path: root/doc/lziprecover.texi

\input texinfo @c -*-texinfo-*-
@c %**start of header
@setfilename lziprecover.info
@documentencoding ISO-8859-15
@settitle Lziprecover Manual
@finalout
@c %**end of header

@set UPDATED 5 April 2014
@set VERSION 1.16-pre1

@dircategory Data Compression
@direntry
* Lziprecover: (lziprecover).   Data recovery tool for lzip files
@end direntry


@ifnothtml
@titlepage
@title Lziprecover
@subtitle Data recovery tool for lzip files
@subtitle for Lziprecover version @value{VERSION}, @value{UPDATED}
@author by Antonio Diaz Diaz

@page
@vskip 0pt plus 1filll
@end titlepage

@contents
@end ifnothtml

@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
* 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
* Unzcrash::               Testing the robustness of decompressors
* Problems::               Reporting bugs
* Concept index::          Index of concepts
@end menu

@sp 1
Copyright @copyright{} 2009, 2010, 2011, 2012, 2013, 2014
Antonio Diaz Diaz.

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


@node Introduction
@chapter Introduction
@cindex introduction

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 data
from damaged files, decompress files and test integrity of files.

The lzip file format is designed for long-term data 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.

@item
The lzip format is as simple as possible (but not simpler). The lzip
manual provides the 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.

@item
Additionally lzip is copylefted, which guarantees that it will remain
free forever.
@end itemize

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 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
@end example

Lziprecover is able to efficiently extract a range of bytes from a
multi-member file, because it only decompresses the members containing
the desired data.

Lziprecover can print correct total file sizes and ratios even for
multi-member files.

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 never modified.

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

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

The format for running lziprecover is:

@example
lziprecover [@var{options}] [@var{files}]
@end example

Lziprecover supports the following options:

@table @samp
@item -h
@itemx --help
Print an informative help message describing the options and exit.

@item -V
@itemx --version
Print the version number of lziprecover on the standard output and exit.

@item -c
@itemx --stdout
Decompress to standard output. Needed when reading from a named pipe
(fifo) or from a device. Use it to recover as much of the uncompressed
data as possible when decompressing a corrupt file.

@item -d
@itemx --decompress
Decompress.

@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}}.
Three formats of @var{range} are recognized, @samp{@var{begin}},
@samp{@var{begin}-@var{end}}, and @samp{@var{begin},@var{size}}. If only
@var{begin} is specified, @var{end} is taken as the end of the file. The
produced bytes are sent to standard output unless the @samp{--output}
option is used. 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. This operation is more
efficient in multi-member files because it only decompresses the members
containing the desired data.

@item -f
@itemx --force
Force overwrite of output files.

@item -i
@itemx --ignore-errors
Make @samp{--range-decompress} ignore data errors and continue
decompressing the remaining members in the file. For example,
@w{@samp{lziprecover -i -D0 file.lz > file}} decompresses all the
recoverable data in all members of @samp{file.lz} without having to
split it first.

@item -k
@itemx --keep
Keep (don't delete) input files during decompression.

@item -l
@itemx --list
Print total file sizes and ratios. The values produced are correct even
for multi-member files. Use it together with @samp{-v} to see
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. 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 @samp{Merging files}
(@pxref{Merging files}) for a complete 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.

@item -q
@itemx --quiet
Quiet operation. Suppress all messages.

@item -R
@itemx --repair
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 @samp{Repairing files} (@pxref{Repairing files}) 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
@samp{.lz} 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.

The names of the files produced are in the form
@samp{rec01@var{file}.lz}, @samp{rec02@var{file}.lz}, etc, and are
designed so that the use of wildcards in subsequent processing, for
example, @w{@samp{lziprecover -cd rec*@var{file}.lz > 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}}.

@item -t
@itemx --test
Check integrity of the specified file(s), 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 file.

@item -v
@itemx --verbose
Verbose mode.@*
When decompressing or testing, further -v's (up to 4) increase the
verbosity level, showing status, compression ratio, dictionary size,
trailer contents (CRC, data size, member size), and up to 6 bytes of
trailing garbage (if any).

@end table

Numbers given as arguments to options may be followed by a multiplier
and an optional @samp{B} for "byte".

Table of SI and binary prefixes (unit multipliers):

@multitable {Prefix} {kilobyte  (10^3 = 1000)} {|} {Prefix} {kibibyte (2^10 = 1024)}
@item Prefix @tab Value               @tab | @tab Prefix @tab Value
@item k @tab kilobyte  (10^3 = 1000)  @tab | @tab Ki @tab kibibyte (2^10 = 1024)
@item M @tab megabyte  (10^6)         @tab | @tab Mi @tab mebibyte (2^20)
@item G @tab gigabyte  (10^9)         @tab | @tab Gi @tab gibibyte (2^30)
@item T @tab terabyte  (10^12)        @tab | @tab Ti @tab tebibyte (2^40)
@item P @tab petabyte  (10^15)        @tab | @tab Pi @tab pebibyte (2^50)
@item E @tab exabyte   (10^18)        @tab | @tab Ei @tab exbibyte (2^60)
@item Z @tab zettabyte (10^21)        @tab | @tab Zi @tab zebibyte (2^70)
@item Y @tab yottabyte (10^24)        @tab | @tab Yi @tab yobibyte (2^80)
@end multitable

@sp 1
Exit status: 0 for a normal exit, 1 for environmental problems (file not
found, invalid flags, I/O errors, etc), 2 to indicate a corrupt or
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.

All the copies must have the same size. If some of them have been
truncated and are therefore smaller than they should, you can extend
them to the correct size with the following command before merging them
with the other copies:

@example
ddrescue --extend-outfile=<correct_size> small_file.lz extended_file.lz
@end example

If some of the copies have got garbage data at the end and are therefore
larger than they should, you can reduce their sizes to the correct value
with the following command before merging them with the other copies:

@example
ddrescue --size=<correct_size> large_file.lz reduced_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.


@node File format
@chapter File format
@cindex file format

Perfection is reached, not when there is no longer anything to add, but
when there is no longer anything to take away.@*
--- Antoine de Saint-Exupery

@sp 1
In the diagram below, a box like this:
@verbatim
+---+
|   | <-- the vertical bars might be missing
+---+
@end verbatim

represents one byte; a box like this:
@verbatim
+==============+
|              |
+==============+
@end verbatim

represents a variable number of bytes.

@sp 1
A lzip file consists of a series of "members" (compressed data sets).
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  |
+--+--+--+--+----+----+=============+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
@end verbatim

All multibyte values are stored in little endian order.

@table @samp
@item ID string
A four byte string, identifying the lzip format, with the value "LZIP"
(0x4C, 0x5A, 0x49, 0x50).

@item VN (version number, 1 byte)
Just in case something needs to be modified in the future. 1 for now.

@item DS (coded dictionary size, 1 byte)
Lzip divides the distance between any two powers of 2 into 8 equally
spaced intervals, named "wedges". The dictionary size is calculated by
taking a power of 2 (the base size) and substracting from it a number of
wedges between 0 and 7. The size of a wedge is (base_size / 16).@*
Bits 4-0 contain the base 2 logarithm of the base size (12 to 29).@*
Bits 7-5 contain the number of wedges (0 to 7) to substract from the
base size to obtain the dictionary size.@*
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. See the lzip manual for a full description.

@item CRC32 (4 bytes)
CRC of the uncompressed original data.

@item Data size (8 bytes)
Size of the uncompressed original data.

@item Member size (8 bytes)
Total size of the member, including header and trailer. This field acts
as a distributed index, allows the verification of stream integrity, and
facilitates safe recovery of undamaged members from multi-member files.

@end table


@node Examples
@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
lziprecover -d file.lz
@end example

@sp 1
@noindent
Example 2: Verify the integrity of the compressed file @samp{file.lz}
and show status.

@example
lziprecover -tv file.lz
@end example

@sp 1
@noindent
Example 3: Decompress @samp{file.lz} partially until 10 KiB of
decompressed data are produced.

@example
lziprecover -D 0,10KiB file.lz
@end example

@sp 1
@noindent
Example 4: Decompress @samp{file.lz} partially from decompressed byte
10000 to decompressed byte 15000 (5000 bytes are produced).

@example
lziprecover -D 10000-15000 file.lz
@end example

@sp 1
@noindent
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
  Copy of input file repaired successfully.
mv file_fixed.lz file.lz
@end example

@sp 1
@noindent
Example 6: Split the multi-member 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 7: 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 -b2048 /dev/cdrom cdimage1 logfile1
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 -b2048 /dev/cdrom cdimage2 logfile2
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
@end example

@sp 1
@noindent
Example 8: 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
@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.

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. This should not cause any invalid memory
accesses. If it does, please, report it as a bug.

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 you can use
unzcrash to test any decompressor (not only lzip), or even other decoder
programs with a suitable command line syntax.

The format for running unzcrash is:

@example
unzcrash [@var{options}] "lzip -tv" @var{filename}.lz
@end example

Unzcrash supports the following options:

@table @samp
@item -h
@itemx --help
Print an informative help message describing the options and exit.

@item -V
@itemx --version
Print the version number of unzcrash on the standard output and exit.

@item -b @var{range}
@itemx --bits=@var{range}
Test N-bit errors only, instead of testing all the 255 wrong values for
each byte. @samp{N-bit error} means any value differing from the
original value in N bit positions, not a value differing from the
original value in the bit position N.@*
The number of N-bit errors per byte (N = 1 to 8) is: 8 28 56 70 56 28 8 1@*
Examples of @var{range}:  1  1,2,3  1-4  1,3-5,8  1-3,5-8

@item -p @var{bytes}
@itemx --position=@var{bytes}
First byte position to test in the file. Defaults to 0.

@item -q
@itemx --quiet
Quiet operation. Suppress all messages.

@item -s @var{bytes}
@itemx --size=@var{bytes}
Number of byte positions to test. If not specified, the whole file is
tested.

@item -v
@itemx --verbose
Verbose mode.

@end table

Exit status: 0 for a normal exit, 1 for environmental problems (file not
found, invalid flags, I/O errors, etc), 2 to indicate a corrupt or
invalid input file, 3 for an internal consistency error (eg, bug) which
caused unzcrash to panic.


@node Problems
@chapter Reporting bugs
@cindex bugs
@cindex getting help

There are probably bugs in lziprecover. There are certainly errors and
omissions in this manual. If you report them, they will get fixed. If
you don't, no one will ever know about them and they will remain unfixed
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{@samp{lziprecover --version}}.


@node Concept index
@unnumbered Concept index

@printindex cp

@bye