linux-imx

SQUASHFS 4.0 FILESYSTEM

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Squashfs is a compressed read-only filesystem for Linux.

It uses zlib, lz4, lzo, or xz compression to compress files, inodes and

directories.  Inodes in the system are very small and all blocks are packed to

minimise data overhead. Block sizes greater than 4K are supported up to a

maximum of 1Mbytes (default block size 128K).

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Squashfs is intended for general read-only filesystem use, for archival

use (i.e. in cases where a .tar.gz file may be used), and in constrained

block device/memory systems (e.g. embedded systems) where low overhead is

needed.

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Mailing list: squashfs-devel@lists.sourceforge.net

Web site: www.squashfs.org

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1. FILESYSTEM FEATURES

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Squashfs filesystem features versus Cramfs:

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                Squashfs        Cramfs

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Max filesystem size:        2^64            256 MiB

Max file size:          ~ 2 TiB         16 MiB

Max files:          unlimited       unlimited

Max directories:        unlimited       unlimited

Max entries per directory:  unlimited       unlimited

Max block size:         1 MiB           4 KiB

Metadata compression:       yes         no

Directory indexes:      yes         no

Sparse file support:        yes         no

Tail-end packing (fragments):   yes         no

Exportable (NFS etc.):      yes         no

Hard link support:      yes         no

"." and ".." in readdir:    yes         no

Real inode numbers:     yes         no

32-bit uids/gids:       yes         no

File creation time:     yes         no

Xattr support:          yes         no

ACL support:            no          no

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Squashfs compresses data, inodes and directories.  In addition, inode and

directory data are highly compacted, and packed on byte boundaries.  Each

compressed inode is on average 8 bytes in length (the exact length varies on

file type, i.e. regular file, directory, symbolic link, and block/char device

inodes have different sizes).

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2. USING SQUASHFS

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As squashfs is a read-only filesystem, the mksquashfs program must be used to

create populated squashfs filesystems.  This and other squashfs utilities

can be obtained from http://www.squashfs.org.  Usage instructions can be

obtained from this site also.

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The squashfs-tools development tree is now located on kernel.org

    git://git.kernel.org/pub/scm/fs/squashfs/squashfs-tools.git

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3. SQUASHFS FILESYSTEM DESIGN

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