This manual is for Lziprecover (version 1.15, 14 September 2013).
Copyright © 2009, 2010, 2011, 2012, 2013 Antonio Diaz Diaz.
This manual is free documentation: you have unlimited permission to copy, distribute and modify it.
Lziprecover is a data recovery tool and decompressor for files in the lzip compressed data format (.lz), able to repair slightly damaged files, recover badly damaged files from two or more copies, extract data from damaged files, decompress files and test integrity of files.
The lzip file format is designed for long-term data archiving. It is clean, provides very safe 4 factor integrity checking, and is backed by the recovery capabilities of lziprecover.
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 makes lzip files resistant to bit-flip (one of the most common forms of data corruption), and can safely merge multiple damaged backup copies.
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.
The format for running lziprecover is:
lziprecover [options] [files]
Lziprecover supports the following options:
The names of the files produced are in the form
`rec01file.lz', `rec02file.lz', etc, and are
designed so that the use of wildcards in subsequent processing, for
example, `lziprecover -cd rec*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 `file'.
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):
|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.
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 `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.
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 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:
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, you can reduce their sizes 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.
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.
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. 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 -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
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
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 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:
unzcrash [options] "lzip -tv" filename.lz
Unzcrash supports the following options:
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.
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 firstname.lastname@example.org. Include the version number, which you can find by running `lziprecover --version'.