Documentation/filesystems/squashfs.txt

   1 SQUASHFS 4.0 FILESYSTEM
   2 =======================
   3
   4 Squashfs is a compressed read-only filesystem for Linux.
   5 It uses zlib/lzo/xz compression to compress files, inodes and directories.
   6 Inodes in the system are very small and all blocks are packed to minimise
   7 data overhead. Block sizes greater than 4K are supported up to a maximum
   8 of 1Mbytes (default block size 128K).
   9
  10 Squashfs is intended for general read-only filesystem use, for archival
  11 use (i.e. in cases where a .tar.gz file may be used), and in constrained
  12 block device/memory systems (e.g. embedded systems) where low overhead is
  13 needed.
  14
  15 Mailing list: squashfs-devel@lists.sourceforge.net
  16 Web site: www.squashfs.org
  17
  18 1. FILESYSTEM FEATURES
  19 ----------------------
  20
  21 Squashfs filesystem features versus Cramfs:
  22
  23                                 Squashfs                Cramfs
  24
  25 Max filesystem size:            2^64                    256 MiB
  26 Max file size:                  ~ 2 TiB                 16 MiB
  27 Max files:                      unlimited               unlimited
  28 Max directories:                unlimited               unlimited
  29 Max entries per directory:      unlimited               unlimited
  30 Max block size:                 1 MiB                   4 KiB
  31 Metadata compression:           yes                     no
  32 Directory indexes:              yes                     no
  33 Sparse file support:            yes                     no
  34 Tail-end packing (fragments):   yes                     no
  35 Exportable (NFS etc.):          yes                     no
  36 Hard link support:              yes                     no
  37 "." and ".." in readdir:        yes                     no
  38 Real inode numbers:             yes                     no
  39 32-bit uids/gids:               yes                     no
  40 File creation time:             yes                     no
  41 Xattr support:                  yes                     no
  42 ACL support:                    no                      no
  43
  44 Squashfs compresses data, inodes and directories.  In addition, inode and
  45 directory data are highly compacted, and packed on byte boundaries.  Each
  46 compressed inode is on average 8 bytes in length (the exact length varies on
  47 file type, i.e. regular file, directory, symbolic link, and block/char device
  48 inodes have different sizes).
  49
  50 2. USING SQUASHFS
  51 -----------------
  52
  53 As squashfs is a read-only filesystem, the mksquashfs program must be used to
  54 create populated squashfs filesystems.  This and other squashfs utilities
  55 can be obtained from http://www.squashfs.org.  Usage instructions can be
  56 obtained from this site also.
  57
  58 The squashfs-tools development tree is now located on kernel.org
  59         git://git.kernel.org/pub/scm/fs/squashfs/squashfs-tools.git
  60
  61 3. SQUASHFS FILESYSTEM DESIGN
  62 -----------------------------
  63
  64 A squashfs filesystem consists of a maximum of nine parts, packed together on a
  65 byte alignment:
  66
  67          ---------------
  68         |  superblock   |
  69         |---------------|
  70         |  compression  |
  71         |    options    |
  72         |---------------|
  73         |  datablocks   |
  74         |  & fragments  |
  75         |---------------|
  76         |  inode table  |
  77         |---------------|
  78         |   directory   |
  79         |     table     |
  80         |---------------|
  81         |   fragment    |
  82         |    table      |
  83         |---------------|
  84         |    export     |
  85         |    table      |
  86         |---------------|
  87         |    uid/gid    |
  88         |  lookup table |
  89         |---------------|
  90         |     xattr     |
  91         |     table     |
  92          ---------------
  93
  94 Compressed data blocks are written to the filesystem as files are read from
  95 the source directory, and checked for duplicates.  Once all file data has been
  96 written the completed inode, directory, fragment, export, uid/gid lookup and
  97 xattr tables are written.
  98
  99 3.1 Compression options
 100 -----------------------
 101
 102 Compressors can optionally support compression specific options (e.g.
 103 dictionary size).  If non-default compression options have been used, then
 104 these are stored here.
 105
 106 3.2 Inodes
 107 ----------
 108
 109 Metadata (inodes and directories) are compressed in 8Kbyte blocks.  Each
 110 compressed block is prefixed by a two byte length, the top bit is set if the
 111 block is uncompressed.  A block will be uncompressed if the -noI option is set,
 112 or if the compressed block was larger than the uncompressed block.
 113
 114 Inodes are packed into the metadata blocks, and are not aligned to block
 115 boundaries, therefore inodes overlap compressed blocks.  Inodes are identified
 116 by a 48-bit number which encodes the location of the compressed metadata block
 117 containing the inode, and the byte offset into that block where the inode is
 118 placed (<block, offset>).
 119
 120 To maximise compression there are different inodes for each file type
 121 (regular file, directory, device, etc.), the inode contents and length
 122 varying with the type.
 123
 124 To further maximise compression, two types of regular file inode and
 125 directory inode are defined: inodes optimised for frequently occurring
 126 regular files and directories, and extended types where extra
 127 information has to be stored.
 128
 129 3.3 Directories
 130 ---------------
 131
 132 Like inodes, directories are packed into compressed metadata blocks, stored
 133 in a directory table.  Directories are accessed using the start address of
 134 the metablock containing the directory and the offset into the
 135 decompressed block (<block, offset>).
 136
 137 Directories are organised in a slightly complex way, and are not simply
 138 a list of file names.  The organisation takes advantage of the
 139 fact that (in most cases) the inodes of the files will be in the same
 140 compressed metadata block, and therefore, can share the start block.
 141 Directories are therefore organised in a two level list, a directory
 142 header containing the shared start block value, and a sequence of directory
 143 entries, each of which share the shared start block.  A new directory header
 144 is written once/if the inode start block changes.  The directory
 145 header/directory entry list is repeated as many times as necessary.
 146
 147 Directories are sorted, and can contain a directory index to speed up
 148 file lookup.  Directory indexes store one entry per metablock, each entry
 149 storing the index/filename mapping to the first directory header
 150 in each metadata block.  Directories are sorted in alphabetical order,
 151 and at lookup the index is scanned linearly looking for the first filename
 152 alphabetically larger than the filename being looked up.  At this point the
 153 location of the metadata block the filename is in has been found.
 154 The general idea of the index is to ensure only one metadata block needs to be
 155 decompressed to do a lookup irrespective of the length of the directory.
 156 This scheme has the advantage that it doesn't require extra memory overhead
 157 and doesn't require much extra storage on disk.
 158
 159 3.4 File data
 160 -------------
 161
 162 Regular files consist of a sequence of contiguous compressed blocks, and/or a
 163 compressed fragment block (tail-end packed block).   The compressed size
 164 of each datablock is stored in a block list contained within the
 165 file inode.
 166
 167 To speed up access to datablocks when reading 'large' files (256 Mbytes or
 168 larger), the code implements an index cache that caches the mapping from
 169 block index to datablock location on disk.
 170
 171 The index cache allows Squashfs to handle large files (up to 1.75 TiB) while
 172 retaining a simple and space-efficient block list on disk.  The cache
 173 is split into slots, caching up to eight 224 GiB files (128 KiB blocks).
 174 Larger files use multiple slots, with 1.75 TiB files using all 8 slots.
 175 The index cache is designed to be memory efficient, and by default uses
 176 16 KiB.
 177
 178 3.5 Fragment lookup table
 179 -------------------------
 180
 181 Regular files can contain a fragment index which is mapped to a fragment
 182 location on disk and compressed size using a fragment lookup table.  This
 183 fragment lookup table is itself stored compressed into metadata blocks.
 184 A second index table is used to locate these.  This second index table for
 185 speed of access (and because it is small) is read at mount time and cached
 186 in memory.
 187
 188 3.6 Uid/gid lookup table
 189 ------------------------
 190
 191 For space efficiency regular files store uid and gid indexes, which are
 192 converted to 32-bit uids/gids using an id look up table.  This table is
 193 stored compressed into metadata blocks.  A second index table is used to
 194 locate these.  This second index table for speed of access (and because it
 195 is small) is read at mount time and cached in memory.
 196
 197 3.7 Export table
 198 ----------------
 199
 200 To enable Squashfs filesystems to be exportable (via NFS etc.) filesystems
 201 can optionally (disabled with the -no-exports Mksquashfs option) contain
 202 an inode number to inode disk location lookup table.  This is required to
 203 enable Squashfs to map inode numbers passed in filehandles to the inode
 204 location on disk, which is necessary when the export code reinstantiates
 205 expired/flushed inodes.
 206
 207 This table is stored compressed into metadata blocks.  A second index table is
 208 used to locate these.  This second index table for speed of access (and because
 209 it is small) is read at mount time and cached in memory.
 210
 211 3.8 Xattr table
 212 ---------------
 213
 214 The xattr table contains extended attributes for each inode.  The xattrs
 215 for each inode are stored in a list, each list entry containing a type,
 216 name and value field.  The type field encodes the xattr prefix
 217 ("user.", "trusted." etc) and it also encodes how the name/value fields
 218 should be interpreted.  Currently the type indicates whether the value
 219 is stored inline (in which case the value field contains the xattr value),
 220 or if it is stored out of line (in which case the value field stores a
 221 reference to where the actual value is stored).  This allows large values
 222 to be stored out of line improving scanning and lookup performance and it
 223 also allows values to be de-duplicated, the value being stored once, and
 224 all other occurrences holding an out of line reference to that value.
 225
 226 The xattr lists are packed into compressed 8K metadata blocks.
 227 To reduce overhead in inodes, rather than storing the on-disk
 228 location of the xattr list inside each inode, a 32-bit xattr id
 229 is stored.  This xattr id is mapped into the location of the xattr
 230 list using a second xattr id lookup table.
 231
 232 4. TODOS AND OUTSTANDING ISSUES
 233 -------------------------------
 234
 235 4.1 Todo list
 236 -------------
 237
 238 Implement ACL support.
 239
 240 4.2 Squashfs internal cache
 241 ---------------------------
 242
 243 Blocks in Squashfs are compressed.  To avoid repeatedly decompressing
 244 recently accessed data Squashfs uses two small metadata and fragment caches.
 245
 246 The cache is not used for file datablocks, these are decompressed and cached in
 247 the page-cache in the normal way.  The cache is used to temporarily cache
 248 fragment and metadata blocks which have been read as a result of a metadata
 249 (i.e. inode or directory) or fragment access.  Because metadata and fragments
 250 are packed together into blocks (to gain greater compression) the read of a
 251 particular piece of metadata or fragment will retrieve other metadata/fragments
 252 which have been packed with it, these because of locality-of-reference may be
 253 read in the near future. Temporarily caching them ensures they are available
 254 for near future access without requiring an additional read and decompress.
 255
 256 In the future this internal cache may be replaced with an implementation which
 257 uses the kernel page cache.  Because the page cache operates on page sized
 258 units this may introduce additional complexity in terms of locking and
 259 associated race conditions.