Documentation/filesystems/squashfs.rst

1 .. SPDX-License-Identifier: GPL-2.0
7 Squashfs is a compressed read-only filesystem for Linux.
14 Squashfs is intended for general read-only filesystem use, for archival
19 Mailing list: squashfs-devel@lists.sourceforge.net
23 ----------------------
39 Tail-end packing (fragments)	yes			no
44 32-bit uids/gids		yes			no
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 -----------------
88                            single  use single-threaded decompression (default)
106                                    decompression is load-balanced across the
128 -----------------------------
133 	 ---------------
135 	|---------------|
138 	|---------------|
141 	|---------------|
142 	|  inode table	|
143 	|---------------|
145 	|     table     |
146 	|---------------|
148 	|    table      |
149 	|---------------|
151 	|    table      |
152 	|---------------|
154 	|  lookup table	|
155 	|---------------|
157 	|     table	|
158 	 ---------------
162 written the completed inode, directory, fragment, export, uid/gid lookup and
166 -----------------------
169 dictionary size).  If non-default compression options have been used, then
173 ----------
177 block is uncompressed.  A block will be uncompressed if the -noI option is set,
182 by a 48-bit number which encodes the location of the compressed metadata block
196 ---------------
199 in a directory table.  Directories are accessed using the start address of
214 file lookup.  Directory indexes store one entry per metablock, each entry
217 and at lookup the index is scanned linearly looking for the first filename
221 decompressed to do a lookup irrespective of the length of the directory.
226 -------------
229 compressed fragment block (tail-end packed block).   The compressed size
238 retaining a simple and space-efficient block list on disk.  The cache
244 3.5 Fragment lookup table
245 -------------------------
248 location on disk and compressed size using a fragment lookup table.  This
249 fragment lookup table is itself stored compressed into metadata blocks.
250 A second index table is used to locate these.  This second index table for
254 3.6 Uid/gid lookup table
255 ------------------------
258 converted to 32-bit uids/gids using an id look up table.  This table is
259 stored compressed into metadata blocks.  A second index table is used to
260 locate these.  This second index table for speed of access (and because it
263 3.7 Export table
264 ----------------
267 can optionally (disabled with the -no-exports Mksquashfs option) contain
268 an inode number to inode disk location lookup table.  This is required to
273 This table is stored compressed into metadata blocks.  A second index table is
274 used to locate these.  This second index table for speed of access (and because
277 3.8 Xattr table
278 ---------------
280 The xattr table contains extended attributes for each inode.  The xattrs
288 to be stored out of line improving scanning and lookup performance and it
289 also allows values to be de-duplicated, the value being stored once, and
293 To reduce overhead in inodes, rather than storing the on-disk
294 location of the xattr list inside each inode, a 32-bit xattr id
296 list using a second xattr id lookup table.
299 -------------------------------
302 -------------
307 ---------------------------
313 the page-cache in the normal way.  The cache is used to temporarily cache
318 which have been packed with it, these because of locality-of-reference may be