Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.
1 2 The Second Extended Filesystem 3 ============================== 4 5 ext2 was originally released in January 1993. Written by R\'emy Card, 6 Theodore Ts'o and Stephen Tweedie, it was a major rewrite of the 7 Extended Filesystem. It is currently still (April 2001) the predominant 8 filesystem in use by Linux. There are also implementations available 9 for NetBSD, FreeBSD, the GNU HURD, Windows 95/98/NT, OS/2 and RISC OS. 10 11 Options 12 ======= 13 14 Most defaults are determined by the filesystem superblock, and can be 15 set using tune2fs(8). Kernel-determined defaults are indicated by (*). 16 17 bsddf (*) Makes `df' act like BSD. 18 minixdf Makes `df' act like Minix. 19 20 check=none, nocheck (*) Don't do extra checking of bitmaps on mount 21 (check=normal and check=strict options removed) 22 23 dax Use direct access (no page cache). See 24 Documentation/filesystems/dax.txt. 25 26 debug Extra debugging information is sent to the 27 kernel syslog. Useful for developers. 28 29 errors=continue Keep going on a filesystem error. 30 errors=remount-ro Remount the filesystem read-only on an error. 31 errors=panic Panic and halt the machine if an error occurs. 32 33 grpid, bsdgroups Give objects the same group ID as their parent. 34 nogrpid, sysvgroups New objects have the group ID of their creator. 35 36 nouid32 Use 16-bit UIDs and GIDs. 37 38 oldalloc Enable the old block allocator. Orlov should 39 have better performance, we'd like to get some 40 feedback if it's the contrary for you. 41 orlov (*) Use the Orlov block allocator. 42 (See http://lwn.net/Articles/14633/ and 43 http://lwn.net/Articles/14446/.) 44 45 resuid=n The user ID which may use the reserved blocks. 46 resgid=n The group ID which may use the reserved blocks. 47 48 sb=n Use alternate superblock at this location. 49 50 user_xattr Enable "user." POSIX Extended Attributes 51 (requires CONFIG_EXT2_FS_XATTR). 52 nouser_xattr Don't support "user." extended attributes. 53 54 acl Enable POSIX Access Control Lists support 55 (requires CONFIG_EXT2_FS_POSIX_ACL). 56 noacl Don't support POSIX ACLs. 57 58 nobh Do not attach buffer_heads to file pagecache. 59 60 grpquota,noquota,quota,usrquota Quota options are silently ignored by ext2. 61 62 63 Specification 64 ============= 65 66 ext2 shares many properties with traditional Unix filesystems. It has 67 the concepts of blocks, inodes and directories. It has space in the 68 specification for Access Control Lists (ACLs), fragments, undeletion and 69 compression though these are not yet implemented (some are available as 70 separate patches). There is also a versioning mechanism to allow new 71 features (such as journalling) to be added in a maximally compatible 72 manner. 73 74 Blocks 75 ------ 76 77 The space in the device or file is split up into blocks. These are 78 a fixed size, of 1024, 2048 or 4096 bytes (8192 bytes on Alpha systems), 79 which is decided when the filesystem is created. Smaller blocks mean 80 less wasted space per file, but require slightly more accounting overhead, 81 and also impose other limits on the size of files and the filesystem. 82 83 Block Groups 84 ------------ 85 86 Blocks are clustered into block groups in order to reduce fragmentation 87 and minimise the amount of head seeking when reading a large amount 88 of consecutive data. Information about each block group is kept in a 89 descriptor table stored in the block(s) immediately after the superblock. 90 Two blocks near the start of each group are reserved for the block usage 91 bitmap and the inode usage bitmap which show which blocks and inodes 92 are in use. Since each bitmap is limited to a single block, this means 93 that the maximum size of a block group is 8 times the size of a block. 94 95 The block(s) following the bitmaps in each block group are designated 96 as the inode table for that block group and the remainder are the data 97 blocks. The block allocation algorithm attempts to allocate data blocks 98 in the same block group as the inode which contains them. 99 100 The Superblock 101 -------------- 102 103 The superblock contains all the information about the configuration of 104 the filing system. The primary copy of the superblock is stored at an 105 offset of 1024 bytes from the start of the device, and it is essential 106 to mounting the filesystem. Since it is so important, backup copies of 107 the superblock are stored in block groups throughout the filesystem. 108 The first version of ext2 (revision 0) stores a copy at the start of 109 every block group, along with backups of the group descriptor block(s). 110 Because this can consume a considerable amount of space for large 111 filesystems, later revisions can optionally reduce the number of backup 112 copies by only putting backups in specific groups (this is the sparse 113 superblock feature). The groups chosen are 0, 1 and powers of 3, 5 and 7. 114 115 The information in the superblock contains fields such as the total 116 number of inodes and blocks in the filesystem and how many are free, 117 how many inodes and blocks are in each block group, when the filesystem 118 was mounted (and if it was cleanly unmounted), when it was modified, 119 what version of the filesystem it is (see the Revisions section below) 120 and which OS created it. 121 122 If the filesystem is revision 1 or higher, then there are extra fields, 123 such as a volume name, a unique identification number, the inode size, 124 and space for optional filesystem features to store configuration info. 125 126 All fields in the superblock (as in all other ext2 structures) are stored 127 on the disc in little endian format, so a filesystem is portable between 128 machines without having to know what machine it was created on. 129 130 Inodes 131 ------ 132 133 The inode (index node) is a fundamental concept in the ext2 filesystem. 134 Each object in the filesystem is represented by an inode. The inode 135 structure contains pointers to the filesystem blocks which contain the 136 data held in the object and all of the metadata about an object except 137 its name. The metadata about an object includes the permissions, owner, 138 group, flags, size, number of blocks used, access time, change time, 139 modification time, deletion time, number of links, fragments, version 140 (for NFS) and extended attributes (EAs) and/or Access Control Lists (ACLs). 141 142 There are some reserved fields which are currently unused in the inode 143 structure and several which are overloaded. One field is reserved for the 144 directory ACL if the inode is a directory and alternately for the top 32 145 bits of the file size if the inode is a regular file (allowing file sizes 146 larger than 2GB). The translator field is unused under Linux, but is used 147 by the HURD to reference the inode of a program which will be used to 148 interpret this object. Most of the remaining reserved fields have been 149 used up for both Linux and the HURD for larger owner and group fields, 150 The HURD also has a larger mode field so it uses another of the remaining 151 fields to store the extra more bits. 152 153 There are pointers to the first 12 blocks which contain the file's data 154 in the inode. There is a pointer to an indirect block (which contains 155 pointers to the next set of blocks), a pointer to a doubly-indirect 156 block (which contains pointers to indirect blocks) and a pointer to a 157 trebly-indirect block (which contains pointers to doubly-indirect blocks). 158 159 The flags field contains some ext2-specific flags which aren't catered 160 for by the standard chmod flags. These flags can be listed with lsattr 161 and changed with the chattr command, and allow specific filesystem 162 behaviour on a per-file basis. There are flags for secure deletion, 163 undeletable, compression, synchronous updates, immutability, append-only, 164 dumpable, no-atime, indexed directories, and data-journaling. Not all 165 of these are supported yet. 166 167 Directories 168 ----------- 169 170 A directory is a filesystem object and has an inode just like a file. 171 It is a specially formatted file containing records which associate 172 each name with an inode number. Later revisions of the filesystem also 173 encode the type of the object (file, directory, symlink, device, fifo, 174 socket) to avoid the need to check the inode itself for this information 175 (support for taking advantage of this feature does not yet exist in 176 Glibc 2.2). 177 178 The inode allocation code tries to assign inodes which are in the same 179 block group as the directory in which they are first created. 180 181 The current implementation of ext2 uses a singly-linked list to store 182 the filenames in the directory; a pending enhancement uses hashing of the 183 filenames to allow lookup without the need to scan the entire directory. 184 185 The current implementation never removes empty directory blocks once they 186 have been allocated to hold more files. 187 188 Special files 189 ------------- 190 191 Symbolic links are also filesystem objects with inodes. They deserve 192 special mention because the data for them is stored within the inode 193 itself if the symlink is less than 60 bytes long. It uses the fields 194 which would normally be used to store the pointers to data blocks. 195 This is a worthwhile optimisation as it we avoid allocating a full 196 block for the symlink, and most symlinks are less than 60 characters long. 197 198 Character and block special devices never have data blocks assigned to 199 them. Instead, their device number is stored in the inode, again reusing 200 the fields which would be used to point to the data blocks. 201 202 Reserved Space 203 -------------- 204 205 In ext2, there is a mechanism for reserving a certain number of blocks 206 for a particular user (normally the super-user). This is intended to 207 allow for the system to continue functioning even if non-privileged users 208 fill up all the space available to them (this is independent of filesystem 209 quotas). It also keeps the filesystem from filling up entirely which 210 helps combat fragmentation. 211 212 Filesystem check 213 ---------------- 214 215 At boot time, most systems run a consistency check (e2fsck) on their 216 filesystems. The superblock of the ext2 filesystem contains several 217 fields which indicate whether fsck should actually run (since checking 218 the filesystem at boot can take a long time if it is large). fsck will 219 run if the filesystem was not cleanly unmounted, if the maximum mount 220 count has been exceeded or if the maximum time between checks has been 221 exceeded. 222 223 Feature Compatibility 224 --------------------- 225 226 The compatibility feature mechanism used in ext2 is sophisticated. 227 It safely allows features to be added to the filesystem, without 228 unnecessarily sacrificing compatibility with older versions of the 229 filesystem code. The feature compatibility mechanism is not supported by 230 the original revision 0 (EXT2_GOOD_OLD_REV) of ext2, but was introduced in 231 revision 1. There are three 32-bit fields, one for compatible features 232 (COMPAT), one for read-only compatible (RO_COMPAT) features and one for 233 incompatible (INCOMPAT) features. 234 235 These feature flags have specific meanings for the kernel as follows: 236 237 A COMPAT flag indicates that a feature is present in the filesystem, 238 but the on-disk format is 100% compatible with older on-disk formats, so 239 a kernel which didn't know anything about this feature could read/write 240 the filesystem without any chance of corrupting the filesystem (or even 241 making it inconsistent). This is essentially just a flag which says 242 "this filesystem has a (hidden) feature" that the kernel or e2fsck may 243 want to be aware of (more on e2fsck and feature flags later). The ext3 244 HAS_JOURNAL feature is a COMPAT flag because the ext3 journal is simply 245 a regular file with data blocks in it so the kernel does not need to 246 take any special notice of it if it doesn't understand ext3 journaling. 247 248 An RO_COMPAT flag indicates that the on-disk format is 100% compatible 249 with older on-disk formats for reading (i.e. the feature does not change 250 the visible on-disk format). However, an old kernel writing to such a 251 filesystem would/could corrupt the filesystem, so this is prevented. The 252 most common such feature, SPARSE_SUPER, is an RO_COMPAT feature because 253 sparse groups allow file data blocks where superblock/group descriptor 254 backups used to live, and ext2_free_blocks() refuses to free these blocks, 255 which would leading to inconsistent bitmaps. An old kernel would also 256 get an error if it tried to free a series of blocks which crossed a group 257 boundary, but this is a legitimate layout in a SPARSE_SUPER filesystem. 258 259 An INCOMPAT flag indicates the on-disk format has changed in some 260 way that makes it unreadable by older kernels, or would otherwise 261 cause a problem if an old kernel tried to mount it. FILETYPE is an 262 INCOMPAT flag because older kernels would think a filename was longer 263 than 256 characters, which would lead to corrupt directory listings. 264 The COMPRESSION flag is an obvious INCOMPAT flag - if the kernel 265 doesn't understand compression, you would just get garbage back from 266 read() instead of it automatically decompressing your data. The ext3 267 RECOVER flag is needed to prevent a kernel which does not understand the 268 ext3 journal from mounting the filesystem without replaying the journal. 269 270 For e2fsck, it needs to be more strict with the handling of these 271 flags than the kernel. If it doesn't understand ANY of the COMPAT, 272 RO_COMPAT, or INCOMPAT flags it will refuse to check the filesystem, 273 because it has no way of verifying whether a given feature is valid 274 or not. Allowing e2fsck to succeed on a filesystem with an unknown 275 feature is a false sense of security for the user. Refusing to check 276 a filesystem with unknown features is a good incentive for the user to 277 update to the latest e2fsck. This also means that anyone adding feature 278 flags to ext2 also needs to update e2fsck to verify these features. 279 280 Metadata 281 -------- 282 283 It is frequently claimed that the ext2 implementation of writing 284 asynchronous metadata is faster than the ffs synchronous metadata 285 scheme but less reliable. Both methods are equally resolvable by their 286 respective fsck programs. 287 288 If you're exceptionally paranoid, there are 3 ways of making metadata 289 writes synchronous on ext2: 290 291 per-file if you have the program source: use the O_SYNC flag to open() 292 per-file if you don't have the source: use "chattr +S" on the file 293 per-filesystem: add the "sync" option to mount (or in /etc/fstab) 294 295 the first and last are not ext2 specific but do force the metadata to 296 be written synchronously. See also Journaling below. 297 298 Limitations 299 ----------- 300 301 There are various limits imposed by the on-disk layout of ext2. Other 302 limits are imposed by the current implementation of the kernel code. 303 Many of the limits are determined at the time the filesystem is first 304 created, and depend upon the block size chosen. The ratio of inodes to 305 data blocks is fixed at filesystem creation time, so the only way to 306 increase the number of inodes is to increase the size of the filesystem. 307 No tools currently exist which can change the ratio of inodes to blocks. 308 309 Most of these limits could be overcome with slight changes in the on-disk 310 format and using a compatibility flag to signal the format change (at 311 the expense of some compatibility). 312 313 Filesystem block size: 1kB 2kB 4kB 8kB 314 315 File size limit: 16GB 256GB 2048GB 2048GB 316 Filesystem size limit: 2047GB 8192GB 16384GB 32768GB 317 318 There is a 2.4 kernel limit of 2048GB for a single block device, so no 319 filesystem larger than that can be created at this time. There is also 320 an upper limit on the block size imposed by the page size of the kernel, 321 so 8kB blocks are only allowed on Alpha systems (and other architectures 322 which support larger pages). 323 324 There is an upper limit of 32000 subdirectories in a single directory. 325 326 There is a "soft" upper limit of about 10-15k files in a single directory 327 with the current linear linked-list directory implementation. This limit 328 stems from performance problems when creating and deleting (and also 329 finding) files in such large directories. Using a hashed directory index 330 (under development) allows 100k-1M+ files in a single directory without 331 performance problems (although RAM size becomes an issue at this point). 332 333 The (meaningless) absolute upper limit of files in a single directory 334 (imposed by the file size, the realistic limit is obviously much less) 335 is over 130 trillion files. It would be higher except there are not 336 enough 4-character names to make up unique directory entries, so they 337 have to be 8 character filenames, even then we are fairly close to 338 running out of unique filenames. 339 340 Journaling 341 ---------- 342 343 A journaling extension to the ext2 code has been developed by Stephen 344 Tweedie. It avoids the risks of metadata corruption and the need to 345 wait for e2fsck to complete after a crash, without requiring a change 346 to the on-disk ext2 layout. In a nutshell, the journal is a regular 347 file which stores whole metadata (and optionally data) blocks that have 348 been modified, prior to writing them into the filesystem. This means 349 it is possible to add a journal to an existing ext2 filesystem without 350 the need for data conversion. 351 352 When changes to the filesystem (e.g. a file is renamed) they are stored in 353 a transaction in the journal and can either be complete or incomplete at 354 the time of a crash. If a transaction is complete at the time of a crash 355 (or in the normal case where the system does not crash), then any blocks 356 in that transaction are guaranteed to represent a valid filesystem state, 357 and are copied into the filesystem. If a transaction is incomplete at 358 the time of the crash, then there is no guarantee of consistency for 359 the blocks in that transaction so they are discarded (which means any 360 filesystem changes they represent are also lost). 361 Check Documentation/filesystems/ext4.txt if you want to read more about 362 ext4 and journaling. 363 364 References 365 ========== 366 367 The kernel source file:/usr/src/linux/fs/ext2/ 368 e2fsprogs (e2fsck) http://e2fsprogs.sourceforge.net/ 369 Design & Implementation http://e2fsprogs.sourceforge.net/ext2intro.html 370 Journaling (ext3) ftp://ftp.uk.linux.org/pub/linux/sct/fs/jfs/ 371 Filesystem Resizing http://ext2resize.sourceforge.net/ 372 Compression (*) http://e2compr.sourceforge.net/ 373 374 Implementations for: 375 Windows 95/98/NT/2000 http://www.chrysocome.net/explore2fs 376 Windows 95 (*) http://www.yipton.net/content.html#FSDEXT2 377 DOS client (*) ftp://metalab.unc.edu/pub/Linux/system/filesystems/ext2/ 378 OS/2 (+) ftp://metalab.unc.edu/pub/Linux/system/filesystems/ext2/ 379 RISC OS client http://www.esw-heim.tu-clausthal.de/~marco/smorbrod/IscaFS/ 380 381 (*) no longer actively developed/supported (as of Apr 2001) 382 (+) no longer actively developed/supported (as of Mar 2009)