diff options
Diffstat (limited to 'doc/tarlz.info')
| -rw-r--r-- | doc/tarlz.info | 232 |
1 files changed, 138 insertions, 94 deletions
diff --git a/doc/tarlz.info b/doc/tarlz.info index f49bb43..edcee44 100644 --- a/doc/tarlz.info +++ b/doc/tarlz.info @@ -11,12 +11,13 @@ File: tarlz.info, Node: Top, Next: Introduction, Up: (dir) Tarlz Manual ************ -This manual is for Tarlz (version 0.15, 11 April 2019). +This manual is for Tarlz (version 0.16, 8 October 2019). * 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 @@ -39,13 +40,19 @@ File: tarlz.info, Node: Introduction, Next: Invoking tarlz, Prev: Top, Up: T 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. +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 @@ -62,7 +69,7 @@ archive, but it has the following advantages: member), and unwanted members can be deleted from the archive. Just like an uncompressed tar archive. - * It is a safe posix-style backup format. In case of corruption, + * 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 @@ -77,10 +84,11 @@ archive, but it has the following advantages: with standard tar tools. *Note crc32::. Tarlz does not understand other tar formats like 'gnu', 'oldgnu', -'star' or 'v7'. +'star' or 'v7'. 'tarlz -tf archive.tar.lz > /dev/null' can be used to +verify that the format of the archive is compatible with tarlz. -File: tarlz.info, Node: Invoking tarlz, Next: File format, Prev: Introduction, Up: Top +File: tarlz.info, Node: Invoking tarlz, Next: Portable character set, Prev: Introduction, Up: Top 2 Invoking tarlz **************** @@ -94,9 +102,9 @@ 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 -filename prefixes containing a '..' component. On extraction, leading +file name 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 +members containing a '..' 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. @@ -149,30 +157,31 @@ equivalent to '-1 --solid' 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. + a new '-C' option appears in the command line. When extracting or + comparing, all the '-C' options are executed in sequence before + reading the archive. 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 + archive if a '-C' option appears after a relative file name 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 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). + 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 '-C' - option may be used in combination with '--diff' when absolute - filenames were used on archive creation: 'tarlz -C / -d'. + option may be used in combination with '--diff' when absolute file + names were used on archive creation: 'tarlz -C / -d'. Alternatively, tarlz may be run from the root directory to perform the comparison. @@ -184,15 +193,22 @@ equivalent to '-1 --solid' 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 ('--no-solid' - archives). To delete a directory without deleting the files under - it, use '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. + archives). Note that files of about '--data-size' or larger are + compressed individually even if '--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 + '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. '--exclude=PATTERN' Exclude files matching a shell pattern like '*.o'. A file is - considered to match if any component of the filename matches. For - example, '*.o' matches 'foo.o', 'foo.o/bar' and 'foo/bar.o'. + considered to match if any component of the file name matches. For + example, '*.o' matches 'foo.o', 'foo.o/bar' and 'foo/bar.o'. If + PATTERN contains a '/', it matches a corresponding '/' in the file + name. For example, 'foo/*.o' matches 'foo/bar.o'. '-f ARCHIVE' '--file=ARCHIVE' @@ -234,13 +250,15 @@ equivalent to '-1 --solid' 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. - 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 FILES have been specified. + 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 FILES have been + specified. '-t' '--list' @@ -351,7 +369,7 @@ equivalent to '-1 --solid' 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 + the POSIX pax format; i.e., the lack of a mandatory check sequence in the extended records. *Note crc32::. '--out-slots=N' @@ -369,9 +387,24 @@ invalid input file, 3 for an internal consistency error (eg, bug) which caused tarlz to panic. -File: tarlz.info, Node: File format, Next: Amendments to pax format, Prev: Invoking tarlz, Up: Top +File: tarlz.info, Node: Portable character set, Next: File format, Prev: Invoking tarlz, Up: Top + +3 POSIX portable filename character set +*************************************** + +The set of characters from which portable file names are constructed. + + 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 . _ - + + The last three characters are the period, underscore, and +hyphen-minus characters, respectively. + + +File: tarlz.info, Node: File format, Next: Amendments to pax format, Prev: Portable character set, Up: Top -3 File format +4 File format ************* In the diagram below, a box like this: @@ -393,7 +426,7 @@ 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 +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. @@ -438,7 +471,7 @@ tar.lz +===============+=================================================+========+ -3.1 Pax header block +4.1 Pax header block ==================== The pax header block is identical to the ustar header block described @@ -492,7 +525,7 @@ conversion to UTF-8 nor any other transformation. swapping of two bytes. -3.2 Ustar header block +4.2 Ustar header block ====================== The ustar header block has a length of 512 bytes and is structured as @@ -519,11 +552,10 @@ 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. +names should only contain characters from the portable character set. +But if an implementation supports the use of characters outside of '/' +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 @@ -618,38 +650,45 @@ character. File: tarlz.info, Node: Amendments to pax format, Next: Multi-threaded tar, Prev: File format, Up: Top -4 The reasons for the differences with pax +5 The reasons for the differences with pax ****************************************** -Tarlz is meant to reliably detect invalid or corrupt metadata during -decoding, and to create safe archives where corrupt metadata can be -reliably detected. 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. +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 '--list' and +'--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. -4.1 Add a CRC of the extended records +5.1 Add a CRC of the extended records ===================================== -The posix pax format has a serious flaw. The metadata stored in pax +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 +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 filename and file size must be always protected in an + 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. *Note key_crc32::. -4.2 Remove flawed backward compatibility +5.2 Remove flawed backward compatibility ======================================== In order to allow the extraction of pax archives by a tar utility @@ -657,12 +696,12 @@ 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 +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 filenames, perhaps overwriting existing files -or directories. It may be a security risk to extract a file with a -truncated filename. +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 @@ -672,28 +711,29 @@ 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 decoding the detection of a tar member split between two lzip -members at the boundary between the extended header and the ustar -header. +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. -4.3 As simple as possible (but not simpler) +5.3 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 +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. +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. -4.4 Avoid misconversions to/from UTF-8 +5.4 Avoid misconversions to/from UTF-8 ====================================== There is no portable way to tell what charset a text string is coded @@ -705,7 +745,7 @@ this behavior will be adjusted with a command line option in the future. File: tarlz.info, Node: Multi-threaded tar, Next: Minimum archive sizes, Prev: Amendments to pax format, Up: Top -5 Limitations of parallel tar decoding +6 Limitations of parallel tar decoding ************************************** Safely decoding an arbitrary tar archive in parallel is impossible. For @@ -753,7 +793,7 @@ example listing the Silesia corpus on a dual core machine: File: tarlz.info, Node: Minimum archive sizes, Next: Examples, Prev: Multi-threaded tar, Up: Top -6 Minimum archive sizes required for multi-threaded block compression +7 Minimum archive sizes required for multi-threaded block compression ********************************************************************* When creating or appending to a compressed archive using multi-threaded @@ -791,7 +831,7 @@ Level File: tarlz.info, Node: Examples, Next: Problems, Prev: Minimum archive sizes, Up: Top -7 A small tutorial with examples +8 A small tutorial with examples ******************************** Example 1: Create a multimember compressed archive 'archive.tar.lz' @@ -850,7 +890,7 @@ Example 8: Copy the contents of directory 'sourcedir' to the directory File: tarlz.info, Node: Problems, Next: Concept index, Prev: Examples, Up: Top -8 Reporting bugs +9 Reporting bugs **************** There are probably bugs in tarlz. There are certainly errors and @@ -881,6 +921,9 @@ Concept index * invoking: Invoking tarlz. (line 6) * minimum archive sizes: Minimum archive sizes. (line 6) * options: Invoking tarlz. (line 6) +* parallel tar decoding: Multi-threaded tar. (line 6) +* portable character set: Portable character set. + (line 6) * usage: Invoking tarlz. (line 6) * version: Invoking tarlz. (line 6) @@ -888,20 +931,21 @@ Concept index Tag Table: Node: Top223 -Node: Introduction1086 -Node: Invoking tarlz3337 -Ref: --data-size5489 -Ref: --bsolid12172 -Node: File format15802 -Ref: key_crc3220622 -Node: Amendments to pax format26039 -Ref: crc3226580 -Ref: flawed-compat27605 -Node: Multi-threaded tar30128 -Node: Minimum archive sizes32667 -Node: Examples34800 -Node: Problems36517 -Node: Concept index37043 +Node: Introduction1155 +Node: Invoking tarlz3841 +Ref: --data-size6006 +Ref: --bsolid13287 +Node: Portable character set16917 +Node: File format17420 +Ref: key_crc3222248 +Node: Amendments to pax format27647 +Ref: crc3228304 +Ref: flawed-compat29564 +Node: Multi-threaded tar32198 +Node: Minimum archive sizes34737 +Node: Examples36870 +Node: Problems38587 +Node: Concept index39113 End Tag Table |