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.txt. 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 62nobh Do not attach buffer_heads to file pagecache. 63 64quota, usrquota Enable user disk quota support 65 (requires CONFIG_QUOTA). 66 67grpquota Enable group disk quota support 68 (requires CONFIG_QUOTA). 69==================== === ================================================ 70 71noquota option ls silently ignored by ext2. 72 73 74Specification 75============= 76 77ext2 shares many properties with traditional Unix filesystems. It has 78the concepts of blocks, inodes and directories. It has space in the 79specification for Access Control Lists (ACLs), fragments, undeletion and 80compression though these are not yet implemented (some are available as 81separate patches). There is also a versioning mechanism to allow new 82features (such as journalling) to be added in a maximally compatible 83manner. 84 85Blocks 86------ 87 88The space in the device or file is split up into blocks. These are 89a fixed size, of 1024, 2048 or 4096 bytes (8192 bytes on Alpha systems), 90which is decided when the filesystem is created. Smaller blocks mean 91less wasted space per file, but require slightly more accounting overhead, 92and also impose other limits on the size of files and the filesystem. 93 94Block Groups 95------------ 96 97Blocks are clustered into block groups in order to reduce fragmentation 98and minimise the amount of head seeking when reading a large amount 99of consecutive data. Information about each block group is kept in a 100descriptor table stored in the block(s) immediately after the superblock. 101Two blocks near the start of each group are reserved for the block usage 102bitmap and the inode usage bitmap which show which blocks and inodes 103are in use. Since each bitmap is limited to a single block, this means 104that the maximum size of a block group is 8 times the size of a block. 105 106The block(s) following the bitmaps in each block group are designated 107as the inode table for that block group and the remainder are the data 108blocks. The block allocation algorithm attempts to allocate data blocks 109in the same block group as the inode which contains them. 110 111The Superblock 112-------------- 113 114The superblock contains all the information about the configuration of 115the filing system. The primary copy of the superblock is stored at an 116offset of 1024 bytes from the start of the device, and it is essential 117to mounting the filesystem. Since it is so important, backup copies of 118the superblock are stored in block groups throughout the filesystem. 119The first version of ext2 (revision 0) stores a copy at the start of 120every block group, along with backups of the group descriptor block(s). 121Because this can consume a considerable amount of space for large 122filesystems, later revisions can optionally reduce the number of backup 123copies by only putting backups in specific groups (this is the sparse 124superblock feature). The groups chosen are 0, 1 and powers of 3, 5 and 7. 125 126The information in the superblock contains fields such as the total 127number of inodes and blocks in the filesystem and how many are free, 128how many inodes and blocks are in each block group, when the filesystem 129was mounted (and if it was cleanly unmounted), when it was modified, 130what version of the filesystem it is (see the Revisions section below) 131and which OS created it. 132 133If the filesystem is revision 1 or higher, then there are extra fields, 134such as a volume name, a unique identification number, the inode size, 135and space for optional filesystem features to store configuration info. 136 137All fields in the superblock (as in all other ext2 structures) are stored 138on the disc in little endian format, so a filesystem is portable between 139machines without having to know what machine it was created on. 140 141Inodes 142------ 143 144The inode (index node) is a fundamental concept in the ext2 filesystem. 145Each object in the filesystem is represented by an inode. The inode 146structure contains pointers to the filesystem blocks which contain the 147data held in the object and all of the metadata about an object except 148its name. The metadata about an object includes the permissions, owner, 149group, flags, size, number of blocks used, access time, change time, 150modification time, deletion time, number of links, fragments, version 151(for NFS) and extended attributes (EAs) and/or Access Control Lists (ACLs). 152 153There are some reserved fields which are currently unused in the inode 154structure and several which are overloaded. One field is reserved for the 155directory ACL if the inode is a directory and alternately for the top 32 156bits of the file size if the inode is a regular file (allowing file sizes 157larger than 2GB). The translator field is unused under Linux, but is used 158by the HURD to reference the inode of a program which will be used to 159interpret this object. Most of the remaining reserved fields have been 160used up for both Linux and the HURD for larger owner and group fields, 161The HURD also has a larger mode field so it uses another of the remaining 162fields to store the extra more bits. 163 164There are pointers to the first 12 blocks which contain the file's data 165in the inode. There is a pointer to an indirect block (which contains 166pointers to the next set of blocks), a pointer to a doubly-indirect 167block (which contains pointers to indirect blocks) and a pointer to a 168trebly-indirect block (which contains pointers to doubly-indirect blocks). 169 170The flags field contains some ext2-specific flags which aren't catered 171for by the standard chmod flags. These flags can be listed with lsattr 172and changed with the chattr command, and allow specific filesystem 173behaviour on a per-file basis. There are flags for secure deletion, 174undeletable, compression, synchronous updates, immutability, append-only, 175dumpable, no-atime, indexed directories, and data-journaling. Not all 176of these are supported yet. 177 178Directories 179----------- 180 181A directory is a filesystem object and has an inode just like a file. 182It is a specially formatted file containing records which associate 183each name with an inode number. Later revisions of the filesystem also 184encode the type of the object (file, directory, symlink, device, fifo, 185socket) to avoid the need to check the inode itself for this information 186(support for taking advantage of this feature does not yet exist in 187Glibc 2.2). 188 189The inode allocation code tries to assign inodes which are in the same 190block group as the directory in which they are first created. 191 192The current implementation of ext2 uses a singly-linked list to store 193the filenames in the directory; a pending enhancement uses hashing of the 194filenames to allow lookup without the need to scan the entire directory. 195 196The current implementation never removes empty directory blocks once they 197have been allocated to hold more files. 198 199Special files 200------------- 201 202Symbolic links are also filesystem objects with inodes. They deserve 203special mention because the data for them is stored within the inode 204itself if the symlink is less than 60 bytes long. It uses the fields 205which would normally be used to store the pointers to data blocks. 206This is a worthwhile optimisation as it we avoid allocating a full 207block for the symlink, and most symlinks are less than 60 characters long. 208 209Character and block special devices never have data blocks assigned to 210them. Instead, their device number is stored in the inode, again reusing 211the fields which would be used to point to the data blocks. 212 213Reserved Space 214-------------- 215 216In ext2, there is a mechanism for reserving a certain number of blocks 217for a particular user (normally the super-user). This is intended to 218allow for the system to continue functioning even if non-privileged users 219fill up all the space available to them (this is independent of filesystem 220quotas). It also keeps the filesystem from filling up entirely which 221helps combat fragmentation. 222 223Filesystem check 224---------------- 225 226At boot time, most systems run a consistency check (e2fsck) on their 227filesystems. The superblock of the ext2 filesystem contains several 228fields which indicate whether fsck should actually run (since checking 229the filesystem at boot can take a long time if it is large). fsck will 230run if the filesystem was not cleanly unmounted, if the maximum mount 231count has been exceeded or if the maximum time between checks has been 232exceeded. 233 234Feature Compatibility 235--------------------- 236 237The compatibility feature mechanism used in ext2 is sophisticated. 238It safely allows features to be added to the filesystem, without 239unnecessarily sacrificing compatibility with older versions of the 240filesystem code. The feature compatibility mechanism is not supported by 241the original revision 0 (EXT2_GOOD_OLD_REV) of ext2, but was introduced in 242revision 1. There are three 32-bit fields, one for compatible features 243(COMPAT), one for read-only compatible (RO_COMPAT) features and one for 244incompatible (INCOMPAT) features. 245 246These feature flags have specific meanings for the kernel as follows: 247 248A COMPAT flag indicates that a feature is present in the filesystem, 249but the on-disk format is 100% compatible with older on-disk formats, so 250a kernel which didn't know anything about this feature could read/write 251the filesystem without any chance of corrupting the filesystem (or even 252making it inconsistent). This is essentially just a flag which says 253"this filesystem has a (hidden) feature" that the kernel or e2fsck may 254want to be aware of (more on e2fsck and feature flags later). The ext3 255HAS_JOURNAL feature is a COMPAT flag because the ext3 journal is simply 256a regular file with data blocks in it so the kernel does not need to 257take any special notice of it if it doesn't understand ext3 journaling. 258 259An RO_COMPAT flag indicates that the on-disk format is 100% compatible 260with older on-disk formats for reading (i.e. the feature does not change 261the visible on-disk format). However, an old kernel writing to such a 262filesystem would/could corrupt the filesystem, so this is prevented. The 263most common such feature, SPARSE_SUPER, is an RO_COMPAT feature because 264sparse groups allow file data blocks where superblock/group descriptor 265backups used to live, and ext2_free_blocks() refuses to free these blocks, 266which would leading to inconsistent bitmaps. An old kernel would also 267get an error if it tried to free a series of blocks which crossed a group 268boundary, but this is a legitimate layout in a SPARSE_SUPER filesystem. 269 270An INCOMPAT flag indicates the on-disk format has changed in some 271way that makes it unreadable by older kernels, or would otherwise 272cause a problem if an old kernel tried to mount it. FILETYPE is an 273INCOMPAT flag because older kernels would think a filename was longer 274than 256 characters, which would lead to corrupt directory listings. 275The COMPRESSION flag is an obvious INCOMPAT flag - if the kernel 276doesn't understand compression, you would just get garbage back from 277read() instead of it automatically decompressing your data. The ext3 278RECOVER flag is needed to prevent a kernel which does not understand the 279ext3 journal from mounting the filesystem without replaying the journal. 280 281For e2fsck, it needs to be more strict with the handling of these 282flags than the kernel. If it doesn't understand ANY of the COMPAT, 283RO_COMPAT, or INCOMPAT flags it will refuse to check the filesystem, 284because it has no way of verifying whether a given feature is valid 285or not. Allowing e2fsck to succeed on a filesystem with an unknown 286feature is a false sense of security for the user. Refusing to check 287a filesystem with unknown features is a good incentive for the user to 288update to the latest e2fsck. This also means that anyone adding feature 289flags to ext2 also needs to update e2fsck to verify these features. 290 291Metadata 292-------- 293 294It is frequently claimed that the ext2 implementation of writing 295asynchronous metadata is faster than the ffs synchronous metadata 296scheme but less reliable. Both methods are equally resolvable by their 297respective fsck programs. 298 299If you're exceptionally paranoid, there are 3 ways of making metadata 300writes synchronous on ext2: 301 302- per-file if you have the program source: use the O_SYNC flag to open() 303- per-file if you don't have the source: use "chattr +S" on the file 304- per-filesystem: add the "sync" option to mount (or in /etc/fstab) 305 306the first and last are not ext2 specific but do force the metadata to 307be written synchronously. See also Journaling below. 308 309Limitations 310----------- 311 312There are various limits imposed by the on-disk layout of ext2. Other 313limits are imposed by the current implementation of the kernel code. 314Many of the limits are determined at the time the filesystem is first 315created, and depend upon the block size chosen. The ratio of inodes to 316data blocks is fixed at filesystem creation time, so the only way to 317increase the number of inodes is to increase the size of the filesystem. 318No tools currently exist which can change the ratio of inodes to blocks. 319 320Most of these limits could be overcome with slight changes in the on-disk 321format and using a compatibility flag to signal the format change (at 322the expense of some compatibility). 323 324===================== ======= ======= ======= ======== 325Filesystem block size 1kB 2kB 4kB 8kB 326===================== ======= ======= ======= ======== 327File size limit 16GB 256GB 2048GB 2048GB 328Filesystem size limit 2047GB 8192GB 16384GB 32768GB 329===================== ======= ======= ======= ======== 330 331There is a 2.4 kernel limit of 2048GB for a single block device, so no 332filesystem larger than that can be created at this time. There is also 333an upper limit on the block size imposed by the page size of the kernel, 334so 8kB blocks are only allowed on Alpha systems (and other architectures 335which support larger pages). 336 337There is an upper limit of 32000 subdirectories in a single directory. 338 339There is a "soft" upper limit of about 10-15k files in a single directory 340with the current linear linked-list directory implementation. This limit 341stems from performance problems when creating and deleting (and also 342finding) files in such large directories. Using a hashed directory index 343(under development) allows 100k-1M+ files in a single directory without 344performance problems (although RAM size becomes an issue at this point). 345 346The (meaningless) absolute upper limit of files in a single directory 347(imposed by the file size, the realistic limit is obviously much less) 348is over 130 trillion files. It would be higher except there are not 349enough 4-character names to make up unique directory entries, so they 350have to be 8 character filenames, even then we are fairly close to 351running out of unique filenames. 352 353Journaling 354---------- 355 356A journaling extension to the ext2 code has been developed by Stephen 357Tweedie. It avoids the risks of metadata corruption and the need to 358wait for e2fsck to complete after a crash, without requiring a change 359to the on-disk ext2 layout. In a nutshell, the journal is a regular 360file which stores whole metadata (and optionally data) blocks that have 361been modified, prior to writing them into the filesystem. This means 362it is possible to add a journal to an existing ext2 filesystem without 363the need for data conversion. 364 365When changes to the filesystem (e.g. a file is renamed) they are stored in 366a transaction in the journal and can either be complete or incomplete at 367the time of a crash. If a transaction is complete at the time of a crash 368(or in the normal case where the system does not crash), then any blocks 369in that transaction are guaranteed to represent a valid filesystem state, 370and are copied into the filesystem. If a transaction is incomplete at 371the time of the crash, then there is no guarantee of consistency for 372the blocks in that transaction so they are discarded (which means any 373filesystem changes they represent are also lost). 374Check Documentation/filesystems/ext4/ if you want to read more about 375ext4 and journaling. 376 377References 378========== 379 380======================= =============================================== 381The kernel source file:/usr/src/linux/fs/ext2/ 382e2fsprogs (e2fsck) http://e2fsprogs.sourceforge.net/ 383Design & Implementation http://e2fsprogs.sourceforge.net/ext2intro.html 384Journaling (ext3) ftp://ftp.uk.linux.org/pub/linux/sct/fs/jfs/ 385Filesystem Resizing http://ext2resize.sourceforge.net/ 386Compression [1]_ http://e2compr.sourceforge.net/ 387======================= =============================================== 388 389Implementations for: 390 391======================= =========================================================== 392Windows 95/98/NT/2000 http://www.chrysocome.net/explore2fs 393Windows 95 [1]_ http://www.yipton.net/content.html#FSDEXT2 394DOS client [1]_ ftp://metalab.unc.edu/pub/Linux/system/filesystems/ext2/ 395OS/2 [2]_ ftp://metalab.unc.edu/pub/Linux/system/filesystems/ext2/ 396RISC OS client http://www.esw-heim.tu-clausthal.de/~marco/smorbrod/IscaFS/ 397======================= =========================================================== 398 399.. [1] no longer actively developed/supported (as of Apr 2001) 400.. [2] no longer actively developed/supported (as of Mar 2009) 401