1.. SPDX-License-Identifier: GPL-2.0 2 3 4============================== 5The Second Extended Filesystem 6============================== 7 8ext2 was originally released in January 1993. Written by R\'emy Card, 9Theodore Ts'o and Stephen Tweedie, it was a major rewrite of the 10Extended Filesystem. It is currently still (April 2001) the predominant 11filesystem in use by Linux. There are also implementations available 12for NetBSD, FreeBSD, the GNU HURD, Windows 95/98/NT, OS/2 and RISC OS. 13 14Options 15======= 16 17Most defaults are determined by the filesystem superblock, and can be 18set using tune2fs(8). Kernel-determined defaults are indicated by (*). 19 20==================== === ================================================ 21bsddf (*) Makes ``df`` act like BSD. 22minixdf Makes ``df`` act like Minix. 23 24check=none, nocheck (*) Don't do extra checking of bitmaps on mount 25 (check=normal and check=strict options removed) 26 27dax Use direct access (no page cache). See 28 Documentation/filesystems/dax.rst. 29 30debug Extra debugging information is sent to the 31 kernel syslog. Useful for developers. 32 33errors=continue Keep going on a filesystem error. 34errors=remount-ro Remount the filesystem read-only on an error. 35errors=panic Panic and halt the machine if an error occurs. 36 37grpid, bsdgroups Give objects the same group ID as their parent. 38nogrpid, sysvgroups New objects have the group ID of their creator. 39 40nouid32 Use 16-bit UIDs and GIDs. 41 42oldalloc 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. 45orlov (*) Use the Orlov block allocator. 46 (See http://lwn.net/Articles/14633/ and 47 http://lwn.net/Articles/14446/.) 48 49resuid=n The user ID which may use the reserved blocks. 50resgid=n The group ID which may use the reserved blocks. 51 52sb=n Use alternate superblock at this location. 53 54user_xattr Enable "user." POSIX Extended Attributes 55 (requires CONFIG_EXT2_FS_XATTR). 56nouser_xattr Don't support "user." extended attributes. 57 58acl Enable POSIX Access Control Lists support 59 (requires CONFIG_EXT2_FS_POSIX_ACL). 60noacl Don't support POSIX ACLs. 61 62quota, usrquota Enable user disk quota support 63 (requires CONFIG_QUOTA). 64 65grpquota Enable group disk quota support 66 (requires CONFIG_QUOTA). 67==================== === ================================================ 68 69noquota option ls silently ignored by ext2. 70 71 72Specification 73============= 74 75ext2 shares many properties with traditional Unix filesystems. It has 76the concepts of blocks, inodes and directories. It has space in the 77specification for Access Control Lists (ACLs), fragments, undeletion and 78compression though these are not yet implemented (some are available as 79separate patches). There is also a versioning mechanism to allow new 80features (such as journalling) to be added in a maximally compatible 81manner. 82 83Blocks 84------ 85 86The space in the device or file is split up into blocks. These are 87a fixed size, of 1024, 2048 or 4096 bytes (8192 bytes on Alpha systems), 88which is decided when the filesystem is created. Smaller blocks mean 89less wasted space per file, but require slightly more accounting overhead, 90and also impose other limits on the size of files and the filesystem. 91 92Block Groups 93------------ 94 95Blocks are clustered into block groups in order to reduce fragmentation 96and minimise the amount of head seeking when reading a large amount 97of consecutive data. Information about each block group is kept in a 98descriptor table stored in the block(s) immediately after the superblock. 99Two blocks near the start of each group are reserved for the block usage 100bitmap and the inode usage bitmap which show which blocks and inodes 101are in use. Since each bitmap is limited to a single block, this means 102that the maximum size of a block group is 8 times the size of a block. 103 104The block(s) following the bitmaps in each block group are designated 105as the inode table for that block group and the remainder are the data 106blocks. The block allocation algorithm attempts to allocate data blocks 107in the same block group as the inode which contains them. 108 109The Superblock 110-------------- 111 112The superblock contains all the information about the configuration of 113the filing system. The primary copy of the superblock is stored at an 114offset of 1024 bytes from the start of the device, and it is essential 115to mounting the filesystem. Since it is so important, backup copies of 116the superblock are stored in block groups throughout the filesystem. 117The first version of ext2 (revision 0) stores a copy at the start of 118every block group, along with backups of the group descriptor block(s). 119Because this can consume a considerable amount of space for large 120filesystems, later revisions can optionally reduce the number of backup 121copies by only putting backups in specific groups (this is the sparse 122superblock feature). The groups chosen are 0, 1 and powers of 3, 5 and 7. 123 124The information in the superblock contains fields such as the total 125number of inodes and blocks in the filesystem and how many are free, 126how many inodes and blocks are in each block group, when the filesystem 127was mounted (and if it was cleanly unmounted), when it was modified, 128what version of the filesystem it is (see the Revisions section below) 129and which OS created it. 130 131If the filesystem is revision 1 or higher, then there are extra fields, 132such as a volume name, a unique identification number, the inode size, 133and space for optional filesystem features to store configuration info. 134 135All fields in the superblock (as in all other ext2 structures) are stored 136on the disc in little endian format, so a filesystem is portable between 137machines without having to know what machine it was created on. 138 139Inodes 140------ 141 142The inode (index node) is a fundamental concept in the ext2 filesystem. 143Each object in the filesystem is represented by an inode. The inode 144structure contains pointers to the filesystem blocks which contain the 145data held in the object and all of the metadata about an object except 146its name. The metadata about an object includes the permissions, owner, 147group, flags, size, number of blocks used, access time, change time, 148modification time, deletion time, number of links, fragments, version 149(for NFS) and extended attributes (EAs) and/or Access Control Lists (ACLs). 150 151There are some reserved fields which are currently unused in the inode 152structure and several which are overloaded. One field is reserved for the 153directory ACL if the inode is a directory and alternately for the top 32 154bits of the file size if the inode is a regular file (allowing file sizes 155larger than 2GB). The translator field is unused under Linux, but is used 156by the HURD to reference the inode of a program which will be used to 157interpret this object. Most of the remaining reserved fields have been 158used up for both Linux and the HURD for larger owner and group fields, 159The HURD also has a larger mode field so it uses another of the remaining 160fields to store the extra more bits. 161 162There are pointers to the first 12 blocks which contain the file's data 163in the inode. There is a pointer to an indirect block (which contains 164pointers to the next set of blocks), a pointer to a doubly-indirect 165block (which contains pointers to indirect blocks) and a pointer to a 166trebly-indirect block (which contains pointers to doubly-indirect blocks). 167 168The flags field contains some ext2-specific flags which aren't catered 169for by the standard chmod flags. These flags can be listed with lsattr 170and changed with the chattr command, and allow specific filesystem 171behaviour on a per-file basis. There are flags for secure deletion, 172undeletable, compression, synchronous updates, immutability, append-only, 173dumpable, no-atime, indexed directories, and data-journaling. Not all 174of these are supported yet. 175 176Directories 177----------- 178 179A directory is a filesystem object and has an inode just like a file. 180It is a specially formatted file containing records which associate 181each name with an inode number. Later revisions of the filesystem also 182encode the type of the object (file, directory, symlink, device, fifo, 183socket) 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 185Glibc 2.2). 186 187The inode allocation code tries to assign inodes which are in the same 188block group as the directory in which they are first created. 189 190The current implementation of ext2 uses a singly-linked list to store 191the filenames in the directory; a pending enhancement uses hashing of the 192filenames to allow lookup without the need to scan the entire directory. 193 194The current implementation never removes empty directory blocks once they 195have been allocated to hold more files. 196 197Special files 198------------- 199 200Symbolic links are also filesystem objects with inodes. They deserve 201special mention because the data for them is stored within the inode 202itself if the symlink is less than 60 bytes long. It uses the fields 203which would normally be used to store the pointers to data blocks. 204This is a worthwhile optimisation as it we avoid allocating a full 205block for the symlink, and most symlinks are less than 60 characters long. 206 207Character and block special devices never have data blocks assigned to 208them. Instead, their device number is stored in the inode, again reusing 209the fields which would be used to point to the data blocks. 210 211Reserved Space 212-------------- 213 214In ext2, there is a mechanism for reserving a certain number of blocks 215for a particular user (normally the super-user). This is intended to 216allow for the system to continue functioning even if non-privileged users 217fill up all the space available to them (this is independent of filesystem 218quotas). It also keeps the filesystem from filling up entirely which 219helps combat fragmentation. 220 221Filesystem check 222---------------- 223 224At boot time, most systems run a consistency check (e2fsck) on their 225filesystems. The superblock of the ext2 filesystem contains several 226fields which indicate whether fsck should actually run (since checking 227the filesystem at boot can take a long time if it is large). fsck will 228run if the filesystem was not cleanly unmounted, if the maximum mount 229count has been exceeded or if the maximum time between checks has been 230exceeded. 231 232Feature Compatibility 233--------------------- 234 235The compatibility feature mechanism used in ext2 is sophisticated. 236It safely allows features to be added to the filesystem, without 237unnecessarily sacrificing compatibility with older versions of the 238filesystem code. The feature compatibility mechanism is not supported by 239the original revision 0 (EXT2_GOOD_OLD_REV) of ext2, but was introduced in 240revision 1. There are three 32-bit fields, one for compatible features 241(COMPAT), one for read-only compatible (RO_COMPAT) features and one for 242incompatible (INCOMPAT) features. 243 244These feature flags have specific meanings for the kernel as follows: 245 246A COMPAT flag indicates that a feature is present in the filesystem, 247but the on-disk format is 100% compatible with older on-disk formats, so 248a kernel which didn't know anything about this feature could read/write 249the filesystem without any chance of corrupting the filesystem (or even 250making it inconsistent). This is essentially just a flag which says 251"this filesystem has a (hidden) feature" that the kernel or e2fsck may 252want to be aware of (more on e2fsck and feature flags later). The ext3 253HAS_JOURNAL feature is a COMPAT flag because the ext3 journal is simply 254a regular file with data blocks in it so the kernel does not need to 255take any special notice of it if it doesn't understand ext3 journaling. 256 257An RO_COMPAT flag indicates that the on-disk format is 100% compatible 258with older on-disk formats for reading (i.e. the feature does not change 259the visible on-disk format). However, an old kernel writing to such a 260filesystem would/could corrupt the filesystem, so this is prevented. The 261most common such feature, SPARSE_SUPER, is an RO_COMPAT feature because 262sparse groups allow file data blocks where superblock/group descriptor 263backups used to live, and ext2_free_blocks() refuses to free these blocks, 264which would leading to inconsistent bitmaps. An old kernel would also 265get an error if it tried to free a series of blocks which crossed a group 266boundary, but this is a legitimate layout in a SPARSE_SUPER filesystem. 267 268An INCOMPAT flag indicates the on-disk format has changed in some 269way that makes it unreadable by older kernels, or would otherwise 270cause a problem if an old kernel tried to mount it. FILETYPE is an 271INCOMPAT flag because older kernels would think a filename was longer 272than 256 characters, which would lead to corrupt directory listings. 273The COMPRESSION flag is an obvious INCOMPAT flag - if the kernel 274doesn't understand compression, you would just get garbage back from 275read() instead of it automatically decompressing your data. The ext3 276RECOVER flag is needed to prevent a kernel which does not understand the 277ext3 journal from mounting the filesystem without replaying the journal. 278 279For e2fsck, it needs to be more strict with the handling of these 280flags than the kernel. If it doesn't understand ANY of the COMPAT, 281RO_COMPAT, or INCOMPAT flags it will refuse to check the filesystem, 282because it has no way of verifying whether a given feature is valid 283or not. Allowing e2fsck to succeed on a filesystem with an unknown 284feature is a false sense of security for the user. Refusing to check 285a filesystem with unknown features is a good incentive for the user to 286update to the latest e2fsck. This also means that anyone adding feature 287flags to ext2 also needs to update e2fsck to verify these features. 288 289Metadata 290-------- 291 292It is frequently claimed that the ext2 implementation of writing 293asynchronous metadata is faster than the ffs synchronous metadata 294scheme but less reliable. Both methods are equally resolvable by their 295respective fsck programs. 296 297If you're exceptionally paranoid, there are 3 ways of making metadata 298writes 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 304the first and last are not ext2 specific but do force the metadata to 305be written synchronously. See also Journaling below. 306 307Limitations 308----------- 309 310There are various limits imposed by the on-disk layout of ext2. Other 311limits are imposed by the current implementation of the kernel code. 312Many of the limits are determined at the time the filesystem is first 313created, and depend upon the block size chosen. The ratio of inodes to 314data blocks is fixed at filesystem creation time, so the only way to 315increase the number of inodes is to increase the size of the filesystem. 316No tools currently exist which can change the ratio of inodes to blocks. 317 318Most of these limits could be overcome with slight changes in the on-disk 319format and using a compatibility flag to signal the format change (at 320the expense of some compatibility). 321 322===================== ======= ======= ======= ======== 323Filesystem block size 1kB 2kB 4kB 8kB 324===================== ======= ======= ======= ======== 325File size limit 16GB 256GB 2048GB 2048GB 326Filesystem size limit 2047GB 8192GB 16384GB 32768GB 327===================== ======= ======= ======= ======== 328 329There is a 2.4 kernel limit of 2048GB for a single block device, so no 330filesystem larger than that can be created at this time. There is also 331an upper limit on the block size imposed by the page size of the kernel, 332so 8kB blocks are only allowed on Alpha systems (and other architectures 333which support larger pages). 334 335There is an upper limit of 32000 subdirectories in a single directory. 336 337There is a "soft" upper limit of about 10-15k files in a single directory 338with the current linear linked-list directory implementation. This limit 339stems from performance problems when creating and deleting (and also 340finding) files in such large directories. Using a hashed directory index 341(under development) allows 100k-1M+ files in a single directory without 342performance problems (although RAM size becomes an issue at this point). 343 344The (meaningless) absolute upper limit of files in a single directory 345(imposed by the file size, the realistic limit is obviously much less) 346is over 130 trillion files. It would be higher except there are not 347enough 4-character names to make up unique directory entries, so they 348have to be 8 character filenames, even then we are fairly close to 349running out of unique filenames. 350 351Journaling 352---------- 353 354A journaling extension to the ext2 code has been developed by Stephen 355Tweedie. It avoids the risks of metadata corruption and the need to 356wait for e2fsck to complete after a crash, without requiring a change 357to the on-disk ext2 layout. In a nutshell, the journal is a regular 358file which stores whole metadata (and optionally data) blocks that have 359been modified, prior to writing them into the filesystem. This means 360it is possible to add a journal to an existing ext2 filesystem without 361the need for data conversion. 362 363When changes to the filesystem (e.g. a file is renamed) they are stored in 364a transaction in the journal and can either be complete or incomplete at 365the 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 367in that transaction are guaranteed to represent a valid filesystem state, 368and are copied into the filesystem. If a transaction is incomplete at 369the time of the crash, then there is no guarantee of consistency for 370the blocks in that transaction so they are discarded (which means any 371filesystem changes they represent are also lost). 372Check Documentation/filesystems/ext4/ if you want to read more about 373ext4 and journaling. 374 375References 376========== 377 378======================= =============================================== 379The kernel source file:/usr/src/linux/fs/ext2/ 380e2fsprogs (e2fsck) http://e2fsprogs.sourceforge.net/ 381Design & Implementation http://e2fsprogs.sourceforge.net/ext2intro.html 382Journaling (ext3) ftp://ftp.uk.linux.org/pub/linux/sct/fs/jfs/ 383Filesystem Resizing http://ext2resize.sourceforge.net/ 384Compression [1]_ http://e2compr.sourceforge.net/ 385======================= =============================================== 386 387Implementations for: 388 389======================= =========================================================== 390Windows 95/98/NT/2000 http://www.chrysocome.net/explore2fs 391Windows 95 [1]_ http://www.yipton.net/content.html#FSDEXT2 392DOS client [1]_ ftp://metalab.unc.edu/pub/Linux/system/filesystems/ext2/ 393OS/2 [2]_ ftp://metalab.unc.edu/pub/Linux/system/filesystems/ext2/ 394RISC 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) 399