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1SQUASHFS 4.0 FILESYSTEM 2======================= 3 4Squashfs is a compressed read-only filesystem for Linux. 5It uses zlib compression to compress files, inodes and directories. 6Inodes in the system are very small and all blocks are packed to minimise 7data overhead. Block sizes greater than 4K are supported up to a maximum 8of 1Mbytes (default block size 128K). 9 10Squashfs is intended for general read-only filesystem use, for archival 11use (i.e. in cases where a .tar.gz file may be used), and in constrained 12block device/memory systems (e.g. embedded systems) where low overhead is 13needed. 14 15Mailing list: email@example.com 16Web site: www.squashfs.org 17 181. FILESYSTEM FEATURES 19---------------------- 20 21Squashfs filesystem features versus Cramfs: 22 23 Squashfs Cramfs 24 25Max filesystem size: 2^64 256 MiB 26Max file size: ~ 2 TiB 16 MiB 27Max files: unlimited unlimited 28Max directories: unlimited unlimited 29Max entries per directory: unlimited unlimited 30Max block size: 1 MiB 4 KiB 31Metadata compression: yes no 32Directory indexes: yes no 33Sparse file support: yes no 34Tail-end packing (fragments): yes no 35Exportable (NFS etc.): yes no 36Hard link support: yes no 37"." and ".." in readdir: yes no 38Real inode numbers: yes no 3932-bit uids/gids: yes no 40File creation time: yes no 41Xattr support: yes no 42ACL support: no no 43 44Squashfs compresses data, inodes and directories. In addition, inode and 45directory data are highly compacted, and packed on byte boundaries. Each 46compressed inode is on average 8 bytes in length (the exact length varies on 47file type, i.e. regular file, directory, symbolic link, and block/char device 48inodes have different sizes). 49 502. USING SQUASHFS 51----------------- 52 53As squashfs is a read-only filesystem, the mksquashfs program must be used to 54create populated squashfs filesystems. This and other squashfs utilities 55can be obtained from http://www.squashfs.org. Usage instructions can be 56obtained from this site also. 57 58 593. SQUASHFS FILESYSTEM DESIGN 60----------------------------- 61 62A squashfs filesystem consists of a maximum of eight parts, packed together on a byte 63alignment: 64 65 --------------- 66 | superblock | 67 |---------------| 68 | datablocks | 69 | & fragments | 70 |---------------| 71 | inode table | 72 |---------------| 73 | directory | 74 | table | 75 |---------------| 76 | fragment | 77 | table | 78 |---------------| 79 | export | 80 | table | 81 |---------------| 82 | uid/gid | 83 | lookup table | 84 |---------------| 85 | xattr | 86 | table | 87 --------------- 88 89Compressed data blocks are written to the filesystem as files are read from 90the source directory, and checked for duplicates. Once all file data has been 91written the completed inode, directory, fragment, export and uid/gid lookup 92tables are written. 93 943.1 Inodes 95---------- 96 97Metadata (inodes and directories) are compressed in 8Kbyte blocks. Each 98compressed block is prefixed by a two byte length, the top bit is set if the 99block is uncompressed. A block will be uncompressed if the -noI option is set, 100or if the compressed block was larger than the uncompressed block. 101 102Inodes are packed into the metadata blocks, and are not aligned to block 103boundaries, therefore inodes overlap compressed blocks. Inodes are identified 104by a 48-bit number which encodes the location of the compressed metadata block 105containing the inode, and the byte offset into that block where the inode is 106placed (<block, offset>). 107 108To maximise compression there are different inodes for each file type 109(regular file, directory, device, etc.), the inode contents and length 110varying with the type. 111 112To further maximise compression, two types of regular file inode and 113directory inode are defined: inodes optimised for frequently occurring 114regular files and directories, and extended types where extra 115information has to be stored. 116 1173.2 Directories 118--------------- 119 120Like inodes, directories are packed into compressed metadata blocks, stored 121in a directory table. Directories are accessed using the start address of 122the metablock containing the directory and the offset into the 123decompressed block (<block, offset>). 124 125Directories are organised in a slightly complex way, and are not simply 126a list of file names. The organisation takes advantage of the 127fact that (in most cases) the inodes of the files will be in the same 128compressed metadata block, and therefore, can share the start block. 129Directories are therefore organised in a two level list, a directory 130header containing the shared start block value, and a sequence of directory 131entries, each of which share the shared start block. A new directory header 132is written once/if the inode start block changes. The directory 133header/directory entry list is repeated as many times as necessary. 134 135Directories are sorted, and can contain a directory index to speed up 136file lookup. Directory indexes store one entry per metablock, each entry 137storing the index/filename mapping to the first directory header 138in each metadata block. Directories are sorted in alphabetical order, 139and at lookup the index is scanned linearly looking for the first filename 140alphabetically larger than the filename being looked up. At this point the 141location of the metadata block the filename is in has been found. 142The general idea of the index is ensure only one metadata block needs to be 143decompressed to do a lookup irrespective of the length of the directory. 144This scheme has the advantage that it doesn't require extra memory overhead 145and doesn't require much extra storage on disk. 146 1473.3 File data 148------------- 149 150Regular files consist of a sequence of contiguous compressed blocks, and/or a 151compressed fragment block (tail-end packed block). The compressed size 152of each datablock is stored in a block list contained within the 153file inode. 154 155To speed up access to datablocks when reading 'large' files (256 Mbytes or 156larger), the code implements an index cache that caches the mapping from 157block index to datablock location on disk. 158 159The index cache allows Squashfs to handle large files (up to 1.75 TiB) while 160retaining a simple and space-efficient block list on disk. The cache 161is split into slots, caching up to eight 224 GiB files (128 KiB blocks). 162Larger files use multiple slots, with 1.75 TiB files using all 8 slots. 163The index cache is designed to be memory efficient, and by default uses 16416 KiB. 165 1663.4 Fragment lookup table 167------------------------- 168 169Regular files can contain a fragment index which is mapped to a fragment 170location on disk and compressed size using a fragment lookup table. This 171fragment lookup table is itself stored compressed into metadata blocks. 172A second index table is used to locate these. This second index table for 173speed of access (and because it is small) is read at mount time and cached 174in memory. 175 1763.5 Uid/gid lookup table 177------------------------ 178 179For space efficiency regular files store uid and gid indexes, which are 180converted to 32-bit uids/gids using an id look up table. This table is 181stored compressed into metadata blocks. A second index table is used to 182locate these. This second index table for speed of access (and because it 183is small) is read at mount time and cached in memory. 184 1853.6 Export table 186---------------- 187 188To enable Squashfs filesystems to be exportable (via NFS etc.) filesystems 189can optionally (disabled with the -no-exports Mksquashfs option) contain 190an inode number to inode disk location lookup table. This is required to 191enable Squashfs to map inode numbers passed in filehandles to the inode 192location on disk, which is necessary when the export code reinstantiates 193expired/flushed inodes. 194 195This table is stored compressed into metadata blocks. A second index table is 196used to locate these. This second index table for speed of access (and because 197it is small) is read at mount time and cached in memory. 198 1993.7 Xattr table 200--------------- 201 202The xattr table contains extended attributes for each inode. The xattrs 203for each inode are stored in a list, each list entry containing a type, 204name and value field. The type field encodes the xattr prefix 205("user.", "trusted." etc) and it also encodes how the name/value fields 206should be interpreted. Currently the type indicates whether the value 207is stored inline (in which case the value field contains the xattr value), 208or if it is stored out of line (in which case the value field stores a 209reference to where the actual value is stored). This allows large values 210to be stored out of line improving scanning and lookup performance and it 211also allows values to be de-duplicated, the value being stored once, and 212all other occurences holding an out of line reference to that value. 213 214The xattr lists are packed into compressed 8K metadata blocks. 215To reduce overhead in inodes, rather than storing the on-disk 216location of the xattr list inside each inode, a 32-bit xattr id 217is stored. This xattr id is mapped into the location of the xattr 218list using a second xattr id lookup table. 219 2204. TODOS AND OUTSTANDING ISSUES 221------------------------------- 222 2234.1 Todo list 224------------- 225 226Implement ACL support. 227 2284.2 Squashfs internal cache 229--------------------------- 230 231Blocks in Squashfs are compressed. To avoid repeatedly decompressing 232recently accessed data Squashfs uses two small metadata and fragment caches. 233 234The cache is not used for file datablocks, these are decompressed and cached in 235the page-cache in the normal way. The cache is used to temporarily cache 236fragment and metadata blocks which have been read as a result of a metadata 237(i.e. inode or directory) or fragment access. Because metadata and fragments 238are packed together into blocks (to gain greater compression) the read of a 239particular piece of metadata or fragment will retrieve other metadata/fragments 240which have been packed with it, these because of locality-of-reference may be 241read in the near future. Temporarily caching them ensures they are available 242for near future access without requiring an additional read and decompress. 243 244In the future this internal cache may be replaced with an implementation which 245uses the kernel page cache. Because the page cache operates on page sized 246units this may introduce additional complexity in terms of locking and 247associated race conditions.