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

@set UPDATED 8 October 2019
@set VERSION 0.16

@dircategory Data Compression
@direntry
* Tarlz: (tarlz).               Archiver with multimember lzip compression
@end direntry


@ifnothtml
@titlepage
@title Tarlz
@subtitle Archiver with multimember lzip compression
@subtitle for Tarlz 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 Tarlz (version @value{VERSION}, @value{UPDATED}).

@menu
* Introduction::              Purpose and features of tarlz
* Invoking tarlz::            Command line interface
* Portable character set::    POSIX portable filename character set
* File format::               Detailed format of the compressed archive
* Amendments to pax format::  The reasons for the differences with pax
* Multi-threaded tar::        Limitations of parallel tar decoding
* Minimum archive sizes::     Sizes required for full multi-threaded speed
* Examples::                  A small tutorial with examples
* Problems::                  Reporting bugs
* Concept index::             Index of concepts
@end menu

@sp 1
Copyright @copyright{} 2013-2019 Antonio Diaz Diaz.

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


@node Introduction
@chapter Introduction
@cindex introduction

@uref{http://www.nongnu.org/lzip/tarlz.html,,Tarlz} is a massively parallel
(multi-threaded) combined implementation of the tar archiver and the
@uref{http://www.nongnu.org/lzip/lzip.html,,lzip} compressor. Tarlz creates,
lists and extracts archives in a simplified and safer variant of the POSIX
pax format compressed with lzip, keeping the alignment between tar members
and lzip members. 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.

Keeping the alignment between tar members and lzip members has two
advantages. It adds an indexed lzip layer on top of the tar archive, making
it possible to decode the archive safely in parallel. It also minimizes the
amount of data lost in case of corruption. Compressing a tar archive with
plzip may even double the amount of files lost for each lzip member damaged
because it does not keep the members aligned.

Tarlz can create tar archives with five levels of compression granularity;
per file (---no-solid), per block (---bsolid, default), per directory
(---dsolid), appendable solid (---asolid), and solid (---solid).

@noindent
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:

@itemize @bullet
@item
The resulting multimember tar.lz archive can be decompressed in
parallel, multiplying the decompression speed.

@item
New members can be appended to the archive (by removing the EOF
member), and unwanted members can be deleted from the archive. Just
like an uncompressed tar archive.

@item
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 @samp{--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.

@item
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.
@end itemize

Tarlz protects the extended records with a CRC in a way compatible with
standard tar tools. @xref{crc32}.

Tarlz does not understand other tar formats like @samp{gnu}, @samp{oldgnu},
@samp{star} or @samp{v7}. @w{@samp{tarlz -tf archive.tar.lz > /dev/null}}
can be used to verify that the format of the archive is compatible with
tarlz.


@node Invoking tarlz
@chapter Invoking tarlz
@cindex invoking
@cindex options
@cindex usage
@cindex version

The format for running tarlz is:

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

@noindent
All operations except @samp{--concatenate} operate on whole trees if any
@var{file} is a directory.

On archive creation or appending tarlz archives the files specified, but
removes from member names any leading and trailing slashes and any file name
prefixes containing a @samp{..} component. On extraction, leading and
trailing slashes are also removed from member names, and archive members
containing a @samp{..} component in the file name 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 @samp{./} strings from
member names in the archive or given in the command line, so that
@w{@samp{tarlz -xf foo ./bar baz}} extracts members @samp{bar} and
@samp{./baz} from archive @samp{foo}.

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

tarlz supports the following options:

@table @code
@item --help
Print an informative help message describing the options and exit.

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

@item -A
@itemx --concatenate
Append one or more archives to the end of an archive. All the archives
involved must be regular (seekable) files, and must be either all compressed
or all uncompressed. Compressed and uncompressed archives can't be mixed.
Compressed archives must be multimember lzip files with the two end-of-file
blocks plus any zero padding contained in the last lzip member of each
archive. The intermediate end-of-file blocks are removed as each new archive
is concatenated. If the archive is uncompressed, tarlz parses and skips tar
headers until it finds the end-of-file blocks. Exit with status 0 without
modifying the archive if no @var{files} have been specified.

@anchor{--data-size}
@item -B @var{bytes}
@itemx --data-size=@var{bytes}
Set target size of input data blocks for the @samp{--bsolid} option.
@xref{--bsolid}. Valid values range from @w{8 KiB} to @w{1 GiB}. Default
value is two times the dictionary size, except for option @samp{-0} where it
defaults to @w{1 MiB}. @xref{Minimum archive sizes}.

@item -c
@itemx --create
Create a new archive from @var{files}.

@item -C @var{dir}
@itemx --directory=@var{dir}
Change to directory @var{dir}. When creating or appending, the position of
each @samp{-C} option in the command line is significant; it will change the
current working directory for the following @var{files} until a new
@samp{-C} option appears in the command line. When extracting or comparing,
all the @samp{-C} options are executed in sequence before reading the
archive. Listing ignores any @samp{-C} options specified. @var{dir} is
relative to the then current working directory, perhaps changed by a
previous @samp{-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 @samp{-C}
option appears after a relative file name in the command line.

@item -d
@itemx --diff
Compare and report differences between archive and file system. For each tar
member in the archive, verify that the corresponding file in the file system
exists and is of the same type (regular file, directory, etc). Report on
standard output 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).

As tarlz removes leading slashes from member names, the @samp{-C} option may
be used in combination with @samp{--diff} when absolute file names were used
on archive creation: @w{@samp{tarlz -C / -d}}. Alternatively, tarlz may be
run from the root directory to perform the comparison.

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

@item --delete
Delete the specified files and directories from an archive in place. It
currently can delete only from uncompressed archives and from archives with
individually compressed files (@samp{--no-solid} archives). Note that files
of about @samp{--data-size} or larger are compressed individually even if
@samp{--bsolid} is used, and can therefore be deleted. Tarlz takes care to
not delete a tar member unless it is possible to do so. For example it won't
try to delete a tar member that is not individually compressed. To delete a
directory without deleting the files under it, use
@w{@samp{tarlz --delete -f foo --exclude='dir/*' dir}}. Deleting in place
may be dangerous. A corrupt archive, a power cut, or an I/O error may cause
data loss.

@item --exclude=@var{pattern}
Exclude files matching a shell pattern like @samp{*.o}. A file is considered
to match if any component of the file name matches. For example, @samp{*.o}
matches @samp{foo.o}, @samp{foo.o/bar} and @samp{foo/bar.o}. If
@var{pattern} contains a @samp{/}, it matches a corresponding @samp{/} in
the file name. For example, @samp{foo/*.o} matches @samp{foo/bar.o}.

@item -f @var{archive}
@itemx --file=@var{archive}
Use archive file @var{archive}. @samp{-} used as an @var{archive} argument
reads from standard input or writes to standard output.

@item -h
@itemx --dereference
Follow symbolic links during archive creation, appending or comparison.
Archive or compare the files they point to instead of the links themselves.

@item -n @var{n}
@itemx --threads=@var{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.
@w{@samp{tarlz --help}} shows the system's default value. See the note about
multi-threaded archive creation in the @samp{-C} option above.
Multi-threaded extraction of files from an archive is not yet implemented.
@xref{Multi-threaded tar}.

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

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

@item -r
@itemx --append
Append files to the end of an archive. The archive must be a regular
(seekable) file either compressed or uncompressed. Compressed members can't
be appended to an uncompressed archive, nor vice versa. If the archive is
compressed, it must be a multimember lzip file with the two end-of-file
blocks plus any zero padding contained in the last lzip member of the
archive. It is possible to append files to an archive with a different
compression granularity. Appending works as follows; first the end-of-file
blocks are removed, then the new members are appended, and finally two new
end-of-file blocks are appended to the archive. If the archive is
uncompressed, tarlz parses and skips tar headers until it finds the
end-of-file blocks. Exit with status 0 without modifying the archive if no
@var{files} have been specified.

@item -t
@itemx --list
List the contents of an archive. If @var{files} are given, list only the
@var{files} given.

@item -v
@itemx --verbose
Verbosely list files processed.

@item -x
@itemx --extract
Extract files from an archive. If @var{files} are given, extract only the
@var{files} given. Else extract all the files in the archive. To extract a
directory without extracting the files under it, use
@w{@samp{tarlz -xf foo --exclude='dir/*' dir}}.

@item -0 .. -9
Set the compression level for @samp{--create} and @samp{--append}. The
default compression level is @samp{-6}. Like lzip, tarlz also minimizes the
dictionary size of the lzip members it creates, reducing the amount of
memory required for decompression.

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

@item --uncompressed
With @samp{--create}, don't compress the tar archive created. Create an
uncompressed tar archive instead. With @samp{--append}, don't compress the
new members appended to the tar archive. Compressed members can't be
appended to an uncompressed archive, nor vice versa.

@item --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.

@anchor{--bsolid}
@item --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 @samp{tar='tarlz --solid'}. @xref{--data-size}, to set the target
block size.

@item --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.

@item --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.

@item --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.

@item --anonymous
Equivalent to @samp{--owner=root --group=root}.

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

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

@item --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.

@item --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
@samp{GNU.crc32} keyword, for example @samp{GNU.crc33}, is reported as a
missing CRC instead of as a corrupt record. This misleading
@samp{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. @xref{crc32}.

@item --out-slots=@var{n}
Number of @w{1 MiB} output packets buffered per worker thread during
multi-threaded creation or appending to compressed archives. Increasing the
number of packets may increase compression speed if the files being archived
are larger than @w{64 MiB} compressed, but requires more memory. Valid
values range from 1 to 1024. The default value is 64.

@ignore
@item --permissive
Allow some violations of the archive format, like consecutive extended
headers preceding a ustar header, or several records with the same
keyword appearing in the same block of extended records.
@end ignore

@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 tarlz to panic.


@node Portable character set
@chapter POSIX portable filename character set
@cindex portable character set

The set of characters from which portable file names are constructed.

@example
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
a b c d e f g h i j k l m n o p q r s t u v w x y z
0 1 2 3 4 5 6 7 8 9 . _ -
@end example

The last three characters are the period, underscore, and hyphen-minus
characters, respectively.


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

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

@sp 1
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
@samp{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:

@itemize @bullet
@item
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 @samp{x}. The extended header records are included as
the data for this header block.

@item
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.

@item
Zero or more blocks that contain the contents of the file.
@end itemize

Each tar member must be contiguously stored in a lzip member for the
parallel decoding operations like @samp{--list} to work. If any tar member
is split over two or more lzip members, the archive must be decoded
sequentially. @xref{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
@uref{http://www.nongnu.org/lzip/manual/lzip_manual.html#File-format,,lzip member}
in the resulting multimember tar.lz archive, when per file compression is
used:
@ifnothtml
@xref{File format,,,lzip}.
@end ifnothtml

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

tar.lz
+===============+=================================================+========+
|     member    |                      member                     | member |
+===============+=================================================+========+
@end verbatim

@ignore
When @samp{--permissive} is used, the following violations of the
archive format are allowed:@*
If several extended headers precede an ustar header, only the last
extended header takes effect. The other extended headers are ignored.
Similarly, if several records with the same keyword appear in the same
block of extended records, only the last record for the repeated keyword
takes effect. The other records for the repeated keyword are ignored.@*
A global header inserted between an extended header and an ustar header.@*
An extended header just before the EOF blocks.
@end ignore

@sp 1
@section Pax header block

The pax header block is identical to the ustar header block described below
except that the typeflag has the value @samp{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. @xref{flawed-compat}.

The pax extended header data consists of one or more records, each of
them constructed as follows:@*
@samp{"%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:

@table @code
@item 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.

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

@item 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
@w{(octal 77777777777)}. This is 2^33 bytes or larger.

@anchor{key_crc32}
@item 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,
@w{@samp{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.
@end table

@sp 1
@section 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.

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

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 character set. But if an implementation
supports the use of characters outside of @samp{/} and the portable
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:

@multitable {Bit Name} {Value} {Bit Name} {Value} {Bit Name} {Value}
@headitem Bit Name @tab Value @tab Bit Name @tab Value @tab Bit Name @tab Value
@item S_ISUID @tab 04000 @tab S_ISGID @tab 02000 @tab S_ISVTX @tab 01000
@item S_IRUSR @tab 00400 @tab S_IWUSR @tab 00200 @tab S_IXUSR @tab 00100
@item S_IRGRP @tab 00040 @tab S_IWGRP @tab 00020 @tab S_IXGRP @tab 00010
@item S_IROTH @tab 00004 @tab S_IWOTH @tab 00002 @tab S_IXOTH @tab 00001
@end multitable

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 @w{(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 @w{(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:

@table @code
@item '0'
Regular file.

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

@item '2'
Symbolic link.

@item '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.

@item '5'
Directory.

@item '6'
FIFO special file.

@item '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).

@end table

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 longer than standard
ustar by not requiring a terminating null character.


@node Amendments to pax format
@chapter The reasons for the differences with pax
@cindex Amendments to pax format

Tarlz creates safe archives that allow the reliable detection of invalid or
corrupt metadata during decoding even when the integrity checking of lzip
can't be used because the lzip members are only decompressed partially, as
it happens in parallel @samp{--list} and @samp{--extract}. In order to
achieve this goal, 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.

@sp 1
@anchor{crc32}
@section 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 file name 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 file name 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.

Headers and metadata must be protected separately from data because the
integrity checking of lzip may not be able to detect the corruption before
the metadata has been used, for example, to create a new file in the wrong
place.

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

@sp 1
@anchor{flawed-compat}
@section 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 file name, 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 file names, perhaps overwriting
existing files or directories. It may be a security risk to extract a file
with a truncated file name.

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 @w{8 GiB} or a link name longer than 100 bytes), tarlz moves the
file name also to the extended header to prevent an ustar tool from trying
to extract the file or link. This also makes easier during parallel decoding
the detection of a tar member split between two lzip members at the boundary
between the extended header and the ustar header.

@sp 1
@section 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 file name 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. Moreover, minimizing the overhead may help recovering the archive
with lziprecover in case of corruption.

Global pax headers are tolerated, but not supported; they are parsed and
ignored. Some operations may not behave as expected if the archive contains
global headers.

@sp 1
@section 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.


@node Multi-threaded tar
@chapter Limitations of parallel tar decoding
@cindex 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 @samp{--list}
option is able to do multi-threaded decoding.

If the files in the archive are large, multi-threaded @samp{--list} on a
regular (seekable) tar.lz archive can be hundreds of times faster than
sequential @samp{--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:

@example
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)
@end example


@node Minimum archive sizes
@chapter Minimum archive sizes required for multi-threaded block compression
@cindex minimum archive sizes

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
(@pxref{--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 archive is limited by the ratio
@w{(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:

@multitable {Processors} {512 MiB} {512 MiB} {512 MiB} {512 MiB} {512 MiB} {512 MiB}
@headitem Processors @tab 2 @tab 4 @tab 8 @tab 16 @tab 64 @tab 256
@item Level
@item -0 @tab   2 MiB @tab   4 MiB @tab   8 MiB @tab  16 MiB @tab  64 MiB @tab 256 MiB
@item -1 @tab   4 MiB @tab   8 MiB @tab  16 MiB @tab  32 MiB @tab 128 MiB @tab 512 MiB
@item -2 @tab   6 MiB @tab  12 MiB @tab  24 MiB @tab  48 MiB @tab 192 MiB @tab 768 MiB
@item -3 @tab   8 MiB @tab  16 MiB @tab  32 MiB @tab  64 MiB @tab 256 MiB @tab   1 GiB
@item -4 @tab  12 MiB @tab  24 MiB @tab  48 MiB @tab  96 MiB @tab 384 MiB @tab 1.5 GiB
@item -5 @tab  16 MiB @tab  32 MiB @tab  64 MiB @tab 128 MiB @tab 512 MiB @tab   2 GiB
@item -6 @tab  32 MiB @tab  64 MiB @tab 128 MiB @tab 256 MiB @tab   1 GiB @tab   4 GiB
@item -7 @tab  64 MiB @tab 128 MiB @tab 256 MiB @tab 512 MiB @tab   2 GiB @tab   8 GiB
@item -8 @tab  96 MiB @tab 192 MiB @tab 384 MiB @tab 768 MiB @tab   3 GiB @tab  12 GiB
@item -9 @tab 128 MiB @tab 256 MiB @tab 512 MiB @tab   1 GiB @tab   4 GiB @tab  16 GiB
@end multitable


@node Examples
@chapter A small tutorial with examples
@cindex examples

@noindent
Example 1: Create a multimember compressed archive @samp{archive.tar.lz}
containing files @samp{a}, @samp{b} and @samp{c}.

@example
tarlz -cf archive.tar.lz a b c
@end example

@sp 1
@noindent
Example 2: Append files @samp{d} and @samp{e} to the multimember
compressed archive @samp{archive.tar.lz}.

@example
tarlz -rf archive.tar.lz d e
@end example

@sp 1
@noindent
Example 3: Create a solidly compressed appendable archive
@samp{archive.tar.lz} containing files @samp{a}, @samp{b} and @samp{c}.
Then append files @samp{d} and @samp{e} to the archive.

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

@sp 1
@noindent
Example 4: Create a compressed appendable archive containing directories
@samp{dir1}, @samp{dir2} and @samp{dir3} with a separate lzip member per
directory. Then append files @samp{a}, @samp{b}, @samp{c}, @samp{d} and
@samp{e} to the archive, all of them contained in a single lzip member.
The resulting archive @samp{archive.tar.lz} contains 5 lzip members
(including the EOF member).

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

@sp 1
@noindent
Example 5: Create a solidly compressed archive @samp{archive.tar.lz}
containing files @samp{a}, @samp{b} and @samp{c}. Note that no more
files can be later appended to the archive.

@example
tarlz --solid -cf archive.tar.lz a b c
@end example

@sp 1
@noindent
Example 6: Extract all files from archive @samp{archive.tar.lz}.

@example
tarlz -xf archive.tar.lz
@end example

@sp 1
@noindent
Example 7: Extract files @samp{a} and @samp{c}, and the whole tree under
directory @samp{dir1} from archive @samp{archive.tar.lz}.

@example
tarlz -xf archive.tar.lz a c dir1
@end example

@sp 1
@noindent
Example 8: Copy the contents of directory @samp{sourcedir} to the
directory @samp{destdir}.

@example
tarlz -C sourcedir -c . | tarlz -C destdir -x
@end example


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

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
@email{lzip-bug@@nongnu.org}. Include the version number, which you can
find by running @w{@samp{tarlz --version}}.


@node Concept index
@unnumbered Concept index

@printindex cp

@bye
