Documentation/filesystems/squashfs.rst

1 .. SPDX-License-Identifier: GPL-2.0
7 Squashfs is a compressed read-only filesystem for Linux.
11 minimise data overhead. Block sizes greater than 4K are supported up to a
12 maximum of 1Mbytes (default block size 128K).
14 Squashfs is intended for general read-only filesystem use, for archival
15 use (i.e. in cases where a .tar.gz file may be used), and in constrained
16 block device/memory systems (e.g. embedded systems) where low overhead is
19 Mailing list: squashfs-devel@lists.sourceforge.net
23 ----------------------
30 Max filesystem size		2^64			256 MiB
31 Max file size			~ 2 TiB			16 MiB
35 Max block size			1 MiB			4 KiB
39 Tail-end packing (fragments)	yes			no
44 32-bit uids/gids		yes			no
53 file type, i.e. regular file, directory, symbolic link, and block/char device
57 -----------------
59 As squashfs is a read-only filesystem, the mksquashfs program must be used to
64 The squashfs-tools development tree is now located on kernel.org
65 	git://git.kernel.org/pub/scm/fs/squashfs/squashfs-tools.git
68 -----------------------------
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98 	 ---------------
106 -----------------------
109 dictionary size).  If non-default compression options have been used, then
113 ----------
116 compressed block is prefixed by a two byte length, the top bit is set if the
117 block is uncompressed.  A block will be uncompressed if the -noI option is set,
118 or if the compressed block was larger than the uncompressed block.
120 Inodes are packed into the metadata blocks, and are not aligned to block
122 by a 48-bit number which encodes the location of the compressed metadata block
123 containing the inode, and the byte offset into that block where the inode is
124 placed (<block, offset>).
136 ---------------
141 decompressed block (<block, offset>).
146 compressed metadata block, and therefore, can share the start block.
148 header containing the shared start block value, and a sequence of directory
149 entries, each of which share the shared start block.  A new directory header
150 is written once/if the inode start block changes.  The directory
156 in each metadata block.  Directories are sorted in alphabetical order,
159 location of the metadata block the filename is in has been found.
160 The general idea of the index is to ensure only one metadata block needs to be
166 -------------
169 compressed fragment block (tail-end packed block).   The compressed size
170 of each datablock is stored in a block list contained within the
174 larger), the code implements an index cache that caches the mapping from
175 block index to datablock location on disk.
177 The index cache allows Squashfs to handle large files (up to 1.75 TiB) while
178 retaining a simple and space-efficient block list on disk.  The cache
181 The index cache is designed to be memory efficient, and by default uses
185 -------------------------
188 location on disk and compressed size using a fragment lookup table.  This
195 ------------------------
198 converted to 32-bit uids/gids using an id look up table.  This table is
204 ----------------
207 can optionally (disabled with the -no-exports Mksquashfs option) contain
218 ---------------
229 also allows values to be de-duplicated, the value being stored once, and
233 To reduce overhead in inodes, rather than storing the on-disk
234 location of the xattr list inside each inode, a 32-bit xattr id
239 -------------------------------
242 -------------
246 4.2 Squashfs Internal Cache
247 ---------------------------
252 The cache is not used for file datablocks, these are decompressed and cached in
253 the page-cache in the normal way.  The cache is used to temporarily cache
255 (i.e. inode or directory) or fragment access.  Because metadata and fragments
258 which have been packed with it, these because of locality-of-reference may be
262 In the future this internal cache may be replaced with an implementation which
263 uses the kernel page cache.  Because the page cache operates on page sized