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Lziprecover Manual

This manual is for Lziprecover (version 1.17, 28 May 2015).


Copyright © 2009-2015 Antonio Diaz Diaz.

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


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1 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:

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 GNU ddrescue + lziprecover is the best option for recovering data from multiple damaged copies. See 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):

     lziprecover -D0 -i -o file -q file.lz

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.


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2 Invoking lziprecover

The format for running lziprecover is:

     lziprecover [options] [files]

Lziprecover supports the following options:

-h
--help
Print an informative help message describing the options and exit.
-V
--version
Print the version number of lziprecover on the standard output and exit.
-c
--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.
-d
--decompress
Decompress.
-D range
--range-decompress=range
Decompress only a range of bytes starting at decompressed byte position 'begin' and up to byte position 'end - 1'. Three formats of range are recognized, 'begin', 'begin-end', and 'begin,size'. If only begin is specified, end is taken as the end of the file. The produced bytes are sent to standard output unless the '--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.
-f
--force
Force overwrite of output files.
-i
--ignore-errors
Make '--range-decompress' ignore data errors and continue decompressing the remaining members in the file. For example, 'lziprecover -i -D0 file.lz > file' decompresses all the recoverable data in all members of 'file.lz' without having to split it first.
-k
--keep
Keep (don't delete) input files during decompression.
-l
--list
Print total file sizes and ratios. The values produced are correct even for multi-member files. Use it together with '-v' to see information about the members in the file.
-m
--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 'file_fixed.lz'. The exit status is 0 if a correct file could be produced, 2 otherwise. See the chapter 'Merging files' (see Merging files) for a complete description of the merge mode.
-o file
--output=file
Place the output into 'file' instead of into 'file_fixed.lz'. If splitting, the names of the files produced are in the form 'rec01file', 'rec02file', etc. If decompressing from standard input and '--stdout' has not been specified, use 'file' as the name of the decompressed file.
-q
--quiet
Quiet operation. Suppress all messages.
-R
--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 'file_fixed.lz'. 'file' is not modified at all. The exit status is 0 if the file could be repaired, 2 otherwise. See the chapter 'Repairing files' (see Repairing files) for a complete description of the repair mode.
-s
--split
Search for members in 'file' and write each member in its own '.lz' file. You can then use '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 'rec01file', 'rec02file', etc, and are designed so that the use of wildcards in subsequent processing, for example, 'lziprecover -cd rec*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 'file'.

-t
--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 '-v' to see information about the file.
-v
--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).

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

Table of SI and binary prefixes (unit multipliers):

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


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.


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3 Protecting data from accidental loss

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 (see 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 (see 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.


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4 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 '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 (see File format). If the error is in the member size, it is enough to ignore the message about '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.


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5 Merging files

If you have several copies of a file but all of them are too damaged to repair them (see 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:

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

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:

     ddrescue --size=<correct_size> large_file.lz reduced_file.lz

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).


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6 Names of the files produced by lziprecover

The name of the fixed file produced by '--merge' and '--repair' is made by appending the string '_fixed.lz' to the original file name. If the original file name ends with one of the extensions '.tar.lz', '.lz' or '.tlz', the string '_fixed' is inserted before the extension.


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7 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


In the diagram below, a box like this:
+---+
|   | <-- the vertical bars might be missing
+---+

represents one byte; a box like this:

+==============+
|              |
+==============+

represents a variable number of bytes.


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:

+--+--+--+--+----+----+=============+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID string | VN | DS | Lzma stream | CRC32 |   Data size   |  Member size  |
+--+--+--+--+----+----+=============+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

All multibyte values are stored in little endian order.

'ID string'
A four byte string, identifying the lzip format, with the value "LZIP" (0x4C, 0x5A, 0x49, 0x50).
'VN (version number, 1 byte)'
Just in case something needs to be modified in the future. 1 for now.
'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.
'Lzma stream'
The lzma stream, finished by an end of stream marker. Uses default values for encoder properties. See Stream format for a complete description.
'CRC32 (4 bytes)'
CRC of the uncompressed original data.
'Data size (8 bytes)'
Size of the uncompressed original data.
'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.


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8 A small tutorial with examples

Example 1: Restore a regular file from its compressed version 'file.lz'. If the operation is successful, 'file.lz' is removed.

     lziprecover -d file.lz

Example 2: Verify the integrity of the compressed file 'file.lz' and show status.
     lziprecover -tv file.lz

Example 3: Decompress 'file.lz' partially until 10 KiB of decompressed data are produced.
     lziprecover -D 0,10KiB file.lz

Example 4: Decompress 'file.lz' partially from decompressed byte 10000 to decompressed byte 15000 (5000 bytes are produced).
     lziprecover -D 10000-15000 file.lz

Example 5: Repair small errors in the file 'file.lz'. (Indented lines are abridged diagnostic messages from lziprecover).
     lziprecover -v -R file.lz
       Copy of input file repaired successfully.
     lziprecover -tv file_fixed.lz
       ok
     mv file_fixed.lz file.lz

Example 6: Split the multi-member file 'file.lz' and write each member in its own 'recXXXfile.lz' file. Then use 'lziprecover -t' to test the integrity of the resulting files.
     lziprecover -s file.lz
     lziprecover -tv rec*file.lz

Example 7: Recover a compressed backup from two copies on CD-ROM with error-checked merging of copies (See the ddrescue manual for details about ddrescue).
     ddrescue -d -r1 -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 -d -r1 -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

Example 8: Recover the first volume of those created with the command 'lzip -b 32MiB -S 650MB big_db' from two copies, 'big_db1_00001.lz' and '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 'big_db_00001.lz'.
     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


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9 Testing the robustness of decompressors

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 '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:

     unzcrash [options] "lzip -tv" filename.lz

Unzcrash supports the following options:

-h
--help
Print an informative help message describing the options and exit.
-V
--version
Print the version number of unzcrash on the standard output and exit.
-b range
--bits=range
Test N-bit errors only, instead of testing all the 255 wrong values for each byte. '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 range Tests errors of N-bit
1 1
1,2,3 1, 2 and 3
2-4 2, 3 and 4
1,3-5,8 1, 3, 4, 5 and 8
1-3,5-8 1, 2, 3, 5, 6, 7 and 8

-p bytes
--position=bytes
First byte position to test in the file. Defaults to 0.
-q
--quiet
Quiet operation. Suppress all messages.
-s bytes
--size=bytes
Number of byte positions to test. If not specified, the whole file is tested.
-v
--verbose
Verbose mode.

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.


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10 Reporting bugs

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 lzip-bug@nongnu.org. Include the version number, which you can find by running lziprecover --version.


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