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Squashfs: documentation 

Signed-off-by: Phillip Lougher <phillip@lougher.demon.co.uk>
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SQUASHFS 4.0 FILESYSTEM
=======================
Squashfs is a compressed read-only filesystem for Linux.
It uses zlib 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).
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.
Mailing list: squashfs-devel@lists.sourceforge.net
Web site: www.squashfs.org
1. FILESYSTEM FEATURES
----------------------
Squashfs filesystem features versus Cramfs:
Squashfs Cramfs
Max filesystem size: 2^64 16 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 and ACL support: no no
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).
2. USING SQUASHFS
-----------------
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.
3. SQUASHFS FILESYSTEM DESIGN
-----------------------------
A squashfs filesystem consists of seven parts, packed together on a byte
alignment:
---------------
| superblock |
|---------------|
| datablocks |
| & fragments |
|---------------|
| inode table |
|---------------|
| directory |
| table |
|---------------|
| fragment |
| table |
|---------------|
| export |
| table |
|---------------|
| uid/gid |
| lookup table |
---------------
Compressed data blocks are written to the filesystem as files are read from
the source directory, and checked for duplicates. Once all file data has been
written the completed inode, directory, fragment, export and uid/gid lookup
tables are written.
3.1 Inodes
----------
Metadata (inodes and directories) are compressed in 8Kbyte blocks. Each
compressed block is prefixed by a two byte length, the top bit is set if the
block is uncompressed. A block will be uncompressed if the -noI option is set,
or if the compressed block was larger than the uncompressed block.
Inodes are packed into the metadata blocks, and are not aligned to block
boundaries, therefore inodes overlap compressed blocks. Inodes are identified
by a 48-bit number which encodes the location of the compressed metadata block
containing the inode, and the byte offset into that block where the inode is
placed (<block, offset>).
To maximise compression there are different inodes for each file type
(regular file, directory, device, etc.), the inode contents and length
varying with the type.
To further maximise compression, two types of regular file inode and
directory inode are defined: inodes optimised for frequently occurring
regular files and directories, and extended types where extra
information has to be stored.
3.2 Directories
---------------
Like inodes, directories are packed into compressed metadata blocks, stored
in a directory table. Directories are accessed using the start address of
the metablock containing the directory and the offset into the
decompressed block (<block, offset>).
Directories are organised in a slightly complex way, and are not simply
a list of file names. The organisation takes advantage of the
fact that (in most cases) the inodes of the files will be in the same
compressed metadata block, and therefore, can share the start block.
Directories are therefore organised in a two level list, a directory
header containing the shared start block value, and a sequence of directory
entries, each of which share the shared start block. A new directory header
is written once/if the inode start block changes. The directory
header/directory entry list is repeated as many times as necessary.
Directories are sorted, and can contain a directory index to speed up
file lookup. Directory indexes store one entry per metablock, each entry
storing the index/filename mapping to the first directory header
in each metadata block. Directories are sorted in alphabetical order,
and at lookup the index is scanned linearly looking for the first filename
alphabetically larger than the filename being looked up. At this point the
location of the metadata block the filename is in has been found.
The general idea of the index is ensure only one metadata block needs to be
decompressed to do a lookup irrespective of the length of the directory.
This scheme has the advantage that it doesn't require extra memory overhead
and doesn't require much extra storage on disk.
3.3 File data
-------------
Regular files consist of a sequence of contiguous compressed blocks, and/or a
compressed fragment block (tail-end packed block). The compressed size
of each datablock is stored in a block list contained within the
file inode.
To speed up access to datablocks when reading 'large' files (256 Mbytes or
larger), the code implements an index cache that caches the mapping from
block index to datablock location on disk.
The index cache allows Squashfs to handle large files (up to 1.75 TiB) while
retaining a simple and space-efficient block list on disk. The cache
is split into slots, caching up to eight 224 GiB files (128 KiB blocks).
Larger files use multiple slots, with 1.75 TiB files using all 8 slots.
The index cache is designed to be memory efficient, and by default uses
16 KiB.
3.4 Fragment lookup table
-------------------------
Regular files can contain a fragment index which is mapped to a fragment
location on disk and compressed size using a fragment lookup table. This
fragment lookup table is itself stored compressed into metadata blocks.
A second index table is used to locate these. This second index table for
speed of access (and because it is small) is read at mount time and cached
in memory.
3.5 Uid/gid lookup table
------------------------
For space efficiency regular files store uid and gid indexes, which are
converted to 32-bit uids/gids using an id look up table. This table is
stored compressed into metadata blocks. A second index table is used to
locate these. This second index table for speed of access (and because it
is small) is read at mount time and cached in memory.
3.6 Export table
----------------
To enable Squashfs filesystems to be exportable (via NFS etc.) filesystems
can optionally (disabled with the -no-exports Mksquashfs option) contain
an inode number to inode disk location lookup table. This is required to
enable Squashfs to map inode numbers passed in filehandles to the inode
location on disk, which is necessary when the export code reinstantiates
expired/flushed inodes.
This table is stored compressed into metadata blocks. A second index table is
used to locate these. This second index table for speed of access (and because
it is small) is read at mount time and cached in memory.
4. TODOS AND OUTSTANDING ISSUES
-------------------------------
4.1 Todo list
-------------
Implement Xattr and ACL support. The Squashfs 4.0 filesystem layout has hooks
for these but the code has not been written. Once the code has been written
the existing layout should not require modification.
4.2 Squashfs internal cache
---------------------------
Blocks in Squashfs are compressed. To avoid repeatedly decompressing
recently accessed data Squashfs uses two small metadata and fragment caches.
The cache is not used for file datablocks, these are decompressed and cached in
the page-cache in the normal way. The cache is used to temporarily cache
fragment and metadata blocks which have been read as a result of a metadata
(i.e. inode or directory) or fragment access. Because metadata and fragments
are packed together into blocks (to gain greater compression) the read of a
particular piece of metadata or fragment will retrieve other metadata/fragments
which have been packed with it, these because of locality-of-reference may be
read in the near future. Temporarily caching them ensures they are available
for near future access without requiring an additional read and decompress.
In the future this internal cache may be replaced with an implementation which
uses the kernel page cache. Because the page cache operates on page sized
units this may introduce additional complexity in terms of locking and
associated race conditions.