1 .. SPDX-License-Identifier: GPL-2.0 2 3 4 ============================== 5 The Second Extended Filesystem 6 ============================== 7 8 ext2 was originally released in January 1993. Written by R\'emy Card, 9 Theodore Ts'o and Stephen Tweedie, it was a major rewrite of the 10 Extended Filesystem. It is currently still (April 2001) the predominant 11 filesystem in use by Linux. There are also implementations available 12 for NetBSD, FreeBSD, the GNU HURD, Windows 95/98/NT, OS/2 and RISC OS. 13 14 Options 15 ======= 16 17 Most defaults are determined by the filesystem superblock, and can be 18 set using tune2fs(8). Kernel-determined defaults are indicated by (*). 19 20 ==================== === ================================================ 21 bsddf (*) Makes ``df`` act like BSD. 22 minixdf Makes ``df`` act like Minix. 23 24 check=none, nocheck (*) Don't do extra checking of bitmaps on mount 25 (check=normal and check=strict options removed) 26 27 dax Use direct access (no page cache). See 28 Documentation/filesystems/dax.rst. 29 30 debug Extra debugging information is sent to the 31 kernel syslog. Useful for developers. 32 33 errors=continue Keep going on a filesystem error. 34 errors=remount-ro Remount the filesystem read-only on an error. 35 errors=panic Panic and halt the machine if an error occurs. 36 37 grpid, bsdgroups Give objects the same group ID as their parent. 38 nogrpid, sysvgroups New objects have the group ID of their creator. 39 40 nouid32 Use 16-bit UIDs and GIDs. 41 42 oldalloc Enable the old block allocator. Orlov should 43 have better performance, we'd like to get some 44 feedback if it's the contrary for you. 45 orlov (*) Use the Orlov block allocator. 46 (See http://lwn.net/Articles/14633/ and 47 http://lwn.net/Articles/14446/.) 48 49 resuid=n The user ID which may use the reserved blocks. 50 resgid=n The group ID which may use the reserved blocks. 51 52 sb=n Use alternate superblock at this location. 53 54 user_xattr Enable "user." POSIX Extended Attributes 55 (requires CONFIG_EXT2_FS_XATTR). 56 nouser_xattr Don't support "user." extended attributes. 57 58 acl Enable POSIX Access Control Lists support 59 (requires CONFIG_EXT2_FS_POSIX_ACL). 60 noacl Don't support POSIX ACLs. 61 62 quota, usrquota Enable user disk quota support 63 (requires CONFIG_QUOTA). 64 65 grpquota Enable group disk quota support 66 (requires CONFIG_QUOTA). 67 ==================== === ================================================ 68 69 noquota option ls silently ignored by ext2. 70 71 72 Specification 73 ============= 74 75 ext2 shares many properties with traditional Unix filesystems. It has 76 the concepts of blocks, inodes and directories. It has space in the 77 specification for Access Control Lists (ACLs), fragments, undeletion and 78 compression though these are not yet implemented (some are available as 79 separate patches). There is also a versioning mechanism to allow new 80 features (such as journalling) to be added in a maximally compatible 81 manner. 82 83 Blocks 84 ------ 85 86 The space in the device or file is split up into blocks. These are 87 a fixed size, of 1024, 2048 or 4096 bytes (8192 bytes on Alpha systems), 88 which is decided when the filesystem is created. Smaller blocks mean 89 less wasted space per file, but require slightly more accounting overhead, 90 and also impose other limits on the size of files and the filesystem. 91 92 Block Groups 93 ------------ 94 95 Blocks are clustered into block groups in order to reduce fragmentation 96 and minimise the amount of head seeking when reading a large amount 97 of consecutive data. Information about each block group is kept in a 98 descriptor table stored in the block(s) immediately after the superblock. 99 Two blocks near the start of each group are reserved for the block usage 100 bitmap and the inode usage bitmap which show which blocks and inodes 101 are in use. Since each bitmap is limited to a single block, this means 102 that the maximum size of a block group is 8 times the size of a block. 103 104 The block(s) following the bitmaps in each block group are designated 105 as the inode table for that block group and the remainder are the data 106 blocks. The block allocation algorithm attempts to allocate data blocks 107 in the same block group as the inode which contains them. 108 109 The Superblock 110 -------------- 111 112 The superblock contains all the information about the configuration of 113 the filing system. The primary copy of the superblock is stored at an 114 offset of 1024 bytes from the start of the device, and it is essential 115 to mounting the filesystem. Since it is so important, backup copies of 116 the superblock are stored in block groups throughout the filesystem. 117 The first version of ext2 (revision 0) stores a copy at the start of 118 every block group, along with backups of the group descriptor block(s). 119 Because this can consume a considerable amount of space for large 120 filesystems, later revisions can optionally reduce the number of backup 121 copies by only putting backups in specific groups (this is the sparse 122 superblock feature). The groups chosen are 0, 1 and powers of 3, 5 and 7. 123 124 The information in the superblock contains fields such as the total 125 number of inodes and blocks in the filesystem and how many are free, 126 how many inodes and blocks are in each block group, when the filesystem 127 was mounted (and if it was cleanly unmounted), when it was modified, 128 what version of the filesystem it is (see the Revisions section below) 129 and which OS created it. 130 131 If the filesystem is revision 1 or higher, then there are extra fields, 132 such as a volume name, a unique identification number, the inode size, 133 and space for optional filesystem features to store configuration info. 134 135 All fields in the superblock (as in all other ext2 structures) are stored 136 on the disc in little endian format, so a filesystem is portable between 137 machines without having to know what machine it was created on. 138 139 Inodes 140 ------ 141 142 The inode (index node) is a fundamental concept in the ext2 filesystem. 143 Each object in the filesystem is represented by an inode. The inode 144 structure contains pointers to the filesystem blocks which contain the 145 data held in the object and all of the metadata about an object except 146 its name. The metadata about an object includes the permissions, owner, 147 group, flags, size, number of blocks used, access time, change time, 148 modification time, deletion time, number of links, fragments, version 149 (for NFS) and extended attributes (EAs) and/or Access Control Lists (ACLs). 150 151 There are some reserved fields which are currently unused in the inode 152 structure and several which are overloaded. One field is reserved for the 153 directory ACL if the inode is a directory and alternately for the top 32 154 bits of the file size if the inode is a regular file (allowing file sizes 155 larger than 2GB). The translator field is unused under Linux, but is used 156 by the HURD to reference the inode of a program which will be used to 157 interpret this object. Most of the remaining reserved fields have been 158 used up for both Linux and the HURD for larger owner and group fields, 159 The HURD also has a larger mode field so it uses another of the remaining 160 fields to store the extra more bits. 161 162 There are pointers to the first 12 blocks which contain the file's data 163 in the inode. There is a pointer to an indirect block (which contains 164 pointers to the next set of blocks), a pointer to a doubly-indirect 165 block (which contains pointers to indirect blocks) and a pointer to a 166 trebly-indirect block (which contains pointers to doubly-indirect blocks). 167 168 The flags field contains some ext2-specific flags which aren't catered 169 for by the standard chmod flags. These flags can be listed with lsattr 170 and changed with the chattr command, and allow specific filesystem 171 behaviour on a per-file basis. There are flags for secure deletion, 172 undeletable, compression, synchronous updates, immutability, append-only, 173 dumpable, no-atime, indexed directories, and data-journaling. Not all 174 of these are supported yet. 175 176 Directories 177 ----------- 178 179 A directory is a filesystem object and has an inode just like a file. 180 It is a specially formatted file containing records which associate 181 each name with an inode number. Later revisions of the filesystem also 182 encode the type of the object (file, directory, symlink, device, fifo, 183 socket) to avoid the need to check the inode itself for this information 184 (support for taking advantage of this feature does not yet exist in 185 Glibc 2.2). 186 187 The inode allocation code tries to assign inodes which are in the same 188 block group as the directory in which they are first created. 189 190 The current implementation of ext2 uses a singly-linked list to store 191 the filenames in the directory; a pending enhancement uses hashing of the 192 filenames to allow lookup without the need to scan the entire directory. 193 194 The current implementation never removes empty directory blocks once they 195 have been allocated to hold more files. 196 197 Special files 198 ------------- 199 200 Symbolic links are also filesystem objects with inodes. They deserve 201 special mention because the data for them is stored within the inode 202 itself if the symlink is less than 60 bytes long. It uses the fields 203 which would normally be used to store the pointers to data blocks. 204 This is a worthwhile optimisation as it we avoid allocating a full 205 block for the symlink, and most symlinks are less than 60 characters long. 206 207 Character and block special devices never have data blocks assigned to 208 them. Instead, their device number is stored in the inode, again reusing 209 the fields which would be used to point to the data blocks. 210 211 Reserved Space 212 -------------- 213 214 In ext2, there is a mechanism for reserving a certain number of blocks 215 for a particular user (normally the super-user). This is intended to 216 allow for the system to continue functioning even if non-privileged users 217 fill up all the space available to them (this is independent of filesystem 218 quotas). It also keeps the filesystem from filling up entirely which 219 helps combat fragmentation. 220 221 Filesystem check 222 ---------------- 223 224 At boot time, most systems run a consistency check (e2fsck) on their 225 filesystems. The superblock of the ext2 filesystem contains several 226 fields which indicate whether fsck should actually run (since checking 227 the filesystem at boot can take a long time if it is large). fsck will 228 run if the filesystem was not cleanly unmounted, if the maximum mount 229 count has been exceeded or if the maximum time between checks has been 230 exceeded. 231 232 Feature Compatibility 233 --------------------- 234 235 The compatibility feature mechanism used in ext2 is sophisticated. 236 It safely allows features to be added to the filesystem, without 237 unnecessarily sacrificing compatibility with older versions of the 238 filesystem code. The feature compatibility mechanism is not supported by 239 the original revision 0 (EXT2_GOOD_OLD_REV) of ext2, but was introduced in 240 revision 1. There are three 32-bit fields, one for compatible features 241 (COMPAT), one for read-only compatible (RO_COMPAT) features and one for 242 incompatible (INCOMPAT) features. 243 244 These feature flags have specific meanings for the kernel as follows: 245 246 A COMPAT flag indicates that a feature is present in the filesystem, 247 but the on-disk format is 100% compatible with older on-disk formats, so 248 a kernel which didn't know anything about this feature could read/write 249 the filesystem without any chance of corrupting the filesystem (or even 250 making it inconsistent). This is essentially just a flag which says 251 "this filesystem has a (hidden) feature" that the kernel or e2fsck may 252 want to be aware of (more on e2fsck and feature flags later). The ext3 253 HAS_JOURNAL feature is a COMPAT flag because the ext3 journal is simply 254 a regular file with data blocks in it so the kernel does not need to 255 take any special notice of it if it doesn't understand ext3 journaling. 256 257 An RO_COMPAT flag indicates that the on-disk format is 100% compatible 258 with older on-disk formats for reading (i.e. the feature does not change 259 the visible on-disk format). However, an old kernel writing to such a 260 filesystem would/could corrupt the filesystem, so this is prevented. The 261 most common such feature, SPARSE_SUPER, is an RO_COMPAT feature because 262 sparse groups allow file data blocks where superblock/group descriptor 263 backups used to live, and ext2_free_blocks() refuses to free these blocks, 264 which would leading to inconsistent bitmaps. An old kernel would also 265 get an error if it tried to free a series of blocks which crossed a group 266 boundary, but this is a legitimate layout in a SPARSE_SUPER filesystem. 267 268 An INCOMPAT flag indicates the on-disk format has changed in some 269 way that makes it unreadable by older kernels, or would otherwise 270 cause a problem if an old kernel tried to mount it. FILETYPE is an 271 INCOMPAT flag because older kernels would think a filename was longer 272 than 256 characters, which would lead to corrupt directory listings. 273 The COMPRESSION flag is an obvious INCOMPAT flag - if the kernel 274 doesn't understand compression, you would just get garbage back from 275 read() instead of it automatically decompressing your data. The ext3 276 RECOVER flag is needed to prevent a kernel which does not understand the 277 ext3 journal from mounting the filesystem without replaying the journal. 278 279 For e2fsck, it needs to be more strict with the handling of these 280 flags than the kernel. If it doesn't understand ANY of the COMPAT, 281 RO_COMPAT, or INCOMPAT flags it will refuse to check the filesystem, 282 because it has no way of verifying whether a given feature is valid 283 or not. Allowing e2fsck to succeed on a filesystem with an unknown 284 feature is a false sense of security for the user. Refusing to check 285 a filesystem with unknown features is a good incentive for the user to 286 update to the latest e2fsck. This also means that anyone adding feature 287 flags to ext2 also needs to update e2fsck to verify these features. 288 289 Metadata 290 -------- 291 292 It is frequently claimed that the ext2 implementation of writing 293 asynchronous metadata is faster than the ffs synchronous metadata 294 scheme but less reliable. Both methods are equally resolvable by their 295 respective fsck programs. 296 297 If you're exceptionally paranoid, there are 3 ways of making metadata 298 writes synchronous on ext2: 299 300 - per-file if you have the program source: use the O_SYNC flag to open() 301 - per-file if you don't have the source: use "chattr +S" on the file 302 - per-filesystem: add the "sync" option to mount (or in /etc/fstab) 303 304 the first and last are not ext2 specific but do force the metadata to 305 be written synchronously. See also Journaling below. 306 307 Limitations 308 ----------- 309 310 There are various limits imposed by the on-disk layout of ext2. Other 311 limits are imposed by the current implementation of the kernel code. 312 Many of the limits are determined at the time the filesystem is first 313 created, and depend upon the block size chosen. The ratio of inodes to 314 data blocks is fixed at filesystem creation time, so the only way to 315 increase the number of inodes is to increase the size of the filesystem. 316 No tools currently exist which can change the ratio of inodes to blocks. 317 318 Most of these limits could be overcome with slight changes in the on-disk 319 format and using a compatibility flag to signal the format change (at 320 the expense of some compatibility). 321 322 ===================== ======= ======= ======= ======== 323 Filesystem block size 1kB 2kB 4kB 8kB 324 ===================== ======= ======= ======= ======== 325 File size limit 16GB 256GB 2048GB 2048GB 326 Filesystem size limit 2047GB 8192GB 16384GB 32768GB 327 ===================== ======= ======= ======= ======== 328 329 There is a 2.4 kernel limit of 2048GB for a single block device, so no 330 filesystem larger than that can be created at this time. There is also 331 an upper limit on the block size imposed by the page size of the kernel, 332 so 8kB blocks are only allowed on Alpha systems (and other architectures 333 which support larger pages). 334 335 There is an upper limit of 32000 subdirectories in a single directory. 336 337 There is a "soft" upper limit of about 10-15k files in a single directory 338 with the current linear linked-list directory implementation. This limit 339 stems from performance problems when creating and deleting (and also 340 finding) files in such large directories. Using a hashed directory index 341 (under development) allows 100k-1M+ files in a single directory without 342 performance problems (although RAM size becomes an issue at this point). 343 344 The (meaningless) absolute upper limit of files in a single directory 345 (imposed by the file size, the realistic limit is obviously much less) 346 is over 130 trillion files. It would be higher except there are not 347 enough 4-character names to make up unique directory entries, so they 348 have to be 8 character filenames, even then we are fairly close to 349 running out of unique filenames. 350 351 Journaling 352 ---------- 353 354 A journaling extension to the ext2 code has been developed by Stephen 355 Tweedie. It avoids the risks of metadata corruption and the need to 356 wait for e2fsck to complete after a crash, without requiring a change 357 to the on-disk ext2 layout. In a nutshell, the journal is a regular 358 file which stores whole metadata (and optionally data) blocks that have 359 been modified, prior to writing them into the filesystem. This means 360 it is possible to add a journal to an existing ext2 filesystem without 361 the need for data conversion. 362 363 When changes to the filesystem (e.g. a file is renamed) they are stored in 364 a transaction in the journal and can either be complete or incomplete at 365 the time of a crash. If a transaction is complete at the time of a crash 366 (or in the normal case where the system does not crash), then any blocks 367 in that transaction are guaranteed to represent a valid filesystem state, 368 and are copied into the filesystem. If a transaction is incomplete at 369 the time of the crash, then there is no guarantee of consistency for 370 the blocks in that transaction so they are discarded (which means any 371 filesystem changes they represent are also lost). 372 Check Documentation/filesystems/ext4/ if you want to read more about 373 ext4 and journaling. 374 375 References 376 ========== 377 378 ======================= =============================================== 379 The kernel source file:/usr/src/linux/fs/ext2/ 380 e2fsprogs (e2fsck) http://e2fsprogs.sourceforge.net/ 381 Design & Implementation http://e2fsprogs.sourceforge.net/ext2intro.html 382 Journaling (ext3) ftp://ftp.uk.linux.org/pub/linux/sct/fs/jfs/ 383 Filesystem Resizing http://ext2resize.sourceforge.net/ 384 Compression [1]_ http://e2compr.sourceforge.net/ 385 ======================= =============================================== 386 387 Implementations for: 388 389 ======================= =========================================================== 390 Windows 95/98/NT/2000 http://www.chrysocome.net/explore2fs 391 Windows 95 [1]_ http://www.yipton.net/content.html#FSDEXT2 392 DOS client [1]_ ftp://metalab.unc.edu/pub/Linux/system/filesystems/ext2/ 393 OS/2 [2]_ ftp://metalab.unc.edu/pub/Linux/system/filesystems/ext2/ 394 RISC OS client http://www.esw-heim.tu-clausthal.de/~marco/smorbrod/IscaFS/ 395 ======================= =========================================================== 396 397 .. [1] no longer actively developed/supported (as of Apr 2001) 398 .. [2] no longer actively developed/supported (as of Mar 2009)
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