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SQUASHFS/squashfs-tools-4.4/RELEASE-READMEs/README-4.1
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SQUASHFS/squashfs-tools-4.4/RELEASE-READMEs/README-4.1
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SQUASHFS 4.1 - A squashed read-only filesystem for Linux
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Copyright 2002-2010 Phillip Lougher <phillip@lougher.demon.co.uk>
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Released under the GPL licence (version 2 or later).
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Welcome to Squashfs 4.1. This is a tools only release, support for Squashfs
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file systems is in mainline (2.6.29 and later).
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New features in Squashfs-tools 4.1
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----------------------------------
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1. Support for extended attributes
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2. Support for LZMA and LZO compression
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3. New pseudo file features
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Compatiblity
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------------
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Mksquashfs 4.1 generates 4.0 filesystems. These filesystems are fully
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compatible/interchangable with filesystems generated by Mksquashfs 4.0 and are
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mountable on 2.6.29 and later kernels.
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Extended attributes (xattrs)
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----------------------------
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Squashfs file systems now have extended attribute support. The
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extended attribute implementation has the following features:
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1. Layout can store up to 2^48 bytes of compressed xattr data.
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2. Number of xattrs per inode unlimited.
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3. Total size of xattr data per inode 2^48 bytes of compressed data.
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4. Up to 4 Gbytes of data per xattr value.
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5. Inline and out-of-line xattr values supported for higher performance
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in xattr scanning (listxattr & getxattr), and to allow xattr value
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de-duplication.
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6. Both whole inode xattr duplicate detection and individual xattr value
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duplicate detection supported. These can obviously nest, file C's
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xattrs can be a complete duplicate of file B, and file B's xattrs
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can be a partial duplicate of file A.
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7. Xattr name prefix types stored, allowing the redundant "user.", "trusted."
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etc. characters to be eliminated and more concisely stored.
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8. Support for files, directories, symbolic links, device nodes, fifos
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and sockets.
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Extended attribute support is in 2.6.35 and later kernels. File systems
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with extended attributes can be mounted on 2.6.29 and later kernels, the
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extended attributes will be ignored with a warning.
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LZMA and LZO compression
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------------------------
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Squashfs now supports LZMA and LZO compression.
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LZO support is in 2.6.36 and newer kernels. LZMA is not yet in mainline.
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New Mksquashfs options
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----------------------
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-comp <comp>
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Select <comp> compression.
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The compression algorithms supported by the build of Mksquashfs can be
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found by typing mksquashfs without any arguments. The compressors available
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are displayed at the end of the help message, e.g.
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Compressors available:
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gzip (default)
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lzma
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lzo
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The default compression used when -comp isn't specified on the command line
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is indicated by "(default)".
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-no-xattrs
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Don't store extended attributes
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-xattrs
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Store extended attributes
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The default behaviour of Mksquashfs with respect to extended attribute
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storage is build time selectable. The Mksquashfs help message indicates
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whether extended attributes are stored or not, e.g.
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-no-xattrs don't store extended attributes
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-xattrs store extended attributes (default)
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shows that extended attributes are stored by default, and can be disabled
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by the -no-xattrs option.
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-no-xattrs don't store extended attributes (default)
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-xattrs store extended attributes
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shows that extended attributes are not stored by default, storage can be
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enabled by the -xattrs option.
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-noX
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-noXattrCompression
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Don't compress extended attributes
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New Unsquashfs options
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----------------------
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-n[o-xattrs]
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Don't extract xattrs in filesystem
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-x[attrs]
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Extract xattrs in filesystem
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The default behaviour of Unsquashfs with respect to extended attributes
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is build time selectable. The Unsquashfs help message indicates whether
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extended attributes are stored or not, e.g.
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-no[-xattrs] don't extract xattrs in file system
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-x[attrs] extract xattrs in file system (default)
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shows that xattrs are extracted by default.
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-no[-xattrs] don't extract xattrs in file system (default)
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-x[attrs] extract xattrs in file system
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shows that xattrs are not extracted by default.
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New pseudo file support
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-----------------------
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Mksquashfs supports pseudo files, these allow fake files, directories, character
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and block devices to be specified and added to the Squashfs filesystem being
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built, rather than requiring them to be present in the source directories.
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This, for example, allows device nodes to be added to the filesystem without
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requiring root access.
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Mksquashfs 4.1 adds support for "dynamic pseudo files" and a modify operation.
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Dynamic pseudo files allow files to be dynamically created when Mksquashfs
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is run, their contents being the result of running a command or piece of
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shell script. The modifiy operation allows the mode/uid/gid of an existing
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file in the source filesystem to be modified.
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Two Mksquashfs options are supported, -p allows one pseudo file to be specified
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on the command line, and -pf allows a pseudo file to be specified containing a
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list of pseduo definitions, one per line.
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Pseudo operations
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-----------------
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1. Creating a dynamic file
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--------------------------
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Pseudo definition
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Filename f mode uid gid command
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mode is the octal mode specifier, similar to that expected by chmod.
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uid and gid can be either specified as a decimal number, or by name.
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command can be an executable or a piece of shell script, and it is executed
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by running "/bin/sh -c command". The stdout becomes the contents of
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"Filename".
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Examples:
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Running a basic command
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-----------------------
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/somedir/dmesg f 444 root root dmesg
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creates a file "/somedir/dmesg" containing the output from dmesg.
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Executing shell script
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----------------------
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RELEASE f 444 root root \
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if [ ! -e /tmp/ver ]; then \
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echo 0 > /tmp/ver; \
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fi; \
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ver=`cat /tmp/ver`; \
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ver=$((ver +1)); \
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echo $ver > /tmp/ver; \
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echo -n `cat /tmp/release`; \
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echo "-dev #"$ver `date` "Build host" `hostname`
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Creates a file RELEASE containing the release name, date, build host, and
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an incrementing version number. The incrementing version is a side-effect
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of executing the shell script, and ensures every time Mksquashfs is run a
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new version number is used without requiring any other shell scripting.
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The above example also shows that commands can be split across multiple lines
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using "\". Obviously as the script will be presented to the shell as a single
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line, a semicolon is need to separate individual shell commands within the
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shell script.
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Reading from a device (or fifo/named socket)
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--------------------------------------------
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input f 444 root root dd if=/dev/sda1 bs=1024 count=10
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Copies 10K from the device /dev/sda1 into the file input. Ordinarily Mksquashfs
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given a device, fifo, or named socket will place that special file within the
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Squashfs filesystem, the above allows input from these special files to be
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captured and placed in the Squashfs filesystem.
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2. Creating a block or character device
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---------------------------------------
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Pseudo definition
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Filename type mode uid gid major minor
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Where type is either
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b - for block devices, and
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c - for character devices
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mode is the octal mode specifier, similar to that expected by chmod.
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uid and gid can be either specified as a decimal number, or by name.
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For example:
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/dev/chr_dev c 666 root root 100 1
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/dev/blk_dev b 666 0 0 200 200
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creates a character device "/dev/chr_dev" with major:minor 100:1 and
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a block device "/dev/blk_dev" with major:minor 200:200, both with root
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uid/gid and a mode of rw-rw-rw.
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3. Creating a directory
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-----------------------
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Pseudo definition
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Filename d mode uid gid
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mode is the octal mode specifier, similar to that expected by chmod.
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uid and gid can be either specified as a decimal number, or by name.
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For example:
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/pseudo_dir d 666 root root
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creates a directory "/pseudo_dir" with root uid/gid and mode of rw-rw-rw.
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4. Modifying attributes of an existing file
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-------------------------------------------
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Pseudo definition
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Filename m mode uid gid
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mode is the octal mode specifier, similar to that expected by chmod.
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uid and gid can be either specified as a decimal number, or by name.
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For example:
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dmesg m 666 root root
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Changes the attributes of the file "dmesg" in the filesystem to have
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root uid/gid and a mode of rw-rw-rw, overriding the attributes obtained
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from the source filesystem.
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