	SQUASHFS-TOOLS 4.7.1 - A squashed read-only filesystem for Linux

	Copyright 2002-2025 Phillip Lougher <phillip@squashfs.org.uk>

	Released under the GPL licence (version 2 or later).

Welcome to Squashfs-Tools 4.7.1.  This is a minor improvements and bug fix
update release to Squashfs-tools 4.7.

The Squashfs-tools 4.6.1 release was more than two years ago, and there are
substantial improvements to the tools, in particular Mksquashfs can now be 20%
to more than ten times faster (dependant on source media and input files).
The help system has also been completely rewritten and improved for Mksquashfs/
Unsquashfs/Sqfstar/Sqfscat.  There are also new options for building
reproducible images, and a lot of other improvements.

Please see the INSTALL file for instructions on installing the tools, and the
USAGE files for information on how to use the tools.

This README has the following sections:

1. Improvements and bug fixes in 4.7.1
2. Summary of changes in 4.7
3. Parallel file reading and options
4. Help system and options
5. Reproducible filesystem images and new options
6. Author info

1. IMPROVEMENTS AND BUG FIXES IN 4.7.1
--------------------------------------

1. Fix regression in -offset (-o) where it stopped working in Mksquashfs and
   Sqfstar.
2. Allow arguments (spaces) in the PAGER environment variable, rather than just
   a command name.  Also support quoted strings, string concatenation and
   backslashes.
3. Don't use a pager to display the help text if the PAGER environment variable
   is empty.
4. Add -no-pager option which if specified means a pager won't be used to
   display the help text.
5. Add -cols option which specifies the number of columns used to display the
   help text.  This is useful if output is not to a terminal.
6. Fix regression in tar file reading, where file exclusion would leak cache
   (memory) leading in some cases to Sqfstar hanging.
7. Handle negative dates in tar files (before the start of the epoch of
   1970-01-01), and round them up to the start of the epoch.
8. Add --version spelling in addition to -version, as this appears to be a
   common mistake.
9. Add missing include files in print_pager.c (Ros Burton).
10. Add missing include file in thread.c (Shiji Yang).
11. Fix BLOCK_READER_THREADS typo in Makefile (Alexandru Ardelean).

2. SUMMARY OF CHANGES in 4.7
----------------------------

1. Mksquashfs now reads files in parallel from the input directories

   1.1. This can significantly increase I/O when reading lots of small files,
        and/or the input media benefits from parallel reading e.g. modern SSD
        drives, or network filesystems etc.
   1.2  In cases where speed of I/O is the bottleneck in Mksquashfs, this can
        make Mksquashfs run significantly faster, in some cases Mksquashfs can
        be more than ten times faster.
   1.3. New -small-readers option to specify number of parallel small file
        reader threads (files less than a block size).  Default 4 threads.
   1.4. New -block-readers option to specify number of parallel block reader
        threads (files one block or larger).  Default 4 threads.
   1.5. New -single-reader option to specify a single reader thread, similar to
        previous Mksquashfs versions.

2. Rewritten and improved help system (Mksquashfs/Unsquashfs/Sqfstar/Sqfscat)

   2.1. Help text now uses the full width of the terminal (rather than being
        pre-formatted to 80 columns).
   2.2. The help text is now automatically paged (using pager, less or more).
   2.3. The tools now print a summary on failure to parse the command line (or
        encountering other errors that prevent the tool from running), rather
        than displaying the help text.
   2.4. The help text can be displayed in full, by section, or by option using
        regex matching.
   2.5. New -help-all option to display all help text
   2.6. New -help-section option to display help for a particular section
   2.7. New -help-option to display all options matching regex.
   2.8. New -help-comp option to display compressor options for given
        compressor.

3. New options for building reproducible filesystems (Mksquash/Sqfstar)

  3.1 Low level timestamp setting options extended
     -mkfs-time inode sets the fs creation time to the latest inode timestamp
     -inode-time inode sets all inode timestamps to the latest inode timestamp
     -root-time inode sets the root dir timestamp to the latest inode timestamp

  3.2 New easier to remember shorthand options
     -repro builds a reproducible fs image, it is shorthand for -mkfs-time inode
     -repro-time <time> builds a reproducible fs image, it is shorthand for
      specifying -mkfs-time <time> and -inode-time <time>.

4. Elimination of "fragment block stall" and -(not-)reproducible options

  A technical issue called "the fragment block stall" has been eliminated in
  this release in a way that generates a reproducible ordering of files in the
  filesystem image.  This can increase performance by 20% or more, in addition
  to the parallel reader performance improvements.

  This "fragment block stall" was introduced in release 4.4 (2019) to produce
  a reproducible ordering of files in the filesystem image, but which led to a
  reduction in parallelisation and performance.  Due to this reduction, the
  previous behaviour was retained and enabled using the -not-reproducible option.
  As the "fragment block stall" has now been removed, the options
  -not-reproducible and -reproducible now do nothing, but are still recognised
  for backwards compatibility.

5. Other improvements for Mksquashfs/Sqfstar

   3.1. New -force-file-mode option, which sets all file (non-directory)
        permissions to the given mode.
   3.2. New -force-dir-mode option, which sets all directory permissions to
        the given mode.
   3.3. -root-mode and above new -force-file-mode/-force-dir-mode options
        now take a symbolic mode in addition to an octal mode.
   3.4. New -info-file option, which prints files written to the filesystem to
        a file rather than stdout.  Allows -info-file to be used in conjunction
        with the progress bar.
   3.5. New -pseudo-dir (or -pd) option which supplies a default directory
        if any directories in a pseudo file definition pathname doesn't exist.
   3.6. New pseudo file 'h' definition which creates a hard link to a file,
        and follows symbolic links.
   3.7. Previously if a directory was missing (or not a directory) in a
        Pseudo file definition pathname, the pseudo file definition would be
        ignored.  This has been hardened to a fatal error.

6. Other improvements for Unsquashfs/Sqfscat

   4.1. New -mem option, which sets the amount of memory to be used,
        K, M and G can be used to specify Kbytes, Mbytes and Gbytes.
   4.2. New -mem-percent option, which sets the amount of memory to be
        used as percentage of available physical memory.
   4.3. Memory specified is limited to 75% of physical memory or less.

7. New environment variable SQFS_CMDLINE (Mksquashfs/Unsquashfs/Sqfstar/Sqfscat)

   If set, this is used as the directory to write the file sqfs_cmdline
   which contains the command line arguments given to Mksquashfs etc.  Intended
   to be used to debug scripts/discover what is being passed to Mksquashfs.


3. PARALLEL FILE READING AND OPTIONS
------------------------------------

Modern computers can have 16 cores/32 threads or more [*], and systems with 8
cores/16 threads are becoming standard.   What this increase in computational
power means is Mksquashfs was increasingly I/O bound rather than CPU bound.

Whether the speed of input I/O has become the bottleneck is essentially entirely
dependant on the nature of the input files, the media they're stored on, the
number of CPUs/threads you have, and the cost of compression.  So it is
difficult to give hard and fast rules, but the following facts can be given.

1. The more CPUs/threads you have the faster the input I/O needs to be to
   occupy all cores/threads.
2. The faster the compression algorithm (i.e. requires less CPU) the faster
   input I/O needs to be to keep up with compression.
3. Small files in general are much slower to read than large files.  So the more
   small files you have the slower I/O will be, and the more difficult it will
   be to occupy all the cores/threads.

Traditionally Mksquashfs used a single reader thread to read input files.  The
reason for that is because up until about 5 or so years ago, mechanical hard
disks were still commonplace, and parallel reading of files from a mechanical
hard disk is very expensive, and much slower than sequential reading.  So
parallel file reading from a mechanical hard disk is completely self defeating.

Mechanical drives are now largely obsolete, and so Mksquashfs has now moved to
reading files in parallel by default.  This can significantly speed up input I/O.

Mksquashfs by default uses four small file reader threads and four block file
reader threads.  A small file reader thread only reads files which are smaller
than a block in size, and a block file reader thread only reads files which
are a block or larger in size (see section 2.1 for an explanation of why there
are specialised reader threads).

Three new options have been added which allow the number of reader threads to
be changed:

	1. -small-readers <n>
	2. -block-readers <n>
	3. -single-reader

The maximum number of small and block reader threads are 1024 each.  But, the
total number of reader threads cannot exceed the open file limit (usually 1024)
less a margin of 10.  Also the more reader threads, the more memory that
Mksquashfs will need.

The -single-reader option makes Mksquashfs behave similarly to previous versions
and files are read sequentially.

The following test matrix was generated by running Mksquashfs over source
directories containing the same amount of data (1.5GB uncompressed Linux
6.12.8 tarball), but with the tarball split into 128 byte files, then 256
byte files, and so on up to 1Mbyte files.  For each directory, Mksquashfs was
run with single-threads, 2 small reader threads, 4 small reader threads, and so
on up to 256 small reader threads.  The compression block size was 1Mbyte (and
hence no block reader threads), and the compression algorithm was Gzip.  The
machine has 14 cores/20 threads, and the media was a SanDisk Extreme 55AE SSD
connected via USB 3, formatted with ext4.

All Mksquashfs tests were performed with a cold cache to ensure speed of
filesystem I/O is measured, rather than speed of memory reading from the cache.

The times are in minutes:seconds.

	1	2	4	8	12	16	20	30	40	50	64	128	256
128	56:39.7	30:35.5	19:30.8	10:30.5	9:34.88	9:04.61	9:07.28	9:07.10	8:25.15	7:59.32	7:35.56	7:01.38	6:21.16
	32%	61%	97%	97%	98%	103%	104%	103%	101%	107%	115%	141%	178%
256	27:47.1	14:42.2	8:43.47	4:47.70	4:07.88	4:00.11	3:59.21	3:58.96	3:38.11	3:23.73	3:11.48	2:52.45	2:38.46
	31%	60%	99%	100%	106%	110%	113%	111%	112%	121%	131%	162%	196%
512	13:46.3	7:13.71	4:07.39	2:17.42	1:57.45	1:52.89	1:52.62	1:52.26	1:41.60	1:35.61	1:29.02	1:18.17	1:12.30
	34%	65%	106%	121%	133%	138%	142%	140%	145%	156%	171%	214%	256%
1K	6:48.55	3:34.53	1:55.55	1:06.17	0:56.06	0:53.98	0:53.87	0:53.39	0:48.31	0:45.16	0:42.41	0:36.53	0:33.43
	41%	76%	118%	166%	188%	196%	198%	195%	211%	232%	254%	324%	385%
2K	3:24.05	1:46.84	0:56.85	0:35.42	0:27.57	0:26.61	0:26.67	0:26.60	0:24.11	0:22.40	0:20.88	0:18.21	0:17.00
	55%	92%	155%	244%	312%	330%	333%	327%	360%	396%	438%	555%	647%
4K	1:42.84	0:53.50	0:29.48	0:16.50	0:13.71	0:13.65	0:13.55	0:13.56	0:12.44	0:11.53	0:11.25	0:09.91	0:09.91
	78%	125%	249%	495%	624%	631%	639%	640%	706%	784%	820%	1021%	1064%
8K	0:51.50	0:28.11	0:15.36	0:10.70	0:09.09	0:10.35	0:08.84	0:09.44	0:08.26	0:09.35	0:07.67	0:07.88	0:08.31
	115%	217%	467%	774%	950%	839%	985%	923%	1061%	958%	1193%	1384%	1353%
16K	0:26.65	0:14.50	0:11.83	0:06.95	0:07.73	0:06.86	0:07.31	0:07.67	0:08.77	0:06.72	0:07.57	0:07.02	0:07.18
	200%	452%	635%	1246%	1133%	1278%	1374%	1312%	1192%	1425%	1406%	1507%	1504%
32K	0:15.09	0:10.55	0:06.48	0:07.64	0:06.17	0:07.48	0:07.77	0:06.18	0:06.68	0:06.01	0:06.73	0:06.58	0:06.64
	384%	672%	1304%	1289%	1554%	1288%	1390%	1524%	1635%	1767%	1639%	1610%	1590%
64K	0:13.45	0:07.15	0:06.92	0:06.09	0:06.32	0:07.09	0:06.60	0:06.13	0:06.29	0:06.31	0:05.88	0:06.54	0:06.50
	433%	1103%	1538%	1660%	1686%	1475%	1507%	1651%	1671%	1672%	1795%	1664%	1608%
128K	0:07.62	0:05.52	0:08.00	0:05.46	0:05.82	0:06.50	0:06.83	0:07.61	0:05.61	0:05.83	0:06.32	0:06.24	0:06.83
	955%	1565%	1325%	1697%	1806%	1622%	1508%	1325%	1685%	1817%	1724%	1689%	1495%
256K	0:06.34	0:05.49	0:06.08	0:06.35	0:05.91	0:06.06	0:06.09	0:06.06	0:05.80	0:06.19	0:06.08	0:06.51	0:08.25
	1251%	1719%	1738%	1662%	1739%	1743%	1728%	1732%	1809%	1739%	1727%	1584%	1080%
512K	0:05.80	0:05.76	0:07.07	0:05.62	0:06.10	0:06.04	0:06.09	0:06.06	0:06.79	0:07.07	0:05.89	0:08.30	0:08.28
	1404%	1738%	1478%	1717%	1740%	1750%	1742%	1741%	1530%	1391%	1613%	1101%	1106%
1M	0:05.29	0:06.10	0:07.00	0:05.18	0:05.47	0:05.93	0:07.02	0:05.28	0:05.86	0:05.44	0:05.95	0:05.83	0:05.85
	1517%	1667%	1415%	1749%	1852%	1748%	1438%	1728%	1718%	1864%	1749%	1728%	1730%

I think the figures largely speak for themselves, but the following points can
be made:

1. Small files are very slow to read and reading in parallel can significantly
   speed up Mksquashfs.  The greatest speed-up seen is reading 1Kbyte files with
   256 small reader threads, which is 12.22 times faster than a single reader
   thread (33.43 seconds vs 6 minutes 48.55 seconds).

2. Obviously the larger the files are, the less performance increase is seen
   reading files in parallel.  But even reading 128Kbyte files in parallel can
   achieve a 1.4 times speed increase over a single reader thread.

3. On this SSD drive (and ext4) there doesn't seem to be any performace gain
   reading files in parallel where the files are 512Kbytes or larger.

4. On this SSD drive (and ext4), with 16Kbyte or larger files, performance
   peaks at about 8 small reader threads.

5. On this SSD drive (and ext4) files 8Kbytes or smaller can benefit from a
   large number of reader threads, but, the increase in performance starts
   to tail off after about 12 small reader threads.

Due to the above, and other performance tests, the default number of small
reader threads and block reader threads is set to four, which is a compromise
between different input files/media and performance.  If you think Mksquashfs
is I/O bound then you should experiment with larger reader threads which may
increase performance.

3.1 Specialised small reader and block reader threads
-----------------------------------------------------

The amount of reader threads you need to maximise I/O when reading small files,
is often different to the amount of reader threads you need when reading larger
files.  For instance file input to Mksquashfs often consists of a large sequence
of small files (less than a block size), and then one or more large files, and
this sequence of small files and large files repeats.

For example, imagine you had a sequence of 256 1Kbyte files, followed by eight
large 4Mbyte files, and this sequence repeats.  To maximise I/O reading the
small files you might want sixteen or more reader threads to read them in
parallel, but, you don't really want or need to have sixteen reader threads to
read the 4Mbyte files in parallel, and often doing so will thrash the
filesystem/media resulting in lower performance.

If you only had reader threads that read both small and large files, it would be
difficult to choose an amount of reader threads that worked well for the above
input.  But, with separate small and large file reader threads, it is easy to
ask Mksquashfs to create sixteen small reader threads, and only one block
reader thread.

There is another advantage to splitting reader threads into those that read
small files, and those that read large files, and that is in doing so, you read
small files in parallel with large files, and that should optimise I/O.  Using
the above example again, the block reader thread will work ahead and read the
4Mbyte files in parallel with the small reader threads.

[*] By this I obviously mean consumer-grade hardware.  There has been 16+ core
    Unix machines around since the early 1990s (such as the Sequent Symmetry),
    but these were multi-user systems typically supporting 50 or more users.


4. HELP SYSTEM AND OPTIONS
--------------------------

The help system has been rewritten to remove the annoyances and limitations
of the previous system.  The previous system printed the entire help text
when a command line parse error occurred (or some other reason that prevented
the tool from running).  This behaviour may have been useful when the tools
had much less options, but when for example the full Mksquashfs help text is
now 389 lines (formatted to 80 columns), the full help text is just too much,
and it is multiple pages in size.  What effectively happens is the actual error
message scrolls off the screen and is lost, and the user is left with a large
help text display, most of which has also scrolled off the screen, and the
remaining part is of no interest whatsoever.

The help system was first rewritten to avoid this annoyance.  Now, rather than
the full help being displayed on an error occurring, only a summary of the help
options available is displayed, where the user is directed to use the most
appropriate help option for their situation.

The second aspect of the rewrite was to move away from help text pre-formatted
to 80 columns, and to use the full width of the terminal, which is typically
much wider than 80 columns.  The effect of this, obviously, is to fit more
help text onto the screen.  Also the help text is now paged, and so if the
output is larger than a screen, no information will be lost.

For example, now if the user correctly spells an option, but, makes a mistake
with the argument, the user will firstly be prompted to look up the help text
for that option, and afterwards the list of other help options will be given,
e.g. (output uses 80 columns because this README is formatted to 80 columns)

*************************************************************************
% mksquasfs dir image.sqfs -b 66
mksquashfs: -b block size not power of two or not between 4096 and 1Mbyte

Run
  "mksquashfs -help-option -b$" to get help on -b option

Or run
  "mksquashfs -help-option <regex>" to get help on all options matching <regex>

Or run
  "mksquashfs -help-section <section-name>" to get help on these sections
        SECTION NAME            SECTION
        compression             Filesystem compression options:
        build                   Filesystem build options:
        time                    Filesystem time options:
        perms                   Filesystem permissions options:
        pseudo                  Filesystem pseudo options:
        filter                  Filesystem filter options:
        xattrs                  Filesystem extended attribute (xattrs) options:
        runtime                 Mksquashfs runtime options:
        append                  Filesystem append options:
        actions                 Filesystem actions options:
        tar                     Tar file only options:
        expert                  Expert options (these may make the filesystem
                                unmountable):
        help                    Help options:
        misc                    Miscellaneous options:
        pseudo-defs             Pseudo file definition format:
        symbolic                Symbolic mode specification:
        environment             Environment:
        exit                    Exit status:
        extra                   See also (extra information elsewhere):

Or run
  "mksquashfs -help-all" to get help on all the sections
*************************************************************************

The above ensures the error message is not lost, and the user is not swamped by
irrelevant information.

In doing so, this has introduced three new help options:

	1. -help-option <regex> (or -ho <regex> for short),
	2. -help-section <section-name> (or -hs <section-name> for short)
	3. -help-all (or -ha for short)

4.1 -help-option <regex>
------------------------

The -help-option option displays all the options that match the <regex> regular
expression, where matching is done on the option itself and the arguments,
e.g.

*************************************************************************
% mksquashfs -ho "regex"
-regex                  allow POSIX regular expressions to be used in exclude
                        dirs/files
-xattrs-exclude <regex> exclude any xattr names matching <regex>.  <regex> is a
                        POSIX regular expression, e.g. -xattrs-exclude '^user.'
                        excludes xattrs from the user namespace
-xattrs-include <regex> include any xattr names matching <regex>.  <regex> is a
                        POSIX regular expression, e.g. -xattrs-include '^user.'
                        includes xattrs from the user namespace
-help-option <regex>    print the help information for Mksquashfs options
                        matching <regex> to stdout
-ho <regex>             shorthand alternative to -help-option
*************************************************************************

Displays all the options that contain regex in their name or argument.

Using a regular expression, multiple things can be matched, for example, if you
wanted to return all the options that operate on uids and gids, you could do

*************************************************************************
% mksquashfs -ho "uid|gid"
-root-uid <user>        set root directory owner to specified <user>, <user> can
                        be either an integer uid or user name
-root-gid <group>       set root directory group to specified <group>, <group>
                        can be either an integer gid or group name
-force-uid <user>       set all file and directory uids to specified <user>,
                        <user> can be either an integer uid or user name
-force-gid <group>      set all file and directory gids to specified <group>,
                        <group> can be either an integer gid or group name
-uid-gid-offset <value> offset all uid and gids by specified <value>.  <value>
                        should be a positive integer
-pd <d mode uid gid>    specify a default pseudo directory which will be used in
                        pseudo definitions if a directory in the pathname does
                        not exist.  This also allows pseudo definitions to be
                        specified without specifying all the directories in the
                        pathname.  The definition should be quoted
-pd <D time mode u g>   specify a default pseudo directory which will be used in
                        pseudo definitions if a directory in the pathname does
                        not exist.  The D type also allows a timestamp to be
                        specified in addition to mode, uid and gid
-default-uid <value>    tar files often do not store uids for intermediate
                        directories.  This option sets the default directory
                        owner to <value>, rather than the user running
                        Mksquashfs.  <value> can be either an integer uid or
                        user name.  This also sets the root directory uid
-default-gid <value>    tar files often do not store gids for intermediate
                        directories.  This option sets the default directory
                        group to <value>, rather than the group of the user
                        running Mksquashfs.  <value> can be either an integer
                        uid or group name.  This also sets the root directory
                        gid
*************************************************************************

4.2 -help-section <section>
---------------------------

The -help-section option displays the section that matches the <section> name.
If <section> does not exactly match a section name, it is treated as a regular
expression, and all section names that match are displayed.  Finally, if
<section> is "list", a list of sections and their names is displayed.

For example:

*************************************************************************
% mksquashfs -help-section compression
Filesystem compression options:
-b <block-size>         set data block to <block-size>.  Default 128 Kbytes.
                        Optionally a suffix of K, KB, Kbytes or M, MB, Mbytes
                        can be given to specify Kbytes or Mbytes respectively
-comp <comp>            select <comp> compression.  Run -help-comp <comp> to get
                        compressor options for <comp>, or <all> for all the
                        compressors.
                        Compressors available:
                                gzip (default)
                                lzo
                                lz4
                                xz
                                zstd
-noI                    do not compress inode table
-noId                   do not compress the uid/gid table (implied by -noI)
-noD                    do not compress data blocks
-noF                    do not compress fragment blocks
-noX                    do not compress extended attributes
-no-compression         do not compress any of the data or metadata.  This is
                        equivalent to specifying -noI -noD -noF and -noX
*************************************************************************

Will display the compression options section.

Using regular expression matching section names can be abbreviated, for example
"comp" will also display the compression options section.  But, it also means
multiple sections can be displayed, for example:

*************************************************************************
% mksquashfs -help-section "comp|build"
Filesystem compression options:
-b <block-size>         set data block to <block-size>.  Default 128 Kbytes.
                        Optionally a suffix of K, KB, Kbytes or M, MB, Mbytes
                        can be given to specify Kbytes or Mbytes respectively
-comp <comp>            select <comp> compression.  Run -help-comp <comp> to get
                        compressor options for <comp>, or <all> for all the
                        compressors.
                        Compressors available:
                                gzip (default)
                                lzo
                                lz4
                                xz
                                zstd
-noI                    do not compress inode table
-noId                   do not compress the uid/gid table (implied by -noI)
-noD                    do not compress data blocks
-noF                    do not compress fragment blocks
-noX                    do not compress extended attributes
-no-compression         do not compress any of the data or metadata.  This is
                        equivalent to specifying -noI -noD -noF and -noX

Filesystem build options:
-tar                    read uncompressed tar file from standard in (stdin)
-no-strip               act like tar, and do not strip leading directories from
                        source files
-tarstyle               alternative name for -no-strip
-cpiostyle              act like cpio, and read file pathnames from standard in
                        (stdin)
-cpiostyle0             like -cpiostyle, but filenames are null terminated.  Can
                        be used with find -print0 action
-reproducible           build filesystems that are reproducible (default)
-not-reproducible       build filesystems that are not reproducible
-no-exports             do not make filesystem exportable via NFS (-tar default)
-exports                make filesystem exportable via NFS (default)
-no-sparse              do not detect sparse files
-no-tailends            do not pack tail ends into fragments (default)
-tailends               pack tail ends into fragments
-no-fragments           do not use fragments
-no-duplicates          do not perform duplicate checking
-no-hardlinks           do not hardlink files, instead store duplicates
-keep-as-directory      if one source directory is specified, create a root
                        directory containing that directory, rather than the
                        contents of the directory
*************************************************************************

Will display the compression options and build options sections.

4.3 -help-all
-------------

The -help-all option displays all the help text, and it is similar to the
behaviour of -help in previous Squashfs tools versions, except that the
output is to a pager and not stdout.

4.4. PAGER environment variable
-------------------------------

By default the tools try pager, /usr/bin/pager, less, /usr/bin/less, more,
/usr/bin/more, cat and /usr/bin/cat in that order.

The pager used can be over-ridden using the PAGER environment variable.  If the
filename given by PAGER doesn't contain slashes, the PATH environment variable
will be used to locate it, otherwise it will be treated as a pathname.


5. REPRODUCIBLE FILESYSTEM IMAGES AND NEW OPTIONS
-------------------------------------------------

If you want Mksquashfs to generate an identical (byte for byte) filesystem on
every run, then the following conditions have to be true:

1. The filesystem source data has to be the same,
2. The timestamps (and other metadata such as permissions), must be the same,
3. The root directory timestamp (and other metadata), must be the same,
4. The filesystem make time (stored in the super-block) must be the same.

Due to point 4, every time you run Mksquashfs, the filesystem will be different,
even if everything else is the same.  But less obviously, if Mksquashfs has to
fabricate a root directory (because the source doesn't supply one, for example
where multiple files are specified on the command line), the timestamp of the
root directory will also change on every run.

To avoid the above, previous versions introduced the -mkfs-time <time>, and
-root-time <time> options:

% mksquashfs source(s) image.sqfs -mkfs-time 0 -root-time 0

Will generate a filesystem image where the timestamps (that can change) have
been set to 0 (the start of the epoch 1970-01-01).

But a problem with this (for many people) is that it ensures reproducibility by
losing information and functionality, akin to using a sledgehammer to crack a
nut.  With a filesystem make time of 0, it is no longer possible to discover the
difference between one filesystem and another without looking at the content, or
know how old the filesystem is without looking at the content either.

Due to this, this release introduces new variants of -mkfs-time, and -root-time.
It also introduces a new variant of -all-time, while also renaming it to
-inode-time.  Lastly, there are some new easy to remember shorthand options
added.

5.1 New -mkfs-time, -root-time and -inode-time variants
-------------------------------------------------------

5.1.1 -mkfs-time inode

This sets the filesystem make time to the latest inode timestamp in the
source(s).  Because this is a relative value (rather than absolute), it ensures
the filesystem is identical on multiple runs of Mksquashfs if the content
doesn't change, it also allows filesystems with newer content to be
distinquished using the filesystem make time, and if the timestamps are updated
(due to changed content) this will produce a newer filesystem make time.

In effect this is a more nuanced way of producing reproducibility than an
absolute value.  Also the latest inode timestamp is taken from the source(s),
ignoring any fabricated timestamps (e.g. root directory), and all fabricated
timestamps are set to the latest inode value too.  This means the -root-time
option is no longer necessary if the -mkfs-time inode option is used.

5.1.2 -root-time inode

This sets the root directory timestamp to the latest inode timestamp in the
source(s).  If -mkfs-time inode is specified this option is no longer
necessary.

5.1.3 -inode-time inode

This option has been renamed from -all-time [*] in previous versions because
all-time was a misnomer (it sets all the inode timestamps, but not also the
filesystem make time as the name suggests).

This sets all the inode timestamps to the latest inode timestamp in the
source(s).  I doubt there are many use-cases for this, but it keeps the
functionality matching between options.

[*] the name -all-time is still recognised for backwards compatibility.

5.2 New easier to remember shorthand options
--------------------------------------------

5.2.1 -repro

This option makes Mksquashfs build a reproducible filesystem image.  This is
equivalent to -mkfs-time inode, which achieves reproducibility by setting the
filesystem build time to the latest inode timestamp.  Obviously the image won't
be reproducible if the timestamps or content changes.

5.2.2 -repro-time <time>

This option makes Mksquashfs build a reproducible filesystem image.  This is
equivalent to specifying -mkfs-time <time> and -inode-time <time>, which
achieves reproducibility by setting all timestamps to <time>.  This option can
be used in cases where timestamps may change, and where -repro cannot be used
for this reason.


6. AUTHOR INFO
--------------

Squashfs was written by Phillip Lougher, email phillip@squashfs.org.uk,
in Chepstow, Wales, UK.   If you like the program, or have any problems,
then please email me, as it's nice to get feedback!
