	MKSQUASHFS 4.7.5 - a tool to create Squashfs filesystems

This file describes how to use Mksquashfs, and it has the following sections:

1. Introduction and basic usage
2. Getting help and displaying Mksquashfs options
3. Changing compression algorithm and compression specific options
4. Changing global compression defaults used in Mksquashfs
5. Building reproducible filesystem images
6. Tar style handling of source pathnames in Mksquashfs
7. CPIO style handling of source pathnames in Mksquashfs
8. Specifying the UID/GIDs used in the filesystem
9. Specifying the file permissions used in the filesystem
10. Excluding files from the filesystem
11. Parallel file reading and options
12. Reducing CPU and I/O usage
13. Filtering and adding extended attributes (xattrs)
14. Pseudo file support
15. Extended pseudo file definitions with timestamps
16. Appending to Squashfs filesystems
17. Appending recovery file feature
18. Mksquashfs Actions introduction
19. Miscellaneous options

1. INTRODUCTION AND BASIC USAGE
-------------------------------

As Squashfs is a read-only filesystem, the Mksquashfs program must be used to
create populated squashfs filesystems.

Mksquashfs uses the following arguments

mksquashfs source1 source2 ...  FILESYSTEM [OPTIONS] [-e list of exclude files]

Where source1 source2 are the directories or files you want to be put into the
filesystem, and FILESYSTEM is the name of the output filesystem.  This can be a
file or a block device.  If the file already exists or it is a block device
Mksquashfs will try append to it (see section 16) unless the -noappend option is
given.

Most simple usage is a single source directory:

% mksquashfs test image.sqfs

This will generate a filesystem containing the contents of the directory test.

example 2:

If you specify more than one source directory, Mksquashfs will generate a
filesystem containing all the directories, rather than the contents.

% mksquashfs test1 /home/phillip/test2 image.sqfs

The top-level root directory will contain test1 and test2.

example 3:

If source is a single file, you'll get a filesystem containing only that file

% mksquashfs /home/phillip/hello image.sqfs

The top-level directory will contain the file hello.

example 4:

If you specify a mixture of multiple directories and files, Mksquashfs will
generate a filesystem containing those directories and files

% mksquashfs dir1 /home/phillip/dir2 file1 /home/phillip/file2 image.sqfs

The top-level directory will contain directories dir1 and dir2, and files file1
and file2.

example 5:

If you don't want Mksquashfs to store the contents of the directory when a
single source directory is specified, and instead want it to store the directory
itself, you can use the -keep-as-directory option

% mksquashfs test image.sqfs -keep-as-directory

The top-level directory will contain the directory test.

example 6:

If you want the path specified on the command line to be retained (like TAR
behaviour), you can specify the -no-strip option.

% mksquashfs /home/phillip/test image.sqfs -no-strip

This will generate a filesystem with home, home/phillip and home/phillip/test
directories, see section 6 for more details.


2. GETTING HELP AND DISPLAYING MKSQUASHFS OPTIONS
-------------------------------------------------

Mksquashfs has fairly detailed built-in help information describing the
available options.  Running:

% mksquashfs -help

Will display the following summary of the help options and information
available:

*******************************************************************
SYNTAX: mksquashfs source1 source2 ...  FILESYSTEM [OPTIONS] [-e list of exclude dirs/files]

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
*******************************************************************

For example to get a list of all the options that operate on uids and gids, you
could do

*************************************************************************
% mksquashfs -help-option "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
*************************************************************************

2.1 -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.

2.2. 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.


3. CHANGING COMPRESSION ALGORITHM AND COMPRESSION SPECIFIC OPTIONS
------------------------------------------------------------------

By default Mksquashfs will compress using the GZIP compression algorithm.  This
algorithm offers a good trade-off between compression ratio, and memory and time
taken to decompress.

Squashfs also supports LZ4, LZO, XZ and ZSTD compression.  LZO offers worse
compression ratio than GZIP, but is faster to decompress.  XZ offers better
compression ratio than GZIP, but at the expense of greater memory and time to
decompress (and significantly more time to compress).  LZ4 is similar to LZO.
ZSTD has been developed by Facebook, and aims to compress well and be fast to
decompress.  You should experiment with the compressors to see which is best for
you.

If you're not building the squashfs-tools and kernel from source, then the tools
and kernel may or may not have been built with support for LZ4, LZO, XZ or ZSTD
compression.

The compression algorithms supported by the build of Mksquashfs can be found
by typing:

% mksquashfs -help-comp list
        gzip (default)
        lzo
        lz4
        xz
        zstd

The list of compressor specific options for a compressor can be found by typing
mksquashfs -help-comp <compressor>, for example:

% mksquashfs -help-comp xz
mksquashfs: compressor "xz".  Options supported:
          -Xbcj filter1,filter2,...,filterN
                Compress using filter1,filter2,...,filterN in turn (in addition
                to no filter), and choose the best compression.  Available
                filters: x86, arm, armthumb, arm64, powerpc, sparc, ia64, riscv
          -Xdict-size <dict-size>
                Use <dict-size> as the XZ dictionary size.  The dictionary size
                can be specified as a percentage of the block size, or as an
                absolute value.  The dictionary size must be less than or equal
                to the block size and 8192 bytes or larger.  It must also be
                storable in the xz header as either 2^n or as 2^n+2^(n+1).
                Example dict-sizes are 75%, 50%, 37.5%, 25%, or 32K, 16K, 8K
                etc.

The compression specific options for all compressors can be found by typing
mksquashfs -help-comp all

The compression specific options are, obviously, specific to the compressor in
question, and the compressor documentation and web sites should be consulted to
understand their behaviour.  In general the Mksquashfs compression defaults for
each compressor are optimised to give the best performance for each compressor,
where what constitutes best depends on the compressor.  For GZIP/XZ best means
highest compression, for LZO/LZ4 best means a tradeoff between compression and
(de)-compression overhead (LZO/LZ4 by definition are intended for weaker
processors).


4. CHANGING GLOBAL COMPRESSION DEFAULTS USED IN MKSQUASHFS
----------------------------------------------------------

There are a large number of options that can be used to control the compression
in Mksquashfs.  By and large the defaults are the most optimum settings and
should rarely need to be changed.

Note, this does not apply to the block size, increasing the block size from the
default of 128 Kbytes will increase compression (especially for the XZ and ZSTD
compressors) and should increase I/O performance too.  However, a block size of
greater than 128 Kbytes may increase latency in certain cases (where the
filesystem contains lots of fragments, and no locality of reference is
observed).  For this reason the block size default is configured to the less
optimal 128 Kbytes.  Users should experiment with 256 Kbyte sizes or above.

The -b option allows the block size to be selected, both "K" and "M" postfixes
are supported, this can be either 4K, 8K, 16K, 32K, 64K, 128K, 256K, 512K or
1M bytes.

The -noI, -noD, -noF and -noX options can be used to force Mksquashfs to not
compress inodes (metadata), data, fragments and extended attributes
respectively.  The -no-compression option generates an uncompressed filesystem,
and it is equivalent to specifying all of the -noI, -noD, -noF and -noX options.

The -no-fragments option tells Mksquashfs to not generate fragment blocks.  A
fragment block consists of multiple small files (all smaller than the block
size) packed and compressed together.  This produces much better compression
than storing and compressing these files separately.  It also typically
improves I/O as multiple files in the same directory are read at the same time.
You don't want to enable this option unless you fully understand the effects.

The -tailends option tells Mksquashfs to always generate fragments for files
irrespective of the file length.  By default only small files less than the data
block size are packed into fragment blocks.  The tail ends of files which do not
fit fully into a block, are NOT by default packed into fragments.  This is a
legacy setting when hard disks were mechanical, and had slow seek times.  In
general setting this option will gain a little more compression, without
affecting I/O performance.

The -no-duplicates option tells Mksquashfs to not check the files being added to
the filesystem for duplicates.  This can result in quicker filesystem
generation although obviously compression will suffer badly if there is a lot
of duplicates.


5. BUILDING REPRODUCIBLE FILESYSTEM IMAGES
------------------------------------------

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 -mkfs-time, -root-time and -inode-time options with a timestamp
-------------------------------------------------------------------

5.1.1 -mkfs-time <time>

Set mkfs time to <time>.  Time can be an integer which is the seconds since
the epoch of 1970-01-01 00:00:00 UTC), or a date string as recognised by the
"date" command.

5.1.2 -root-time <time>

Set root directory timestamp to <time>.  Time can be an integer which is the
seconds since the epoch of 1970-01-01 00:00:00 UTC), or a date string as
recognised by the "date" command.

5.1.3 -inode-time <time>

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).

Set all file timestamps to <time>.  Time can be an integer which is the seconds
since the epoch of 1970-01-01 00:00:00 UTC), or a date string as recognised by
the "date" command.

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

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

5.2.1 -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.

5.2.2 -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.

5.2.3 -inode-time inode

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.


5.3 New easier to remember shorthand options
--------------------------------------------

5.3.1 -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.

5.3.2 -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.

5.4 the environment variable SOURCE_DATE_EPOCH
----------------------------------------------

As an alternative to the above command line options, you can set the environment
variable SOURCE_DATE_EPOCH to a time value.

This value will be used to set the mkfs time.  Also any file timestamps which
are after SOURCE_DATE_EPOCH will be clamped to SOURCE_DATE_EPOCH.

See https://reproducible-builds.org/docs/source-date-epoch/ for more
information.


6. TAR STYLE HANDLING OF SOURCE PATHNAMES IN MKSQUASHFS
-------------------------------------------------------

Mksquashfs has always stripped the leading directories of any source pathnames
given on the command line.

For example given the command line

% mksquashfs dir-a/dir-b/dir-c/file1 dir-A/dir-B/file2 sqfs

Mksquashfs will strip the leading directories, and put file1 and file2 into
the same root directory.  If file1 and file2 are directories it will place the
directories into the same root directory, but, if only one directory is
specified, it will put the contents of that directory into the root directory.
Obviously, for a lot of the time this is what you want.  But, if it isn't what
you want it can be quite difficult to get Mksquashfs to do something different.

A lot of people don't like this, and would prefer Mksquashfs acted like "tar",
which does not strip leading directories.  This allows you to create a directory
hierarchy from the pathnames of the supplied files.  In the above example, the
tar archive would contain the pathnames "dir-a/dir-b/dir-c/file1" and
"dir-A/dir-B/file2".

Mksquashfs will act like tar if given the option -no-strip, or -tarstyle.


7. CPIO STYLE HANDLING OF SOURCE PATHNAMES IN MKSQUASHFS
--------------------------------------------------------

Mksquashfs allows you to pipe the set of files to be added to the filesystem to
standard in (stdin), if the -cpiostyle option is given.

As with cpio, directories are not recursively scanned and their contents added.
Evey file to be added to the filesystem must be explicitly specified.

Typically the list of files to be added will be produced via find, or a similar
utility.

For example

% find /home/phillip/squashfs-tools | mksquashfs - img.sqfs -cpiostyle

Will create an image containing everything in squashfs-tools and its
sub-directories.  Note, "-" is given as the source pathname in Mksquashfs, and
indicates no commmand line sources.

The following will add just the files ending in .c, .h and .o.

% find /home/phillip/squashfs-tools -name "*.[cho]" | mksquashfs - img.sqfs -cpiostyle


8. SPECIFYING THE UIDs/GIDs USED IN THE FILESYSTEM
--------------------------------------------------

By default files in the generated filesystem inherit the UID and GID ownership
of the original file.  However,  Mksquashfs provides a number of options which
can be used to override the ownership.

The -all-root option forces all file uids/gids in the generated Squashfs
filesystem to be root.  This allows root owned filesystems to be built without
root access on the host machine.

The "-force-uid <user>"  option forces all files in the generated Squashfs
filesystem to be owned by the specified <user>.  The user can be specified
either by name (i.e. "root") or by number.

The "-force-gid <group>" option forces all files in the generated Squashfs
filesystem to be group owned by the specified <group>.  The group can be
specified either by name (i.e. "root") or by number.

For example:

% mksquashfs source image.sqfs -force-uid phillip -force-gid phillip

Will set all file and directory ownership and group ownership to phillip.

If you only want to set the uid and gid ownership on a subset of files rather
than globally, the Actions system allows you to do that (see section 18).


9. SPECIFYING THE FILE PERMISSIONS USED IN THE FILESYSTEM
---------------------------------------------------------

By default files and directories in the generated filesystem inherit the
permissions of the original files and directories.  However, Mksquashfs provides
a number of options which can be used to override the permissions.

The "-force-file-mode <mode>" option sets all the file (non directories)
permissions to <mode>.  <Mode> can be symbolic or octal (see the next subsection
for the Symbolic mode specification).  The octal mode sets the permissions to
that value, and the symbolic mode specification can either set the permissions,
or add or subtract permissions from the existing file permissions.

The "-force-dir-mode <mode>" option sets all the directory permissions to
<mode>.  <Mode> can be symbolic or octal (see the next subsection for the
Symbolic mode specification).  The octal mode sets the permissions to that
value, and the symbolic mode specification can either set the permissions, or
add or subtract permissions from the existing directory permissions.

The reason why the options are split into two, one that acts on files and one
that acts on directories, is because the permission bits have different
semantics for files and directories.  For example the 'x' bit means execute for
files and search for directories.  Hence you might want to ensure the 'x'
bit is set for directories for user/group/other but not want to make all files
executable.

Some examples:

% mksquashfs source image.sqfs -force-dir-mode 0700

Will set all directory permissions to drwx------, or read, write and search
for owner, and nothing for everyone else.

% mksquashfs source image.sqfs -force-file-mode 0666

Will set all file permissions to -rw-rw-rw-, or read and write by everyone,
but executable by no-one.

% mksquashfs source image.sqfs -force-file-mode go-w,u+rw

Will modify each file's permissions, by removing write ('w') for group and
other, but also ensure the owner can read and write to the files by adding 'r'
and 'w'.

If you only want to set the permissions on a subset of files rather than
globally, the Actions system allows you to do that (see section 18).

9.1 SYMBOLIC MODE SPECIFICATION
-------------------------------

The symbolic mode is of the format [ugoa]*[[+-=]PERMS]+.  PERMS = [rwxXst]+ or
[ugo], and the sequence can be repeated separated with commas.

A combination of the letters ugoa specify which permission bits will be
affected, u means user, g means group, o means other, and a means all or ugo.

The next letter is +, - or =.  The letter + means add to the existing permission
bits, - means remove the bits from the existing permission bits, and = means set
the permission bits.

The permission bits (PERMS) are a combination of [rwxXst] which
sets/adds/removes those bits for the specified ugoa combination, r means read, w
means write and x means execute for files or search for directories.  X has a
special meaning, if the file is a directory it is equivalent to x or search, but
if it is a non-directory, it only takes effect if execute is already set for
user, group or other.  The s flag sets user or group ID on execution, and the t
flag on a directory sets restricted deletion, or historically made the file
sticky if a non-directory.

The permission bits can also be u, g or o, which takes the permission bits from
the user, group or other of the file respectively.


10. EXCLUDING FILES FROM THE FILESYSTEM
---------------------------------------

The -e and -ef options allow files/directories to be specified which are
excluded from the output filesystem.  The -e option takes the exclude
files/directories from the command line, the -ef option takes the
exlude files/directories from the specified exclude file, one file/directory
per line.

Two styles of exclude file matching are supported: basic exclude matching, and
extended wildcard matching.  Basic exclude matching is a legacy feature
retained for backwards compatibility with earlier versions of Mksquashfs.
Extended wildcard matching should be used in preference.

10.1 BASIC EXCLUDE MATCHING
---------------------------

Each exclude file is treated as an exact match of a file/directory in
the source directories.  If an exclude file/directory is absolute (i.e.
prefixed with /, ../, or ./) the entry is treated as absolute, however, if an
exclude file/directory is relative, it is treated as being relative to each of
the sources in turn, i.e.

% mksquashfs /tmp/source1 source2  output_fs -e ex1 /tmp/source1/ex2 out/ex3

Will generate exclude files /tmp/source1/ex2, /tmp/source1/ex1, source2/ex1,
/tmp/source1/out/ex3 and source2/out/ex3.

10.2 EXTENDED EXCLUDE FILE HANDLING
-----------------------------------

Extended exclude file matching treats each exclude file as a wildcard or
regex expression.  To enable wildcard matching specify the -wildcards
option, and to enable regex matching specify the -regex option.  In most
cases the -wildcards option should be used rather than -regex because wildcard
matching behaviour is significantly easier to understand!

In addition to wildcards/regex expressions, exclude files can be "anchored" or
"non-anchored".  An anchored exclude is one which matches from the root of the
directory and nowhere else, a non-anchored exclude matches anywhere.  For
example given the directory hierarchy "a/b/c/a/b", the anchored exclude "a/b"
will match "a/b" at the root of the directory hierarchy, but it will not match
the "/a/b" sub-directory within directory "c", whereas a non-anchored exclude
would.

A couple of examples should make this clearer.

Anchored excludes

  1. mksquashfs example image.sqsh -wildcards -e 'test/*.gz'

     Exclude all files matching "*.gz" in the top level directory "test".

  2. mksquashfs example image.sqsh -wildcards -e '*/[Tt]est/example*'

     Exclude all files beginning with "example" inside directories called
     "Test" or "test", that occur inside any top level directory.

  Using extended wildcards, negative matching is also possible.

  3. mksquashfs example image.sqsh -wildcards -e 'test/!(*data*).gz'

     Exclude all files matching "*.gz" in top level directory "test", except
     those with "data" in the name.

Non-anchored excludes

  By default excludes match from the top level directory, but it is often useful
  to exclude a file matching anywhere in the source directories.  For this
  non-anchored excludes can be used, specified by pre-fixing the exclude with
  "...".

  Examples:

  1. mksquashfs example image.sqsh -wildcards -e '... *.gz'

     Exclude files matching "*.gz" anywhere in the source directories.
     For example this will match "example.gz", "test/example.gz", and
     "test/test/example.gz".

  2. mksquashfs example image.sqsh -wildcards -e '... [Tt]est/*.gz'

     Exclude files matching "*.gz" inside directories called "Test" or
     "test" that occur anywhere in the source directories.

  Again, using extended wildcards, negative matching is also possible.

  3. mksquashfs example image.sqsh -wildcards -e '... !(*data*).gz'

     Exclude all files matching "*.gz" anywhere in the source directories,
     except those with "data" in the name.


11. 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 11.1 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. The percentage is overall CPU usage (out of
2000%).  The X-axis is number of reader threads, and the Y-axis is file size.

	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.

11.1 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.


12. REDUCING CPU AND I/O USAGE
------------------------------

By default Mksquashfs will use all the CPUs available to compress and create the
filesystem, and will read from the source files and write to the output
filesystem as fast as possible.  This maximises both CPU usage and I/O.

Sometimes you don't want Mksquashfs to use all CPU and I/O bandwidth.  For those
cases Mksquashfs supports two complementary options, -processors and -throttle.

The -processors option can be used to reduce the number of parallel compression
threads used by Mksquashfs.  Reducing this to 1 will create the minimum number
of threads, and this will reduce CPU usage, and that in turn will reduce I/O
(because CPUs are normally the bottleneck).

The -throttle option reduces the speed Mksquashfs reads from the source files.
The value is a percentage (obviously from 1 - 100), and 50 will reduce the read
rate by half (the read thread will spend half its time idling), and 75 by three
quarters.  Reducing the speed of I/O will also reduce the CPU usage as there is
insufficient data rate to use all cores.

Which option should you use?  Both will effectively reduce CPU and I/O in
normal cases where intensive use is being made of both I/O and CPUs.  But
in edge cases there can be an imbalance where reducing one has no effect, or
it can't be reduced any further.  For instance when there is only 1 or 2 cores
available, setting -processors to the minimum of 1 may still use too much
CPU.  Additionally if your input source is slow Mksquashfs may still max it out
with -processors set to the minimum of 1.  In this case you can use -throttle
in addition to -processors or on its own.


13. FILTERING AND ADDING EXTENDED ATTRIBUTES (XATTRs)
-----------------------------------------------------

Mksquashfs has a number of options which allow extended attributes (xattrs) to
be filtered from the source files or added to the created Squashfs filesystem.

The -no-xattrs option removes any extended attributes which may exist in the
source files, and creates a filesystem without any extended attributes.

The -xattrs-exclude option specifies a regular expression (regex), which removes
any extended attribute that matches the regular expression from all files.
For example the regex '^user.' will remove all User extended attributes.

The -xattrs-include option instead specifies a regular expression (regex) which
includes any extended attribute that matches, and removes anything that does't
match.  For example the regex '^user.' will only keep User extended attributes
and anything else will be removed.

Mksquashfs also allows you to add extended attributes to files in the Squashfs
filesystem using the -xattrs-add option.  This option takes an xattr name and
value pair separated by the '=' character.

The extended attribute name can be any valid name and can be in the namespaces
security, system, trusted, or user.  User extended attributes are added to files
and directories (see man 7 xattr for explanation), and the others are added to
all files.

The extended attribute value by default will be treated as binary (i.e. an
uninterpreted byte sequence), but it can be prefixed with 0s, where it will be
treated as base64 encoded, or prefixed with 0x, where it will be treated as
hexidecimal.

Obviously using base64 or hexidecimal allows values to be used that cannot be
entered on the command line such as non-printable characters etc.  But it
renders the string non-human readable.  To keep readability and to allow
non-printable characters to be entered, the 0t prefix is supported.  This
encoding is similar to binary encoding, except backslashes are specially
treated, and a backslash followed by three octal digits can be used to encode
any ASCII character, which obviously can be used to encode non-printable values.
The following four command lines are equivalent

mksquashfs dir image.sqfs -xattrs-add "user.comment=hello world"
mksquashfs dir image.sqfs -xattrs-add "user.comment=0saGVsbG8gd29ybGQ="
mksquashfs dir image.sqfs -xattrs-add "user.comment=0x68656c6c6f20776f726c64"
mksquashfs dir image.sqfs -xattrs-add "user.comment=0thello world"

Obviously in the above example there are no non-printable characters and so
the 0t prefixed string is identical to the first line.  The following three
command lines are identical, but where the space has been replaced by the
non-printable NUL '\0' (null character).

mksquashfs dir image.sqfs -xattrs-add "user.comment=0thello\000world"
mksquashfs dir image.sqfs -xattrs-add "user.comment=0saGVsbG8Ad29ybGQ="
mksquashfs dir image.sqfs -xattrs-add "user.comment=0x68656c6c6f00776f726c64"


14. PSEUDO FILE SUPPORT
-----------------------

Mksquashfs supports pseudo files, these allow files, directories, character
devices, block devices, fifos, symbolic links, hard links and extended
attributes to be specified and added to the Squashfs filesystem being built,
rather than requiring them to be present in the source files.  This, for
example, allows device nodes to be added to the filesystem without requiring
root access.

Pseudo files also support "dynamic pseudo files" and a modify operation.
Dynamic pseudo files allow files to be dynamically created when Mksquashfs is
run, their contents being the result of running a command or piece of shell
script.  The modify operation allows the mode/uid/gid of an existing file in the
source filesystem to be modified.

Two Mksquashfs options are supported, -p allows one pseudo file to be specified
on the command line, and -pf allows a pseudo file to be specified containing a
list of pseduo definitions, one per line.

14.1 CREATING A DYNAMIC FILE
----------------------------

Pseudo definition

Filename f mode uid gid command

mode is the octal mode specifier, similar to that expected by chmod.

uid and gid can be either specified as a decimal number, or by name.

command can be an executable or a piece of shell script, and it is executed by
running "/bin/sh -c command".   The stdout becomes the contents of "Filename".

Examples:

Running a basic command
-----------------------

/somedir/dmesg f 444 root root dmesg

creates a file "/somedir/dmesg" containing the output from dmesg.

Executing shell script
----------------------

RELEASE f 444 root root \
		if [ ! -e /tmp/ver ]; then \
			echo 0 > /tmp/ver; \
		fi; \
                ver=`cat /tmp/ver`; \
                ver=$((ver +1)); \
                echo $ver > /tmp/ver; \
                echo -n `cat /tmp/release`; \
                echo "-dev #"$ver `date` "Build host" `hostname`

Creates a file RELEASE containing the release name, date, build host, and an
incrementing version number.  The incrementing version is a side-effect of
executing the shell script, and ensures every time Sqfstar is run a new version
number is used without requiring any other shell scripting.

The above example also shows that commands can be split across multiple lines
using "\".  Obviously as the script will be presented to the shell as a single
line, a semicolon is need to separate individual shell commands within the shell
script.

Reading from a device (or fifo/named socket)
--------------------------------------------

input f 444 root root dd if=/dev/sda1 bs=1024 count=10

Copies 10K from the device /dev/sda1 into the file input.  Ordinarily Sqfstar
given a device, fifo, or named socket will place that special file within the
Squashfs filesystem, the above allows input from these special files to be
captured and placed in the Squashfs filesystem.

14.2 CREATING A BLOCK OR CHARACTER DEVICE
-----------------------------------------

Pseudo definition

Filename type mode uid gid major minor

Where type is either
	b - for block devices, and
	c - for character devices

mode is the octal mode specifier, similar to that expected by chmod.

uid and gid can be either specified as a decimal number, or by name.

For example:

/dev/chr_dev c 666 root root 100 1
/dev/blk_dev b 666 0 0 200 200

creates a character device "/dev/chr_dev" with major:minor 100:1 and a block
device "/dev/blk_dev" with major:minor 200:200, both with root uid/gid and a
mode of rw-rw-rw.

14.3 CREATING A DIRECTORY
--------------------------

Pseudo definition

Filename d mode uid gid

mode is the octal mode specifier, similar to that expected by chmod.

uid and gid can be either specified as a decimal number, or by name.

For example:

/pseudo_dir d 666 root root

creates a directory "/pseudo_dir" with root uid/gid and mode of rw-rw-rw.

14.4 CREATING A SYMBOLIC LINK
------------------------------

Pseudo definition

Filename s mode uid gid symlink

uid and gid can be either specified as a decimal number, or by name.

Note mode is ignored, as symlinks always have "rwxrwxrwx" permissions.

For example:

symlink s 0 root root example

Creates a symlink "symlink" to file "example" with root uid/gid.

14.5 CREATING REFERENCES TO FILES
---------------------------------

The "f" Pseudo definition allows a regular file to be created from the output of
a command (or shell).  Often this is used to reference a file outside the source
directories by executing "cat", e.g.

README f 0555 0 0 cat /home/phillip/latest-version/README

Because this is a quite frequent use of the definition, alternative faster
"File reference" or Hard Link Pseudo definitions exist:

README l pathname

and

README h pathname

If pathname was "/home/phillip/latest-version/README", then both will create a
reference to "/home/phillip/latest-version/README", and obviously the
timestamp/mode and owership will be used.

The difference between the 'l' and 'h' definitions is in the handling of
symbolic links, if the pathname points to a symbolic link, the 'l' definition
won't follow it, and so you'll get a reference to the symbolic link, whereas
the 'h' definition will follow it, and you'll get a reference to whatever
the symbolic link points to.

The definitions can also be used to create additional references to files within
the source directories.  For instance if "phillip/latest/README" was a file
being added to the filesystem, then

README l phillip/latest/README

Will create a Hard Link (and increment the nlink count on the inode).

In both cases, the path to the file being referenced is the system filesystem
path, and can be absolute (prefixed with /), or relative to the current working
directory.

There is an additional 'L' Pseudo definition, which closes a loophole in the
above 'l' definition.  The 'l' Pseudo definition cannot create references or
Hard Links to files created by Pseudo definitions, because by definition they do
not exist in the system filesystem.

with 'L' the referenced file is expected to be a Pseudo file, and in this case
the path is taken to be from the root of the Squashfs filesystem being created,
e.g.

char-dev c 0555 0 0 1 2

link L char-dev

Will create a Hard Link named "link" to the character device called "char-dev"
created by the previous Pseudo definition.

14.6 CREATING SOCKETS/FIFOS
----------------------------

Pseudo definition

filename i mode uid gid [s|f]

To create a Unix domain socket, 's' should be used, i.e.

filename i 0777 root root s

and to create a FIFO, 'f' should be used, i.e.

filename i 0777 root root f

14.7 ADDING EXTENDED ATTRIBUTES TO FILES
----------------------------------------

Pseudo definition

filename x name=val

Will add the extended attribute <name> to <filename> with <val> contents.  See
Section 13 for a description of the <val> formats supported.

14.8 MODIFYING ATTRIBUTES OF AN EXISTING FILE
---------------------------------------------

Pseudo definition

Filename m mode uid gid

mode is the octal mode specifier, similar to that expected by chmod.

uid and gid can be either specified as a decimal number, or by name.

For example:

dmesg m 666 root root

Changes the attributes of the file "dmesg" in the filesystem to have root
uid/gid and a mode of rw-rw-rw, overriding the attributes obtained from the
source filesystem.

14.9 SPECIFYING A DEFAULT PSEUDO DIRECTORY DEFINITION
-----------------------------------------------------

The option

-pd <d mode uid gid>

Specifies 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.

For example this provides an alternative way of specifying any leading
directories in a pseudo file definition.  We could create a pseudo file
/dir1/dir2/file on the command line using no other sources like this:

% mksquashfs - image.sqfs -p "/ d 0777 0 0" -p "/dir1 d 0777 0 0"
  -p "/dir1/dir2 d 0777 0 0" -p "/dir1/dir2/file f 0777 0 0 echo hello_world"

Here we have to create each directory explicitly including the root directory.

The new -pd option allows us to specify a default pseudo directory and then
just specify the pseudo definition for the file, leaving Mksquashfs to
automatically create the leading directories:

% mksquashfs - image.sqfs -pd "d 0777 0 0" -p "/dir1/dir2/file f 0777 0 0 echo hello world"


15. EXTENDED PSEUDO FILE DEFINITIONS WITH TIMESTAMPS
----------------------------------------------------

The Pseudo file definitions described above do not allow the timestamp of the
created file to be specified, and the files will be timestamped with the current
time.

Extended versions of the Pseudo file definitions are supported which take a
<time> timestamp.  These are distinquished from the previous definitions by
using an upper-case type character.  For example the "D" definition is identical
to the "d" definition, but it takes a <time> timestamp.

The list of extended definitions are:

	filename F time mode uid gid command
	filename D time mode uid gid
	filename B time mode uid gid major minor
	filename C time mode uid gid major minor
	filename S time mode uid gid symlink
	filename I time mode uid gid [s|f]
	filename M time mode uid gid

<time> can be either an unsigned decimal integer (which represents the seconds
since the epoch of 1970-01-01 00:00 UTC), or a "date string" which is parsed and
converted into an integer since the epoch, by calling the "date" command.

Because most date strings have spaces, they will need to be quoted, and if
entered on the command line, these quotes will need to be protected from the
shell by backslashes, i.e.

% mksquashfs dir image.sqsh -p "file D \"1 jan 1980\" 0777 phillip phillip"

Obviously anything "date" accepts as a valid string can be used, such as
"yesterday", "last week" etc.

15.1 SPECIFYING A DEFAULT PSEUDO DIRECTORY DEFINITION WITH TIMESTAMP
--------------------------------------------------------------------

The option

-pd <D time mode uid gid>

Specifies a default pseudo directory which will be used in pseudo definitions if
a directory in the pathname does not exist.

The upper-case D indicates this is an extended pseudo definition which takes
a <time> timestamp.  <time> can be either an unsigned decimal integer or a
"date string" which is parsed and converted into an integer by calling the
"date" command.


16. APPENDING TO SQUASHFS FILESYSTEMS
-------------------------------------

Running Mksquashfs with the output file containing an existing Squashfs
filesystem will add the source items to the existing filesystem.  By default,
the source items are added to the existing root directory.

To make this clear... An existing filesystem image contains root entries
"old1", and "old2".  Source directory "/home/phillip/test" contains  "file1",
"file2" and "dir1".

example 1:

% mksquashfs /home/phillip/test image

Will create a new image with root entries "old1", "old2", "file1", "file2" and
"dir1"

example 2:

% mksquashfs /home/phillip/test image -keep-as-directory

Will create a new image with root entries "old1", "old2", and "test".  As shown
in section 1, for single source directories '-keep-as-directory' adds the source
directory rather than the contents of the directory.

example 3:

% mksquashfs /home/phillip/test image -keep-as-directory -root-becomes
original-root

Will create a new image with root entries "original-root", and "test".  The
'-root-becomes' option specifies that the original root becomes a subdirectory
in the new root, with the specified name.

The append option with file duplicate detection, means squashfs can be used as a
simple versioning archiving filesystem. A squashfs filesystem can be created
with for example the linux-2.4.19 source.  Appending the linux-2.4.20 source
will create a filesystem with the two source trees, but only the changed files
will take extra room, the unchanged files will be detected as duplicates.


17. APPENDING RECOVERY FILE FEATURE
-----------------------------------

Recovery files are created when appending to existing Squashfs filesystems.
This allows the original filesystem to be recovered if Mksquashfs aborts
unexpectedly (i.e. power failure).

The recovery files are called squashfs_recovery_xxx_yyy, where "xxx" is the name
of the filesystem being appended to, and "yyy" is a number to guarantee filename
uniqueness (the PID of the parent Mksquashfs process).

Normally if Mksquashfs exits correctly the recovery file is deleted to avoid
cluttering the filesystem.  If Mksquashfs aborts, the "-recover" option can be
used to recover the filesystem, giving the previously created recovery file as a
parameter, i.e.

mksquashfs dummy image.sqsh -recover squashfs_recovery_image.sqsh_1234

The writing of the recovery file can be disabled by specifying the
"-no-recovery" option.


18. MKSQUASHFS ACTIONS INTRODUCTION
-----------------------------------

The Mksquashfs Actions code allows an "action" to be executed on a file if one
or more "tests" succeed.  If you're familiar with the "find" command, then an
action is similar to "-print", and a test is similar to say "-name" or "-type".

To illustrate this it is useful to give two concrete examples.

example 1: the fragment action

% mksquashfs /home/phillip/github github.sqsh -action "fragment(cfiles) @ name(*.[ch])" -action "fragment(ofiles) @ name(*.o)"

This example defines two "fragment actions" which control the packing of files
within fragments.  Specifically, it creates a specialised fragment called
"cfiles" which packs files matching the wildcard name "*.[ch]".

It also creates another specialised fragment called "ofiles" which packs files
matching the wilcard name "*.o".

Producing specialised fragments which only pack files which match a range of
tests, can produce better compression and/or I/O performance as it can optimise
similarity or access patterns.  But it can also produce worse compression, and
so you should always test the effect.

Additionally, you should be able to see that an action definition is split into
an action function before the "@", and one or more test functions after the @.
Quoting is necessary here to protect it from interpretation by the shell.  Also
the spacing before and after the "@" isn't necessary and is used here for added
readability.

example 2: the uncompressed action

% mksquashfs /home/phillip backup.sqsh -action "uncompressed @ ( name(*.jpg) || name(*.mpg) ) || ( name(*.img) && filesize(+1G) )"

This is a more complex example.  It tells Mksquashfs to not try and compress any
file which matches the wildcard names "*.jpg" and "*.mpg".  But it also tells
Mksquashfs not to try and compress files which match the wildcard name "*.img"
and are also 1 Gigabyte in size or larger.

This example introduces the fact that tests can be combined using the logical
operators && (and), || (or) and ! (not), and can be bracketed.

Please see the ACTIONS-README file for syntax and extra information.


19. MISCELLANEOUS OPTIONS
------------------------

The -info option displays the files/directories as they are compressed and
added to the filesystem.  The original uncompressed size of each file is
printed, along with DUPLICATE if the file is a duplicate of a file in the
filesystem.

The info-file option does the same except that the output is to a file rather
than to stdout.  This allows the -info-file option to be used together with the
progressbar.

The -nopad option informs Mksquashfs to not pad the filesystem to a 4K multiple.
This is performed by default to enable the output filesystem file to be mounted
by loopback, which requires files to be a 4K multiple.  If the filesystem is
being written to a block device, or is to be stored in a bootimage, the extra
pad bytes are not needed.
