This manual is for Tarlz (version 0.13, 27 February 2019).

Copyright © 2013-2019 Antonio Diaz Diaz.

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

Introduction
************

Tarlz is a massively parallel (multi-threaded) combined implementation of the tar archiver and the lzip compressor. Tarlz creates, lists and extracts archives in a simplified posix pax format compressed with lzip, keeping the alignment between tar members and lzip members. This method adds an indexed lzip layer on top of the tar archive, making it possible to decode the archive safely in parallel. 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. Tarlz can append files to the end of such compressed archives.

Tarlz can create tar archives with five levels of compression granularity; per file, per block (default), per directory, appendable solid, and solid.

Of course, compressing each file (or each directory) individually can't achieve a compression ratio as high as compressing solidly the whole tar archive, but it has the following advantages:

  * The resulting multimember tar.lz archive can be decompressed in parallel, multiplying the decompression speed.

  * New members can be appended to the archive (by removing the EOF member) just like to an uncompressed tar archive.

  * It is a safe posix-style backup format. 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. Moreover, the option --keep-damaged can be used to recover as much data as possible from each damaged member, and lziprecover can be used to recover some of the damaged members.

  * A multimember tar.lz archive is usually smaller than the corresponding solidly compressed tar.gz archive, except when individually compressing files smaller than about 32 KiB.

Tarlz protects the extended records with a CRC in a way compatible with standard tar tools. See crc32.

Tarlz does not understand other tar formats like gnu, oldgnu, star or v7.

Invoking tarlz
**************

The format for running tarlz is:

     tarlz [OPTIONS] [FILES]

On archive creation or appending tarlz archives the files specified, but removes from member names any leading and trailing slashes and any filename prefixes containing a .. component. On extraction, leading and trailing slashes are also removed from member names, and archive members containing a .. component in the filename are skipped. Tarlz detects when the archive being created or enlarged is among the files to be dumped, appended or concatenated, and skips it.

On extraction and listing, tarlz removes leading ./ strings from member names in the archive or given in the command line, so that tarlz -xf foo ./bar baz extracts members bar and ./baz from archive foo.

If several compression levels or --*solid options are given, the last setting is used. For example -9 --solid --uncompressed -1 is equivalent to -1 --solid

tarlz supports the following options:

-h
--help
     Print an informative help message describing the options and exit.

-V
--version
     Print the version number of tarlz on the standard output and exit. This version number should be included in all bug reports.

-A
--concatenate
     Append tar.lz archives to the end of a tar.lz archive. All the archives involved must be regular (seekable) files compressed as multimember lzip files, and the two end-of-file blocks plus any zero padding must be contained in the last lzip member of each archive. The intermediate end-of-file blocks are removed as each new archive is concatenated. Exit with status 0 without modifying the archive if no FILES have been specified. Tarlz can't concatenate uncompressed tar archives.

-B BYTES
--data-size=BYTES
     Set target size of input data blocks for the --bsolid option. See --bsolid. Valid values range from 8 KiB to 1 GiB. Default value is two times the dictionary size, except for option -0 where it defaults to 1 MiB. See Minimum archive sizes.

-c
--create
     Create a new archive from FILES.

-C DIR
--directory=DIR
     Change to directory DIR. When creating or appending, the position of each -C option in the command line is significant; it will change the current working directory for the following FILES until a new -C option appears in the command line. When extracting, all the -C options are executed in sequence before starting the extraction. Listing ignores any -C options specified. DIR is relative to the then current working directory, perhaps changed by a previous -C option.

     Note that a process can only have one current working directory (CWD). Therefore multi-threading can't be used to create an archive if a -C option appears after a relative filename in the command line.

-d
--diff
     Find differences between archive and file system. For each tar member in the archive, verify that the corresponding file exists and is of the same type (regular file, directory, etc). Report the differences found in type, mode (permissions), owner and group IDs, modification time, file size, file contents (of regular files), target (of symlinks) and device number (of block/character special files).

--ignore-ids
     Make --diff ignore differences in owner and group IDs. This option is useful when comparing an --anonymous archive.

-f ARCHIVE
--file=ARCHIVE
     Use archive file ARCHIVE. - used as an ARCHIVE argument reads from standard input or writes to standard output.

-n N
--threads=N
     Set the number of (de)compression threads, overriding the system's default. Valid values range from 0 to "as many as your system can support". A value of 0 disables threads entirely. If this option is not used, tarlz tries to detect the number of processors in the system and use it as default value. tarlz --help shows the system's default value. See the note about multi-threaded archive creation in the -C option above. Multi-threaded extraction of files from an archive is not yet implemented. See Multi-threaded tar.

     Note that the number of usable threads is limited during compression to ceil( uncompressed_size / data_size ) (see Minimum archive sizes), and during decompression to the number of lzip members in the tar.lz archive, which you can find by running lzip -lv archive.tar.lz.

-q
--quiet
     Quiet operation. Suppress all messages.

-r
--append
     Append files to the end of a tar.lz archive. The archive must be a regular (seekable) file compressed as a multimember lzip file, and the two end-of-file blocks plus any zero padding must be contained in the last lzip member of the archive. First this last member is removed, then the new members are appended, and then a new end-of-file member is appended to the archive. Exit with status 0 without modifying the archive if no FILES have been specified. Tarlz can't append files to an uncompressed tar archive.

-t
--list
     List the contents of an archive. If FILES are given, list only the FILES given.

-v
--verbose
     Verbosely list files processed.

-x
--extract
     Extract files from an archive. If FILES are given, extract only the FILES given. Else extract all the files in the archive.

-0 .. -9
     Set the compression level. The default compression level is -6. Like lzip, tarlz also minimizes the dictionary size of the lzip members it creates, reducing the amount of memory required for decompression.

     Level   Dictionary size   Match length limit
     -0   64 KiB    16 bytes
     -1    1 MiB     5 bytes
     -2    1.5 MiB     6 bytes
     -3    2 MiB     8 bytes
     -4    3 MiB    12 bytes
     -5    4 MiB    20 bytes
     -6    8 MiB    36 bytes
     -7   16 MiB    68 bytes
     -8   24 MiB   132 bytes
     -9   32 MiB   273 bytes

--uncompressed
     With --create, don't compress the created tar archive. Create an uncompressed tar archive instead.

--asolid
     When creating or appending to a compressed archive, use appendable solid compression. All the files being added to the archive are compressed into a single lzip member, but the end-of-file blocks are compressed into a separate lzip member. This creates a solidly compressed appendable archive. Solid archives can't be created nor decoded in parallel.

--bsolid
     When creating or appending to a compressed archive, use block compression. Tar members are compressed together in a lzip member until they approximate a target uncompressed size. The size can't be exact because each solidly compressed data block must contain an integer number of tar members. Block compression is the default because it improves compression ratio for archives with many files smaller than the block size. This option allows tarlz revert to default behavior if, for example, it is invoked through an alias like tar='tarlz --solid'. See --data-size, to set the target block size.

--dsolid
     When creating or appending to a compressed archive, compress each file specified in the command line separately in its own lzip member, and use solid compression for each directory specified in the command line. The end-of-file blocks are compressed into a separate lzip member. This creates a compressed appendable archive with a separate lzip member for each file or top-level directory specified.

--no-solid
     When creating or appending to a compressed archive, compress each file separately in its own lzip member. The end-of-file blocks are compressed into a separate lzip member. This creates a compressed appendable archive with a lzip member for each file.

--solid
     When creating or appending to a compressed archive, use solid compression. The files being added to the archive, along with the end-of-file blocks, are compressed into a single lzip member. The resulting archive is not appendable. No more files can be later appended to the archive. Solid archives can't be created nor decoded in parallel.

--anonymous
     Equivalent to --owner=root --group=root.

--owner=OWNER
     When creating or appending, use OWNER for files added to the archive. If OWNER is not a valid user name, it is decoded as a decimal numeric user ID.

--group=GROUP
     When creating or appending, use GROUP for files added to the archive. If GROUP is not a valid group name, it is decoded as a decimal numeric group ID.

--keep-damaged
     Don't delete partially extracted files. If a decompression error happens while extracting a file, keep the partial data extracted. Use this option to recover as much data as possible from each damaged member.

--missing-crc
     Exit with error status 2 if the CRC of the extended records is missing. When this option is used, tarlz detects any corruption in the extended records (only limited by CRC collisions). But note that a corrupt GNU.crc32 keyword, for example GNU.crc33, is reported as a missing CRC instead of as a corrupt record. This misleading Missing CRC message is the consequence of a flaw in the posix pax format; i.e., the lack of a mandatory check sequence in the extended records. See crc32.

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 tarlz to panic.

File format
***********

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 or a fixed but large number of bytes (for example 512).

A tar.lz file consists of a series of lzip members (compressed data sets). The members simply appear one after another in the file, with no additional information before, between, or after them.

Each lzip member contains one or more tar members in a simplified posix pax interchange format; the only pax typeflag value supported by tarlz (in addition to the typeflag values defined by the ustar format) is x. The pax format is an extension on top of the ustar format that removes the size limitations of the ustar format.

Each tar member contains one file archived, and is represented by the following sequence:

  * An optional extended header block with extended header records. This header block is of the form described in pax header block, with a typeflag value of x. The extended header records are included as the data for this header block.

  * A header block in ustar format that describes the file. Any fields defined in the preceding optional extended header records override the associated fields in this header block for this file.

  * Zero or more blocks that contain the contents of the file.

Each tar member must be contiguously stored in a lzip member for the parallel decoding operations like --list to work. If any tar member is split over two or more lzip members, the archive must be decoded sequentially. See Multi-threaded tar.

At the end of the archive file there are two 512-byte blocks filled with binary zeros, interpreted as an end-of-archive indicator. These EOF blocks are either compressed in a separate lzip member or compressed along with the tar members contained in the last lzip member.

The diagram below shows the correspondence between each tar member (formed by one or two headers plus optional data) in the tar archive and each lzip member in the resulting multimember tar.lz archive, when per file compression is used: See File format (lzip).

tar
+========+======+=================+===============+========+======+========+
| header | data | extended header | extended data | header | data |   EOF  |
+========+======+=================+===============+========+======+========+

tar.lz
+===============+=================================================+========+
|     member    |                      member                     | member |
+===============+=================================================+========+

Pax header block
================

The pax header block is identical to the ustar header block described below except that the typeflag has the value x (extended). The size field is the size of the extended header data in bytes. Most other fields in the pax header block are zeroed on archive creation to prevent trouble if the archive is read by an ustar tool, and are ignored by tarlz on archive extraction. See flawed-compat.

The pax extended header data consists of one or more records, each of them constructed as follows:

"%d %s=%s\n", <length>, <keyword>, <value>

The <length>, <blank>, <keyword>, <equals-sign>, and <newline> in the record must be limited to the portable character set. The <length> field contains the decimal length of the record in bytes, including the trailing <newline>. The <value> field is stored as-is, without conversion to UTF-8 nor any other transformation.

These are the <keyword> fields currently supported by tarlz:

linkpath
     The pathname of a link being created to another file, of any type, previously archived. This record overrides the linkname field in the following ustar header block. The following ustar header block determines the type of link created. If typeflag of the following header block is 1, it will be a hard link. If typeflag is 2, it will be a symbolic link and the linkpath value will be used as the contents of the symbolic link.

path
     The pathname of the following file. This record overrides the name and prefix fields in the following ustar header block.

size
     The size of the file in bytes, expressed as a decimal number using digits from the ISO/IEC 646:1991 (ASCII) standard. This record overrides the size field in the following ustar header block. The size record is used only for files with a size value greater than 8_589_934_591 (octal 77777777777). This is 2^33 bytes or larger.

GNU.crc32
     CRC32-C (Castagnoli) of the extended header data excluding the 8 bytes representing the CRC <value> itself. The <value> is represented as 8 hexadecimal digits in big endian order, 22 GNU.crc32=00000000\n. The keyword of the CRC record is protected by the CRC to guarante that corruption is always detected (except in case of CRC collision). A CRC was chosen because a checksum is too weak for a potentially large list of variable sized records. A checksum can't detect simple errors like the swapping of two bytes.

Ustar header block
==================

The ustar header block has a length of 512 bytes and is structured as shown in the following table. All lengths and offsets are in decimal.

Field Name   Offset   Length (in bytes)
name         0   100
mode       100     8
uid        108     8
gid        116     8
size       124    12
mtime      136    12
chksum     148     8
typeflag   156     1
linkname   157   100
magic      257     6
version    263     2
uname      265    32
gname      297    32
devmajor   329     8
devminor   337     8
prefix     345   155

All characters in the header block are coded using the ISO/IEC 646:1991 (ASCII) standard, except in fields storing names for files, users, and groups. For maximum portability between implementations, names should only contain characters from the portable filename character set. But if an implementation supports the use of characters outside of / and the portable filename character set in names for files, users, and groups, tarlz will use the byte values in these names unmodified.

The fields name, linkname, and prefix are null-terminated character strings except when all characters in the array contain non-null characters including the last character.

The name and the prefix fields produce the pathname of the file. A new pathname is formed, if prefix is not an empty string (its first character is not null), by concatenating prefix (up to the first null character), a <slash> character, and name; otherwise, name is used alone. In either case, name is terminated at the first null character. If prefix begins with a null character, it is ignored. In this manner, pathnames of at most 256 characters can be supported. If a pathname does not fit in the space provided, an extended record is used to store the pathname.

The linkname field does not use the prefix to produce a pathname. If the linkname does not fit in the 100 characters provided, an extended record is used to store the linkname.

The mode field provides 12 access permission bits. The following table shows the symbolic name of each bit and its octal value:

Bit Name   Value   Bit Name   Value   Bit Name   Value
S_ISUID   04000   S_ISGID   02000   S_ISVTX   01000
S_IRUSR   00400   S_IWUSR   00200   S_IXUSR   00100
S_IRGRP   00040   S_IWGRP   00020   S_IXGRP   00010
S_IROTH   00004   S_IWOTH   00002   S_IXOTH   00001

The uid and gid fields are the user and group ID of the owner and group of the file, respectively.

The size field contains the octal representation of the size of the file in bytes. If the typeflag field specifies a file of type '0' (regular file) or '7' (high performance regular file), the number of logical records following the header is (size / 512) rounded to the next integer. For all other values of typeflag, tarlz either sets the size field to 0 or ignores it, and does not store or expect any logical records following the header. If the file size is larger than 8_589_934_591 bytes (octal 77777777777), an extended record is used to store the file size.

The mtime field contains the octal representation of the modification time of the file at the time it was archived, obtained from the stat() function.

The chksum field contains the octal representation of the value of the simple sum of all bytes in the header logical record. Each byte in the header is treated as an unsigned value. When calculating the checksum, the chksum field is treated as if it were all <space> characters.

The typeflag field contains a single character specifying the type of file archived:

'0'
     Regular file.

'1'
     Hard link to another file, of any type, previously archived.

'2'
     Symbolic link.

'3', '4'
     Character special file and block special file respectively. In this case the devmajor and devminor fields contain information defining the device in unspecified format.

'5'
     Directory.

'6'
     FIFO special file.

'7'
     Reserved to represent a file to which an implementation has associated some high-performance attribute. Tarlz treats this type of file as a regular file (type 0).

The magic field contains the ASCII null-terminated string "ustar". The version field contains the characters "00" (0x30,0x30). The fields uname, and gname are null-terminated character strings except when all characters in the array contain non-null characters including the last character. Each numeric field contains a leading space- or zero-filled, optionally null-terminated octal number using digits from the ISO/IEC 646:1991 (ASCII) standard. Tarlz is able to decode numeric fields 1 byte larger than standard ustar by not requiring a terminating null character.

The reasons for the differences with pax
****************************************

Tarlz is meant to reliably detect invalid or corrupt metadata during extraction and to not create safety risks in the archives it creates. In order to achieve these goals, tarlz makes some changes to the variant of the pax format that it uses. This chapter describes these changes and the concrete reasons to implement them.

Add a CRC of the extended records
=================================

The posix pax format has a serious flaw. The metadata stored in pax extended records are not protected by any kind of check sequence. Corruption in a long filename may cause the extraction of the file in the wrong place without warning. Corruption in a large file size may cause the truncation of the file or the appending of garbage to the file, both followed by a spurious warning about a corrupt header far from the place of the undetected corruption.

Metadata like filename and file size must be always protected in an archive format because of the adverse effects of undetected corruption in them, potentially much worse that undetected corruption in the data. Even more so in the case of pax because the amount of metadata it stores is potentially large, making undetected corruption more probable.

Because of the above, tarlz protects the extended records with a CRC in a way compatible with standard tar tools. See key_crc32.

Remove flawed backward compatibility
====================================

In order to allow the extraction of pax archives by a tar utility conforming to the POSIX-2:1993 standard, POSIX.1-2008 recommends selecting extended header field values that allow such tar to create a regular file containing the extended header records as data. This approach is broken because if the extended header is needed because of a long filename, the name and prefix fields will be unable to contain the full pathname of the file. Therefore the files corresponding to both the extended header and the overridden ustar header will be extracted using truncated filenames, perhaps overwriting existing files or directories. It may be a security risk to extract a file with a truncated filename.

To avoid this problem, tarlz writes extended headers with all fields zeroed except size, chksum, typeflag, magic and version. This prevents old tar programs from extracting the extended records as a file in the wrong place. Tarlz also sets to zero those fields of the ustar header overridden by extended records.

If an extended header is required for any reason (for example a file size larger than 8 GiB or a link name longer than 100 bytes), tarlz moves the filename also to the extended header to prevent an ustar tool from trying to extract the file or link. This also makes easier during parallel extraction or listing the detection of a tar member split between two lzip members at the boundary between the extended header and the ustar header.

As simple as possible (but not simpler)
=======================================

The tarlz format is mainly ustar. Extended pax headers are used only when needed because the length of a filename or link name, or the size of a file exceed the limits of the ustar format. Adding extended headers to each member just to record subsecond timestamps seems wasteful for a backup format.

Avoid misconversions to/from UTF-8
==================================

There is no portable way to tell what charset a text string is coded into. Therefore, tarlz stores all fields representing text strings unmodified, without conversion to UTF-8 nor any other transformation. This prevents accidental double UTF-8 conversions. If the need arises this behavior will be adjusted with a command line option in the future.

Limitations of parallel tar decoding
************************************

Safely decoding an arbitrary tar archive in parallel is impossible. For example, if a tar archive containing another tar archive is decoded starting from some position other than the beginning, there is no way to know if the first header found there belongs to the outer tar archive or to the inner tar archive. Tar is a format inherently serial; it was designed for tapes.

In the case of compressed tar archives, the start of each compressed block determines one point through which the tar archive can be decoded in parallel. Therefore, in tar.lz archives the decoding operations can't be parallelized if the tar members are not aligned with the lzip members. Tar archives compressed with plzip can't be decoded in parallel because tar and plzip do not have a way to align both sets of members. Certainly one can decompress one such archive with a multi-threaded tool like plzip, but the increase in speed is not as large as it could be because plzip must serialize the decompressed data and pass them to tar, which decodes them sequentially, one tar member at a time.

On the other hand, if the tar.lz archive is created with a tool like tarlz, which can guarantee the alignment between tar members and lzip members because it controls both archiving and compression, then the lzip format becomes an indexed layer on top of the tar archive which makes possible decoding it safely in parallel.

Tarlz is able to automatically decode aligned and unaligned multimember tar.lz archives, keeping backwards compatibility. If tarlz finds a member misalignment during multi-threaded decoding, it switches to single-threaded mode and continues decoding the archive. Currently only the --list option is able to do multi-threaded decoding.

If the files in the archive are large, multi-threaded --list on a regular (seekable) tar.lz archive can be hundreds of times faster than sequential --list because, in addition to using several processors, it only needs to decompress part of each lzip member. See the following example listing the Silesia corpus on a dual core machine:

     tarlz -9 --no-solid -cf silesia.tar.lz silesia
     time lzip -cd silesia.tar.lz | tar -tf -            (5.032s)
     time plzip -cd silesia.tar.lz | tar -tf -           (3.256s)
     time tarlz -tf silesia.tar.lz                       (0.020s)

Minimum archive sizes required for multi-threaded block compression
*******************************************************************

When creating or appending to a compressed archive using multi-threaded block compression, tarlz puts tar members together in blocks and compresses as many blocks simultaneously as worker threads are chosen, creating a multimember compressed archive.

For this to work as expected (and roughly multiply the compression speed by the number of available processors), the uncompressed archive must be at least as large as the number of worker threads times the block size (see --data-size). Else some processors will not get any data to compress, and compression will be proportionally slower. The maximum speed increase achievable on a given file is limited by the ratio (uncompressed_size / data_size). For example, a tarball the size of gcc or linux will scale up to 10 or 12 processors at level -9.

The following table shows the minimum uncompressed archive size needed for full use of N processors at a given compression level, using the default data size for each level:

Processors   2   4   8   16   64   256
Level
-0     2 MiB     4 MiB     8 MiB    16 MiB    64 MiB   256 MiB
-1     4 MiB     8 MiB    16 MiB    32 MiB   128 MiB   512 MiB
-2     6 MiB    12 MiB    24 MiB    48 MiB   192 MiB   768 MiB
-3     8 MiB    16 MiB    32 MiB    64 MiB   256 MiB     1 GiB
-4    12 MiB    24 MiB    48 MiB    96 MiB   384 MiB   1.5 GiB
-5    16 MiB    32 MiB    64 MiB   128 MiB   512 MiB     2 GiB
-6    32 MiB    64 MiB   128 MiB   256 MiB     1 GiB     4 GiB
-7    64 MiB   128 MiB   256 MiB   512 MiB     2 GiB     8 GiB
-8    96 MiB   192 MiB   384 MiB   768 MiB     3 GiB    12 GiB
-9   128 MiB   256 MiB   512 MiB     1 GiB     4 GiB    16 GiB

A small tutorial with examples
******************************

Example 1: Create a multimember compressed archive archive.tar.lz containing files a, b and c.

     tarlz -cf archive.tar.lz a b c

Example 2: Append files d and e to the multimember compressed archive archive.tar.lz.

     tarlz -rf archive.tar.lz d e

Example 3: Create a solidly compressed appendable archive archive.tar.lz containing files a, b and c. Then append files d and e to the archive.

     tarlz --asolid -cf archive.tar.lz a b c
     tarlz --asolid -rf archive.tar.lz d e

Example 4: Create a compressed appendable archive containing directories dir1, dir2 and dir3 with a separate lzip member per directory. Then append files a, b, c, d and e to the archive, all of them contained in a single lzip member. The resulting archive archive.tar.lz contains 5 lzip members (including the EOF member).

     tarlz --dsolid -cf archive.tar.lz dir1 dir2 dir3
     tarlz --asolid -rf archive.tar.lz a b c d e

Example 5: Create a solidly compressed archive archive.tar.lz containing files a, b and c. Note that no more files can be later appended to the archive.

     tarlz --solid -cf archive.tar.lz a b c

Example 6: Extract all files from archive archive.tar.lz.

     tarlz -xf archive.tar.lz

Example 7: Extract files a and c from archive archive.tar.lz.

     tarlz -xf archive.tar.lz a c

Example 8: Copy the contents of directory sourcedir to the directory destdir.

     tarlz -C sourcedir -c . | tarlz -C destdir -x

Reporting bugs
**************

There are probably bugs in tarlz. 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 tarlz, please send electronic mail to lzip-bug@nongnu.org. Include the version number, which you can find by running tarlz --version.