decompress +it, increasing 256 times each byte of the subset of the compressed data +positions specified by @var{range}, so as to test all possible one-byte +errors. For each decompression error find the error detection delay and +print to standard output the maximum delay. The error detection delay is the +difference between the position of the error and the position where the +decoder realized that the data contains an error. @xref{range-format}, for a +description of @var{range}. + +@item -Z @var{position},@var{value} +@itemx --debug-repair=@var{position},@var{value} +Load the compressed @var{file} into memory, set the byte at @var{position} +to @var{value}, and then try to repair the error. @xref{--repair}. + +@end table + Numbers given as arguments to options may be followed by a multiplier and an optional @samp{B} for "byte". @@ -446,48 +572,117 @@ caused lziprecover to panic. @chapter Protecting data from accidental loss @cindex data safety +It is a fact of life that sometimes data will become corrupt. Software has +errors. Hardware may misbehave or fail. RAM may be struck by a cosmic ray. +This is why a safe enough integrity checking is needed in compressed +formats, and the reason why a data recovery tool is sometimes needed. + There are 3 main types of data corruption that may cause data loss: single-byte errors, multibyte errors (generally affecting a whole sector in a block device), and total device failure. -Lziprecover protects natively against single-byte errors -(@pxref{Repairing files}), as long as file integrity is checked -frequently enough that a second single-byte error does not develop in -the same member before the first one is repaired. +Lziprecover protects natively against single-byte errors as long as file +integrity is checked frequently enough that a second single-byte error does +not develop in the same member before the first one is repaired. +@xref{Repairing one byte}. + +Lziprecover also protects against multibyte errors if at least one backup +copy of the file is made (@pxref{Merging files}), or if the error is a +zeroed sector and the uncompressed data corresponding to the zeroed sector +are available (@pxref{Reproducing one sector}). If you can choose between +merging and reproducing, try merging first because it is usually faster, +easier to use, and has a high probability of success. + +Lziprecover can't help in case of device failure. The only remedy for total +device failure is storing backup copies in separate media. -Lziprecover also protects against multibyte errors (@pxref{Merging -files}), if at least one backup copy of the file is made. +The extraordinary safety of the lzip format allows lziprecover to exploit +the redundance that occurrs naturally when making compressed backups. +Lziprecover can recover data that would not be recoverable from files +compressed in other formats. Let's see two examples of how much better is +lzip compared with gzip and bzip2 with respect to data safety: + +@menu +* Merging with a backup:: Recovering a file using a damaged backup +* Reproducing a mailbox:: Recovering new messages using an old backup +@end menu -The only remedy for total device failure is storing backup copies in -separate media. -How does lzip compare with gzip and bzip2 with respect to data safety? -Let's suppose that you made a backup of your valuable scientific data, -compressed it, and stored two copies on separate media. Years later you -notice that both copies are corrupt. +@node Merging with a backup +@section Recovering a file using a damaged backup +@cindex merging with a backup -If you compressed with gzip and both copies suffer any damage in the -data stream, even if it is just one altered bit, the original data can +Let's suppose that you made a compressed backup of your valuable scientific +data and stored two copies on separate media. Years later you notice that +both copies are corrupt. + +If you compressed the data with gzip and both copies suffer any damage in +the data stream, even if it is just one altered bit, the original data can only be recovered by an expert, if at all. -If you used bzip2, and if the file is large enough to contain more than -one compressed data block (usually larger than @w{900 kB} uncompressed), -and if no block is damaged in both files, then the data can be manually -recovered by splitting the files with bzip2recover, verifying every -block and then copying the right blocks in the right order into another -file. +If you used bzip2, and if the file is large enough to contain more than one +compressed data block (usually larger than @w{900 kB} uncompressed), and if +no block is damaged in both files, then the data can be manually recovered +by splitting the files with bzip2recover, verifying every block, and then +copying the right blocks in the right order into another file. + +But if you used lzip, the data can be automatically recovered with +@w{@samp{lziprecover --merge}} as long as the damaged areas don't overlap. + +Note that each error in a bzip2 file makes a whole block unusable, but each +error in a lzip file only affects the damaged bytes, making it possible to +recover a file with thousands of errors. + + +@node Reproducing a mailbox +@section Recovering new messages using an old backup +@cindex reproducing a mailbox -But if you used lzip, the data can be automatically recovered as long as -the damaged areas don't overlap. +Let's suppose that you make periodic backups of your email messages stored +in one or more mailboxes. (A mailbox is a file containing a possibly large +number of email messages). New messages are appended to the end of each +mailbox, therefore the initial part of two consecutive backups is identical +unless some messages have been changed or deleted in the meantime. The new +messages added to each backup are usually a small part of the whole mailbox. -Note that each error in a bzip2 file makes a whole block unusable, but -each error in a lzip file only affects the damaged bytes, making it -possible to recover a file with thousands of errors. +@verbatim ++========================================================+ +| Older backup containing some messages | ++========================================================+ ++========================================================+================+ +| Newer backup containing the messages above plus some | new messages | ++========================================================+================+ +@end verbatim + +One day you discover that your mailbox has disappeared because you deleted +it inadvertently or because of a bug in your email reader. Not only that. +You need to recover a recent message, but the last backup you made of the +mailbox (the newer backup above) has lost the data corresponding to a whole +sector because of an I/O error in the part containing the old messages. + +If you compressed the mailbox with gzip, usually none of the new messages +can be recovered even if they are intact because all the data beyond the +missing sector can't be decoded. + +If you used bzip2, and if the newer backup is large enough that the new +messages are in a different compressed data block than the one damaged +(usually larger than @w{900 kB} uncompressed), then you can recover the new +messages manually with bzip2recover. If the backups are identical except for +the new messages appended, you may even recover the whole newer backup by +combining the good blocks from both backups. + +But if you used lzip, the whole newer backup can be automatically recovered +with @w{@samp{lziprecover --reproduce}} as long as the missing bytes can be +recovered from the older backup, even if other messages in the common part +have been changed or deleted. Mailboxes seem to be specially easy to +reproduce. The probability of reproducing a mailbox +(@pxref{performance-of-reproduce}) is almost as high as that of merging two +identical backups (@pxref{performance-of-merge}). -@node Repairing files -@chapter Repairing files -@cindex repairing files +@node Repairing one byte +@chapter Repairing one byte +@cindex repairing one byte Lziprecover can repair perfectly most files with small errors (up to one single-byte error per member), without the need of any extra redundance @@ -495,6 +690,9 @@ at all. If the reparation is successful, the repaired file will be identical bit for bit to the original. This makes lzip files resistant to bit flip, one of the most common forms of data corruption. +The file is repaired in memory. Therefore, enough virtual memory +@w{(RAM + swap)} to contain the largest damaged member is required. + The error may be located anywhere in the file except in the first 5 bytes of each member header or in the @samp{Member size} field of the trailer (last 8 bytes of each member). If the error is in the header it @@ -510,13 +708,12 @@ One byte may seem small, but most file corruptions not produced by transmission errors or I/O errors just affect one byte, or even one bit, of the file. Also, unlike magnetic media, where errors usually affect a whole sector, solid-state storage devices tend to produce single-byte -errors, making of lzip the perfect format for data stored on such -devices. +errors, making of lzip the perfect format for data stored on such devices. Repairing a file can take some time. Small files or files with the error located near the beginning can be repaired in a few seconds. But repairing a large file compressed with a large dictionary size and with -the error located far from the beginning, can take hours. +the error located far from the beginning, may take hours. On the other hand, errors located near the beginning of the file cause much more loss of data than errors located near the end. So lziprecover @@ -528,7 +725,7 @@ repairs more efficiently the worst errors. @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 +repair them (@pxref{Repairing one byte}), lziprecover can try to produce a correct file by merging the good parts of the damaged copies. The merge may succeed even if some copies of the file have all the @@ -550,13 +747,14 @@ with the other copies: ddrescue -s<correct_size> -x<correct_size> file.lz correct_size_file.lz @end example -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). +@anchor{performance-of-merge} +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). Some types of solid-state device (NAND flash, for example) can produce bursts of scattered single-bit errors. Lziprecover is able to merge @@ -572,7 +770,7 @@ single-bit errors scattered in an area of 1020 bytes, and the other had correct file, identical to the original, in just 5 seconds: @example -$ lziprecover -vvm a/icecat-3.5.3-x86.tar.lz b/icecat-3.5.3-x86.tar.lz +lziprecover -vvm a/icecat-3.5.3-x86.tar.lz b/icecat-3.5.3-x86.tar.lz Merging member 1 of 1 (2552 errors) 2552 errors have been grouped in 16 clusters. Trying variation 2 of 2, block 2 @@ -583,61 +781,324 @@ Note that the number of errors reported by lziprecover (2552) is lower than the number of corrupt bytes (3104) because contiguous corrupt bytes are counted as a single multibyte error. +@sp 1 +@anchor{ddrescue-example} +@noindent +Example 1: Recover a compressed backup from two copies on CD-ROM with +error-checked merging of copies. +@ifnothtml +@xref{Top,GNU ddrescue manual,,ddrescue}, +@end ifnothtml +@ifhtml +See the +@uref{http://www.gnu.org/software/ddrescue/manual/ddrescue_manual.html,,ddrescue manual} +@end ifhtml +for details about ddrescue. + +@example +ddrescue -d -r1 -b2048 /dev/cdrom cdimage1 mapfile1 +mount -t iso9660 -o loop,ro cdimage1 /mnt/cdimage +cp /mnt/cdimage/backup.tar.lz rescued1.tar.lz +umount /mnt/cdimage + (insert second copy in the CD drive) +ddrescue -d -r1 -b2048 /dev/cdrom cdimage2 mapfile2 +mount -t iso9660 -o loop,ro cdimage2 /mnt/cdimage +cp /mnt/cdimage/backup.tar.lz rescued2.tar.lz +umount /mnt/cdimage +lziprecover -m -v -o backup.tar.lz rescued1.tar.lz rescued2.tar.lz + Input files merged successfully. +lziprecover -tv backup.tar.lz + backup.tar.lz: ok +@end example + +@sp 1 +@noindent +Example 2: Recover the first volume of those created with the command +@w{@samp{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. The correct file produced is saved in +@samp{big_db_00001.lz}. + +@example +lziprecover -m -v -o big_db_00001.lz big_db1_00001.lz big_db2_00001.lz + Input files merged successfully. +lziprecover -tv big_db_00001.lz + big_db_00001.lz: ok +@end example + + +@node Reproducing one sector +@chapter Reproducing one sector +@cindex reproducing one sector + +Lziprecover can recover a zeroed sector in a lzip file by concatenating the +decompressed contents of the file up to the beginning of the zeroed sector +and the uncompressed data corresponding to the zeroed sector, and then +feeding the concatenated data to the same version of lzip that created the +file. For this to work, a reference file is required containing the +uncompressed data corresponding to the missing compressed data of the zeroed +sector, plus some context data before and after them. It is possible to +recover a large file using just a few KB of reference data. + +The difficult part is finding a suitable reference file. It must contain the +exact data required (possibly mixed with other data). Containing similar +data is not enough. + +A zeroed sector may be caused by the incomplete recovery of a damaged +storage device (with I/O errors) using, for example, ddrescue. The +reproduction can't be done if the zeroed sector overlaps with the first 15 +bytes of a member, or if the zeroed sector is smaller than 8 bytes. + +The file is reproduced in memory. Therefore, enough virtual memory +@w{(RAM + swap)} to contain the damaged member is required. + +To understand how it works, take any lzipped file, say @samp{foo.lz}, +decompress it (keeping the original), and try to reproduce an artificially +zeroed sector in it by running the following commands: + +@example +lzip -kd foo.lz +lziprecover -vv --debug-reproduce=65536,512 --reference-file=foo foo.lz +@end example + +@noindent +which should produce an output like the following: + +@example +Reproducing: foo.lz +Reference file: foo +Testing sectors of size 512 at file positions 65536 to 66047 + (master mpos = 65536, dpos = 296892) +foo: Match found at offset 296892 +Reproduction succeeded at pos 65536 + + 1 sectors tested + 1 reproductions returned with zero status + all comparisons passed +@end example + +Using @samp{foo} as reference file guarantees that any zeroed sector in +@samp{foo.lz} can be reproduced because both files contain the same data. In +real use, the reference file needs to contain the data corresponding to the +zeroed sector, but the rest of the data (if any) may differ between both +files. The reference data may be obtained from the partial decompression of +the damaged file itself if it contains repeated data. For example if the +damaged file is a compressed tarball containing several partially modified +versions of the same file. + +The offset reported by lziprecover is the position in the reference file of +the first byte that could not be decompressed. This is the first byte that +will be compressed to reproduce the zeroed sector. + +The reproduce mode tries to reproduce the missing compressed data originally +present in the zeroed sector. It is based on the perfect reproducibility of +lzip files (lzip produces identical compressed output from identical input). +Therefore, the same version of lzip that created the file to be reproduced +should be used to reproduce the zeroed sector. Near versions may also work +because the output of lzip changes infrequently. If reproducing a tar.lz +archive created with tarlz, the version of lzip, clzip, or minilzip +corresponding to the version of the lzlib library used by tarlz to create +the archive should be used. + +When recovering a tar.lz archive and using as reference a file from the +filesystem, if the zeroed sector encodes (part of) a tar header, the archive +can't be reproduced. Therefore, the less overhead (smaller headers) a tar +archive has, the more probable is that the zeroed sector does not include a +header, and that the archive can be reproduced. The tarlz format has minimum +overhead. It uses basic ustar headers, and only adds extended pax headers +when they are required. + +@anchor{performance-of-reproduce} +@section Performance of @samp{--reproduce} +Reproduce mode is specially useful when recovering a corrupt backup (or a +corrupt source tarball) that is part of a series. Usually only a small +fraction of the data changes from one backup to the next or from one version +of a source tarball to the next. This makes sometimes possible to reproduce +a given corrupted version using reference data from a near version. The +following two tables show the fraction of reproducible sectors (reproducible +sectors divided by total sectors in archive) for some archives, using sector +sizes of 512 and 4096 bytes. @samp{mailbox-aug.tar.lz} is a backup of some +of my mailboxes. @samp{backup-feb.tar.lz} and @samp{backup-apr.tar.lz} are +real backups of my own working directory: + +@multitable {Reference file} {gawk-5.0.1.tar.lz} {4369 / 5844 = 74.76%} +@headitem Reference file @tab File @tab Reproducible (512) +@item backup-feb.tar @tab backup-apr.tar.lz @tab 3273 / 4342 = 75.38% +@item backup-apr.tar @tab backup-feb.tar.lz @tab 3259 / 4161 = 78.32% +@item gawk-5.0.0.tar @tab gawk-5.0.1.tar.lz @tab 4369 / 5844 = 74.76% +@item gawk-5.0.1.tar @tab gawk-5.0.0.tar.lz @tab 4379 / 5603 = 78.15% +@item gmp-6.1.1.tar @tab gmp-6.1.2.tar.lz @tab 2454 / 3787 = 64.8% +@item gmp-6.1.2.tar @tab gmp-6.1.1.tar.lz @tab 2461 / 3782 = 65.07% +@end multitable + +@multitable {mailbox-mar.tar} {mailbox-aug.tar.lz} {4036 / 4252 = 94.92%} +@headitem Reference file @tab File @tab Reproducible (4096) +@item mailbox-mar.tar @tab mailbox-aug.tar.lz @tab 4036 / 4252 = 94.92% +@item backup-feb.tar @tab backup-apr.tar.lz @tab 264 / 542 = 48.71% +@item backup-apr.tar @tab backup-feb.tar.lz @tab 264 / 520 = 50.77% +@item gawk-5.0.0.tar @tab gawk-5.0.1.tar.lz @tab 327 / 730 = 44.79% +@item gawk-5.0.1.tar @tab gawk-5.0.0.tar.lz @tab 326 / 700 = 46.57% +@item gmp-6.1.1.tar @tab gmp-6.1.2.tar.lz @tab 175 / 473 = 37% +@item gmp-6.1.2.tar @tab gmp-6.1.1.tar.lz @tab 181 / 472 = 38.35% +@end multitable + +Note that the "performance of reproduce" is a probability, not a partial +recovery. The data is either fully recovered (with the probability X shown +in the last column of the tables above) or not recovered at all (with +probability @w{1 - X}). + +Example 1: Recover a damaged source tarball with a zeroed sector of 512 +bytes at file position 1019904, using as reference another source tarball +for a different version of the software. + +@example +lziprecover -vv -e --reference-file=gmp-6.1.1.tar gmp-6.1.2.tar.lz +Reproducing bad area in member 1 of 1 + (begin = 1019904, size = 512, value = 0x00) + (master mpos = 1019904, dpos = 6292134) +warning: gmp-6.1.1.tar: Partial match found at offset 6277798, len 8716. +Reference data may be mixed with other data. +Trying level -9 + Reproducing position 1015808 +Member reproduced successfully. +Copy of input file reproduced successfully. +@end example + +@sp 1 +@anchor{ddrescue-example2} +@noindent +Example 2: Recover a damaged backup with a zeroed sector of 4096 bytes at +file position 1019904, using as reference a previous backup. The damaged +backup comes from a damaged partition copied with ddrescue. + +@example +ddrescue -b4096 -r10 /dev/sdc1 hdimage mapfile +mount -o loop,ro hdimage /mnt/hdimage +cp /mnt/hdimage/backup.tar.lz backup.tar.lz +umount /mnt/hdimage +lzip -t backup.tar.lz + backup.tar.lz: Decoder error at pos 1020530 +lziprecover -vv -e --reference-file=old_backup.tar backup.tar.lz +Reproducing bad area in member 1 of 1 + (begin = 1019904, size = 4096, value = 0x00) + (master mpos = 1019903, dpos = 5857954) +warning: old_backup.tar: Partial match found at offset 5743778, len 9546. +Reference data may be mixed with other data. +Trying level -9 + Reproducing position 1015808 +Member reproduced successfully. +Copy of input file reproduced successfully. +@end example + +@sp 1 +@noindent +Example 3: Recover a damaged backup with a zeroed sector of 4096 bytes at +file position 1019904, using as reference a file from the filesystem. (If +the zeroed sector encodes (part of) a tar header, the tarball can't be +reproduced). + +@example +# List the contents of the backup tarball to locate the damaged member. +tarlz -n0 -tvf backup.tar.lz + [...] + example.txt +tarlz: Skipping to next header. +tarlz: backup.tar.lz: Archive ends unexpectedly. +# Find in the filesystem the last file listed and use it as reference. +lziprecover -vv -e --reference-file=/somedir/example.txt backup.tar.lz +Reproducing bad area in member 1 of 1 + (begin = 1019904, size = 4096, value = 0x00) + (master mpos = 1019903, dpos = 5857954) +/somedir/example.txt: Match found at offset 9378 +Trying level -9 + Reproducing position 1015808 +Member reproduced successfully. +Copy of input file reproduced successfully. +@end example + +If @samp{backup.tar.lz} is a multimember file with more than one member +damaged and lziprecover shows the message @samp{One member reproduced. Copy +of input file still contains errors.}, the procedure shown in the example +above can be repeated until all the members have been reproduced. + +@samp{tarlz --keep-damaged -n0 -xf backup.tar.lz example.txt} produces a +partial copy of the reference file @samp{example.txt} that may help locate a +complete copy in the filesystem or in another backup, even if +@samp{example.txt} has been renamed. + @node Tarlz @chapter Options supporting the tar.lz format @cindex tarlz -@uref{http://www.nongnu.org/lzip/manual/tarlz_manual.html,,Tarlz} is an -implementation of the tar archiver which by default creates archives -compressed with lzip on a per file basis. Tarlz can append files to the -end of such compressed archives because each tar member is compressed in -its own lzip member, as well as the end-of-file blocks. Thus tarlz -archives are multimember lzip files, which has some safety advantages -over solidly compressed tar.lz archives. For example, 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. In this chapter we'll explain the ways in which -lziprecover can recover and process multimember tar.lz archives. +@uref{http://www.nongnu.org/lzip/manual/tarlz_manual.html,,Tarlz} is a +massively parallel (multi-threaded) combined implementation of the tar +archiver and the +@uref{http://www.nongnu.org/lzip/manual/lzip_manual.html,,lzip} compressor. + +Tarlz creates tar archives using a simplified and safer variant of the POSIX +pax format compressed in lzip format, keeping the alignment between tar +members and lzip members. The resulting multimember tar.lz archive is fully +backward compatible with standard tar tools like GNU tar, which treat it +like any other tar.lz archive. @ifnothtml -@xref{Top,tarlz manual,,tarlz}. +@xref{Top,tarlz manual,,tarlz}, and @ref{Top,lzip manual,,lzip}. @end ifnothtml +Multimember tar.lz archives have some safety advantages over solidly +compressed tar.lz archives. For example, 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. Keeping the +alignment between tar members and lzip members minimizes the amount of data +lost in case of corruption. In this chapter we'll explain the ways in which +lziprecover can recover and process multimember tar.lz archives. + @sp 1 @section Recovering damaged multimember tar.lz archives -If you have several copies of the damaged archive, try merging -them first because merging has a high probability of success. If the -command below prints something like -@w{@code{Input files merged successfully.}} you are done and -@code{archive.tar.lz} now contains the recovered archive: +If you have several copies of the damaged archive, try merging them first +because merging has a high probability of success. @xref{Merging files}. If +the command below prints something like +@w{@samp{Input files merged successfully.}} you are done and +@samp{archive.tar.lz} now contains the recovered archive: @example lziprecover -m -v -o archive.tar.lz a/archive.tar.lz b/archive.tar.lz @end example -If you only have one copy of the damaged archive, you may try to repair -the archive, but this has a lower probability of success. If the command -below prints something like -@w{@code{Copy of input file repaired successfully.}} you are done and -@code{archive_fixed.tar.lz} now contains the recovered archive: +If you only have one copy of the damaged archive with a zeroed block of data +caused by an I/O error, you may try to reproduce the archive. +@xref{Reproducing one sector}. If the command below prints something like +@w{@samp{Copy of input file reproduced successfully.}} you are done and +@samp{archive_fixed.tar.lz} now contains the recovered archive: + +@example +lziprecover -vv -e --reference-file=old_archive.tar archive.tar.lz +@end example + +If you only have one copy of the damaged archive, you may try to repair the +archive, but this has a lower probability of success. @xref{Repairing one +byte}. If the command below prints something like +@w{@samp{Copy of input file repaired successfully.}} you are done and +@samp{archive_fixed.tar.lz} now contains the recovered archive: @example lziprecover -v -R archive.tar.lz @end example -If all the above fails, you may save the damaged members for later and -then copy the good members to another archive. If the two commands below -succeed, @code{bad_members.tar.lz} will contain all the damaged members -and @code{archive_cleaned.tar.lz} will contain a good archive with the -damaged members removed: +If all the above fails, and the archive was created with tarlz, you may save +the damaged members for later and then copy the good members to another +archive. If the two commands below succeed, @samp{bad_members.tar.lz} will +contain all the damaged members and @samp{archive_cleaned.tar.lz} will +contain a good archive with the damaged members removed: @example lziprecover -v --dump=damaged -o bad_members.tar.lz archive.tar.lz lziprecover -v --strip=damaged -o archive_cleaned.tar.lz archive.tar.lz @end example -You can then use @code{tarlz --keep-damaged} to recover as much data as +You can then use @samp{tarlz --keep-damaged} to recover as much data as possible from each damaged member in @samp{bad_members.tar.lz}: @example @@ -651,22 +1112,22 @@ tarlz --keep-damaged -xvf ../bad_members.tar.lz Lziprecover is able to copy a list of members from a file to another. For example the command -@w{@code{lziprecover --dump=1-10:r1:tdata archive.tar.lz > subarch.tar.lz}} -creates a subset archive containing the first ten members, the -end-of-file blocks, and the trailing data (if any) of -@code{archive.tar.lz}. The @code{r1} part selects the last member, which -in an appendable tar.lz archive contains the end-of-file blocks. +@w{@samp{lziprecover --dump=1-10:r1:tdata archive.tar.lz > subarch.tar.lz}} +creates a subset archive containing the first ten members, the end-of-file +blocks, and the trailing data (if any) of @samp{archive.tar.lz}. The +@samp{r1} part selects the last member, which in an appendable tar.lz +archive contains the end-of-file blocks. @node File names @chapter Names of the files produced by lziprecover @cindex file names -The name of the fixed file produced by @samp{--merge} and -@samp{--repair} is made by appending the string @samp{_fixed.lz} to the -original file name. If the original file name ends with one of the -extensions @samp{.tar.lz}, @samp{.lz} or @samp{.tlz}, the string -@samp{_fixed} is inserted before the extension. +The name of the fixed file produced by @samp{--merge} and @samp{--repair} is +made by appending the string @samp{_fixed.lz} to the original file name. If +the original file name ends with one of the extensions @samp{.tar.lz}, +@samp{.lz}, or @samp{.tlz}, the string @samp{_fixed} is inserted before the +extension. @node File format @@ -679,6 +1140,7 @@ when there is no longer anything to take away.@* @sp 1 In the diagram below, a box like this: + @verbatim +---+ | | <-- the vertical bars might be missing @@ -686,6 +1148,7 @@ In the diagram below, a box like this: @end verbatim represents one byte; a box like this: + @verbatim +==============+ | | @@ -700,6 +1163,7 @@ The members simply appear one after another in the file, with no additional information before, between, or after them. Each member has the following structure: + @verbatim +--+--+--+--+----+----+=============+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ID string | VN | DS | LZMA stream | CRC32 | Data size | Member size | @@ -718,8 +1182,7 @@ Just in case something needs to be modified in the future. 1 for now. @item DS (coded dictionary size, 1 byte) The dictionary size is calculated by taking a power of 2 (the base size) -and subtracting from it a fraction between 0/16 and 7/16 of the base -size.@* +and subtracting from it a fraction between 0/16 and 7/16 of the base size.@* Bits 4-0 contain the base 2 logarithm of the base size (12 to 29).@* Bits 7-5 contain the numerator of the fraction (0 to 7) to subtract from the base size to obtain the dictionary size.@* @@ -727,8 +1190,8 @@ Example: 0xD3 = 2^19 - 6 * 2^15 = 512 KiB - 6 * 32 KiB = 320 KiB@* Valid values for dictionary size range from 4 KiB to 512 MiB. @item LZMA stream -The LZMA stream, finished by an end of stream marker. Uses default -values for encoder properties. +The LZMA stream, finished by an end of stream marker. Uses default values +for encoder properties. @ifnothtml @xref{Stream format,,,lzip}, @end ifnothtml @@ -739,7 +1202,7 @@ See for a complete description. @item CRC32 (4 bytes) -CRC of the uncompressed original data. +Cyclic Redundancy Check (CRC) of the uncompressed original data. @item Data size (8 bytes) Size of the uncompressed original data. @@ -836,9 +1299,18 @@ lziprecover --strip=tdata file.lz | sha256sum -c \ @chapter A small tutorial with examples @cindex examples -Example 1: Restore a regular file from its compressed version -@samp{file.lz}. If the operation is successful, @samp{file.lz} is -removed. +Example 1: Extract all the files from archive @samp{foo.tar.lz}. + +@example + tar -xf foo.tar.lz +or + lziprecover -cd foo.tar.lz | tar -xf - +@end example + +@sp 1 +@noindent +Example 2: Restore a regular file from its compressed version +@samp{file.lz}. If the operation is successful, @samp{file.lz} is removed. @example lziprecover -d file.lz @@ -846,8 +1318,8 @@ lziprecover -d file.lz @sp 1 @noindent -Example 2: Verify the integrity of the compressed file @samp{file.lz} -and show status. +Example 3: Verify the integrity of the compressed file @samp{file.lz} and +show status. @example lziprecover -tv file.lz @@ -856,8 +1328,8 @@ lziprecover -tv file.lz @sp 1 @anchor{concat-example} @noindent -Example 3: The right way of concatenating the decompressed output of two -or more compressed files. @xref{Trailing data}. +Example 4: The right way of concatenating the decompressed output of two or +more compressed files. @xref{Trailing data}. @example Don't do this @@ -872,7 +1344,7 @@ Or keeping the trailing data of the last file like this @sp 1 @noindent -Example 4: Decompress @samp{file.lz} partially until @w{10 KiB} of +Example 5: Decompress @samp{file.lz} partially until @w{10 KiB} of decompressed data are produced. @example @@ -881,8 +1353,8 @@ lziprecover -D 0,10KiB file.lz @sp 1 @noindent -Example 5: Decompress @samp{file.lz} partially from decompressed byte -10000 to decompressed byte 15000 (5000 bytes are produced). +Example 6: Decompress @samp{file.lz} partially from decompressed byte at +offset 10000 to decompressed byte at offset 14999 (5000 bytes are produced). @example lziprecover -D 10000-15000 file.lz @@ -890,8 +1362,8 @@ lziprecover -D 10000-15000 file.lz @sp 1 @noindent -Example 6: Repair small errors in the file @samp{file.lz}. (Indented -lines are abridged diagnostic messages from lziprecover). +Example 7: Repair small errors in the file @samp{file.lz}. (Indented lines +are abridged diagnostic messages from lziprecover). @example lziprecover -v -R file.lz @@ -903,126 +1375,74 @@ mv file_fixed.lz file.lz @sp 1 @noindent -Example 7: Split the multimember file @samp{file.lz} and write each -member in its own @samp{recXXXfile.lz} file. Then use -@w{@samp{lziprecover -t}} to test the integrity of the resulting files. +Example 8: Split the multimember file @samp{file.lz} and write each member +in its own @samp{recXXXfile.lz} file. Then use @w{@samp{lziprecover -t}} to +test the integrity of the resulting files. @example lziprecover -s file.lz lziprecover -tv rec*file.lz @end example -@sp 1 -@anchor{ddrescue-example} -@noindent -Example 8: Recover a compressed backup from two copies on CD-ROM with -error-checked merging of copies. -@ifnothtml -@xref{Top,GNU ddrescue manual,,ddrescue}, -@end ifnothtml -@ifhtml -See the -@uref{http://www.gnu.org/software/ddrescue/manual/ddrescue_manual.html,,ddrescue manual} -@end ifhtml -for details about ddrescue. - -@example -ddrescue -d -r1 -b2048 /dev/cdrom cdimage1 mapfile1 -mount -t iso9660 -o loop,ro cdimage1 /mnt/cdimage -cp /mnt/cdimage/backup.tar.lz rescued1.tar.lz -umount /mnt/cdimage - (insert second copy in the CD drive) -ddrescue -d -r1 -b2048 /dev/cdrom cdimage2 mapfile2 -mount -t iso9660 -o loop,ro cdimage2 /mnt/cdimage -cp /mnt/cdimage/backup.tar.lz rescued2.tar.lz -umount /mnt/cdimage -lziprecover -m -v -o backup.tar.lz rescued1.tar.lz rescued2.tar.lz - Input files merged successfully. -lziprecover -tv backup.tar.lz - backup.tar.lz: ok -@end example - -@sp 1 -@noindent -Example 9: Recover the first volume of those created with the command -@w{@samp{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. The correct file produced is saved in -@samp{big_db_00001.lz}. - -@example -lziprecover -m -v -o big_db_00001.lz big_db1_00001.lz big_db2_00001.lz - Input files merged successfully. -lziprecover -tv big_db_00001.lz - big_db_00001.lz: ok -@end example - @node Unzcrash @chapter Testing the robustness of decompressors @cindex unzcrash -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 @samp{make unzcrash} in the -lziprecover source directory to build it. - -By default, unzcrash reads the specified file and then repeatedly -decompresses it, increasing 256 times each byte of the compressed data, -so as to test all possible one-byte errors. Note that it may take years -or even centuries to test all possible one-byte errors in a large file -(tens of MB). - -If the @code{--block} option is given, unzcrash reads the specified file -and then repeatedly decompresses it, setting all bytes in each -successive block to the value given, so as to test all possible full -sector errors. - -If the @code{--truncate} option is given, unzcrash reads the specified -file and then repeatedly decompresses it, truncating the file to -increasing lengths, so as to test all possible truncation points. - -None of the three test modes described above should cause any invalid -memory accesses. If any of them does, please, report it as a bug to the -maintainers of the decompressor being tested. - -Unzcrash really executes as a subprocess the shell command specified in -the first non-option argument, and then writes the file specified in the -second non-option argument to the standard input of the subprocess, -modifying the corresponding byte each time. Therefore unzcrash can be -used to test any decompressor (not only lzip), or even other decoder -programs having a suitable command line syntax. - -If the decompressor returns with zero status, unzcrash compares the -output of the decompressor for the original and corrupt files. If the -outputs differ, it means that the decompressor returned a false -negative; it failed to recognize the corruption and produced garbage -output. The only exception is when a multimember file is truncated just -after the last byte of a member, producing a shorter but valid -compressed file. Except in this latter case, please, report any false -negative as a bug. +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 @samp{make unzcrash} in the lziprecover source +directory to build it. + +By default, unzcrash reads the file specified and then repeatedly +decompresses it, increasing 256 times each byte of the compressed data, so +as to test all possible one-byte errors. Note that it may take years or even +centuries to test all possible one-byte errors in a large file (tens of MB). + +If the option @samp{--block} is given, unzcrash reads the file specified and +then repeatedly decompresses it, setting all bytes in each successive block +to the value given, so as to test all possible full sector errors. + +If the option @samp{--truncate} is given, unzcrash reads the file specified +and then repeatedly decompresses it, truncating the file to increasing +lengths, so as to test all possible truncation points. + +None of the three test modes described above should cause any invalid memory +accesses. If any of them does, please, report it as a bug to the maintainers +of the decompressor being tested. + +Unzcrash really executes as a subprocess the shell command specified in the +first non-option argument, and then writes the file specified in the second +non-option argument to the standard input of the subprocess, modifying the +corresponding byte each time. Therefore unzcrash can be used to test any +decompressor (not only lzip), or even other decoder programs having a +suitable command line syntax. + +If the decompressor returns with zero status, unzcrash compares the output +of the decompressor for the original and corrupt files. If the outputs +differ, it means that the decompressor returned a false negative; it failed +to recognize the corruption and produced garbage output. The only exception +is when a multimember file is truncated just after the last byte of a +member, producing a shorter but valid compressed file. Except in this latter +case, please, report any false negative as a bug. In order to compare the outputs, unzcrash needs a @samp{zcmp} program able to understand the format being tested. For example the @samp{zcmp} provided -by @samp{zutils}. +by @uref{http://www.nongnu.org/zutils/manual/zutils_manual.html#Zcmp,,zutils}. +Use @samp{--zcmp=false} to disable comparisons. @ifnothtml @xref{Zcmp,,,zutils}. @end ifnothtml -@ifhtml -See -@uref{http://www.nongnu.org/zutils/manual/zutils_manual.html#Zcmp,,zcmp}. -@end ifhtml The format for running unzcrash is: @example -unzcrash [@var{options}] 'lzip -t' @var{file}.lz +unzcrash [@var{options}] 'lzip -t' @var{file} @end example @noindent -@var{file}.lz must not contain errors and must be correctly decompressed -by the decompressor being tested for the comparisons to work. +The compressed @var{file} must not contain errors and the decompressor being +tested must decompress it correctly for the comparisons to work. unzcrash supports the following options: @@ -1045,45 +1465,44 @@ original value in the bit position N.@* The number of N-bit errors per byte (N = 1 to 8) is: @w{8 28 56 70 56 28 8 1} -@multitable {Examples of @var{range}} {1, 2, 3, 5, 6, 7 and 8} -@item Examples of @var{range} @tab Tests errors of N-bit +@multitable {Examples of @var{range}} {Tests errors of N-bits} +@item Examples of @var{range} @tab Tests errors of N-bits @item 1 @tab 1 -@item 1,2,3 @tab 1, 2 and 3 -@item 2-4 @tab 2, 3 and 4 -@item 1,3-5,8 @tab 1, 3, 4, 5 and 8 -@item 1-3,5-8 @tab 1, 2, 3, 5, 6, 7 and 8 +@item 1,2,3 @tab 1, 2, 3 +@item 2-4 @tab 2, 3, 4 +@item 1,3-5,8 @tab 1, 3, 4, 5, 8 +@item 1-3,5-8 @tab 1, 2, 3, 5, 6, 7, 8 @end multitable @item -B[@var{size}][,@var{value}] @itemx --block[=@var{size}][,@var{value}] -Test block errors of given @var{size}, simulating a whole sector I/O -error. Block @var{size} defaults to 512 bytes. @var{value} defaults to -0. By default, only blocks aligned to a @var{size}-byte boundary are -tested, but this may be changed with the @code{--delta} option. +Test block errors of given @var{size}, simulating a whole sector I/O error. +@var{size} defaults to 512 bytes. @var{value} defaults to 0. By default, +only contiguous, non-overlapping blocks are tested, but this may be changed +with the option @samp{--delta}. @item -d @var{n} @itemx --delta=@var{n} -Test only one byte, block, or truncation size every @var{n} bytes, -instead of all of them. If the @code{--block} option is given, @var{n} -defaults to the block size. Else @var{n} defaults to 1. Values of -@var{n} smaller than the block size will result in overlappinng blocks. -(Which is convenient for testing because there are usually too few -non-overlappinng blocks in a file). +Test one byte, block, or truncation size every @var{n} bytes. If +@samp{--delta} is not specified, unzcrash tests all the bytes, +non-overlapping blocks, or truncation sizes. Values of @var{n} smaller than +the block size will result in overlapping blocks. (Which is convenient for +testing because there are usually too few non-overlapping blocks in a file). @item -e @var{position},@var{value} @itemx --set-byte=@var{position},@var{value} Set byte at @var{position} to @var{value} in the internal buffer after -reading and testing @var{file}.lz but before the first test call to the -decompressor. If @var{value} is preceded by @samp{+}, it is added to the -original value of the byte at @var{position}. If @var{value} is preceded -by @samp{f} (flip), it is XORed with the original value of the byte at -@var{position}. This option can be used to run tests with a changed -dictionary size, for example. +reading and testing @var{file} but before the first test call to the +decompressor. Byte positions start at 0. If @var{value} is preceded by +@samp{+}, it is added to the original value of the byte at @var{position}. +If @var{value} is preceded by @samp{f} (flip), it is XORed with the original +value of the byte at @var{position}. This option can be used to run tests +with a changed dictionary size, for example. @item -n @itemx --no-verify -Skip initial verification of @var{file}.lz and @samp{zcmp}. May speed up -things a lot when testing many (or large) known good files. +Skip initial verification of @var{file} and @samp{zcmp}. May speed up things +a lot when testing many (or large) known good files. @item -p @var{bytes} @itemx --position=@var{bytes} @@ -1097,13 +1516,13 @@ Quiet operation. Suppress all messages. @item -s @var{bytes} @itemx --size=@var{bytes} Number of byte positions to test. If not specified, the rest of the file -is tested (from @code{--position} to end of file). Negative values are +is tested (from @samp{--position} to end of file). Negative values are relative to the rest of the file. @item -t @itemx --truncate Test all possible truncation points in the range specified by -@code{--position} and @code{--size}. +@samp{--position} and @samp{--size}. @item -v @itemx --verbose @@ -1111,11 +1530,11 @@ Verbose mode. @item -z @itemx --zcmp=<command> -Set zcmp command name and options. Defaults to @code{zcmp}. Use -@code{--zcmp=false} to disable comparisons. If testing a decompressor +Set zcmp command name and options. Defaults to @samp{zcmp}. Use +@samp{--zcmp=false} to disable comparisons. If testing a decompressor different from the one used by default by zcmp, it is needed to force unzcrash and zcmp to use the same decompressor with a command like -@w{@code{unzcrash --zcmp='zcmp --lz=plzip' 'plzip -t' @var{file}.lz}} +@w{@samp{unzcrash --zcmp='zcmp --lz=plzip' 'plzip -t' @var{file}}} @end table @@ -1137,7 +1556,7 @@ for all eternity, if not longer. If you find a bug in lziprecover, please send electronic mail to @email{lzip-bug@@nongnu.org}. Include the version number, which you can -find by running @w{@code{lziprecover --version}}. +find by running @w{@samp{lziprecover --version}}. @node Concept index |