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 8 April 2015
@set VERSION 1.17-rc2

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


@ifnothtml
@titlepage
@title Lziprecover
@subtitle Data recovery tool for the lzip format
@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
* Data safety::            Protecting data from accidental loss
* Repairing files::        Fixing bit-flip and similar errors
* Merging files::          Fixing several damaged copies
* File names::             Names of the files produced by lziprecover
* 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{} 2009-2015 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, 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.

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.

@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 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.

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 by 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}},
@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}}.

@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 Data safety
@chapter Protecting data from accidental loss
@cindex data safety

There are 3 main types of data corruption that may cause data loss:
single-byte errors, multi-byte 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 also protects against multi-byte errors (@pxref{Merging
files}), if at least one backup copy of the file is made.

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?
Lets suppose that you made a backup copy of your valuable scientific
data, compressed it, and stored two copies on separate media. Years
later you notice that both copies are corrupt.

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't
be recovered.

If you used bzip2, and if the file is large enough to contain more than
one compressed data block (usually larger than 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 in another
file.

But if you used lzip, the data can be automatically recovered as long as
no byte is damaged in both files.

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 Repairing files
@chapter Repairing files
@cindex repairing files

Lziprecover can repair perfectly most files with small errors (up to one
single-byte error per member), without the need of any extra redundance
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 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). If the error is in the header it
can be easily repaired with a text editor like GNU Moe (@pxref{File
format}). If the error is in the member size, it is enough to ignore the
message about @samp{bad member size} when decompressing.

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 on an USB flash drive.

One byte may seem small, but most file corruptions not produced by 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.

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.

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
repairs more efficiently the worst errors.


@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 by 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, they can be
extended 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, their sizes can be reduced 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. (Supposing that the
errors are randomly located inside each copy).


@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.


@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)
The dictionary size is calculated by taking a power of 2 (the base size)
and substracting 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 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.
@ifnothtml
@xref{Stream format,,,lzip},
@end ifnothtml
@ifhtml
See
@uref{http://www.nongnu.org/lzip/manual/lzip_manual.html#Stream-format,,Stream format}
@end ifhtml
for a complete 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.
lziprecover -tv file_fixed.lz
  ok
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
  Input files merged successfully.
lziprecover -tv backup.tar.lz
  ok
@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.
lziprecover -tv 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.

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 unzcrash can be
used to test any decompressor (not only lzip), or even other decoder
programs having 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:
@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
@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
@end multitable

@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{@code{lziprecover --version}}.


@node Concept index
@unnumbered Concept index

@printindex cp

@bye