1.. SPDX-License-Identifier: GPL-2.0 2 3 4The Second Extended Filesystem 5============================== 6 7ext2 was originally released in January 1993. Written by R\'emy Card, 8Theodore Ts'o and Stephen Tweedie, it was a major rewrite of the 9Extended Filesystem. It is currently still (April 2001) the predominant 10filesystem in use by Linux. There are also implementations available 11for NetBSD, FreeBSD, the GNU HURD, Windows 95/98/NT, OS/2 and RISC OS. 12 13Options 14======= 15 16Most defaults are determined by the filesystem superblock, and can be 17set using tune2fs(8). Kernel-determined defaults are indicated by (*). 18 19==================== === ================================================ 20bsddf (*) Makes ``df`` act like BSD. 21minixdf Makes ``df`` act like Minix. 22 23check=none, nocheck (*) Don't do extra checking of bitmaps on mount 24 (check=normal and check=strict options removed) 25 26dax Use direct access (no page cache). See 27 Documentation/filesystems/dax.txt. 28 29debug Extra debugging information is sent to the 30 kernel syslog. Useful for developers. 31 32errors=continue Keep going on a filesystem error. 33errors=remount-ro Remount the filesystem read-only on an error. 34errors=panic Panic and halt the machine if an error occurs. 35 36grpid, bsdgroups Give objects the same group ID as their parent. 37nogrpid, sysvgroups New objects have the group ID of their creator. 38 39nouid32 Use 16-bit UIDs and GIDs. 40 41oldalloc Enable the old block allocator. Orlov should 42 have better performance, we'd like to get some 43 feedback if it's the contrary for you. 44orlov (*) Use the Orlov block allocator. 45 (See http://lwn.net/Articles/14633/ and 46 http://lwn.net/Articles/14446/.) 47 48resuid=n The user ID which may use the reserved blocks. 49resgid=n The group ID which may use the reserved blocks. 50 51sb=n Use alternate superblock at this location. 52 53user_xattr Enable "user." POSIX Extended Attributes 54 (requires CONFIG_EXT2_FS_XATTR). 55nouser_xattr Don't support "user." extended attributes. 56 57acl Enable POSIX Access Control Lists support 58 (requires CONFIG_EXT2_FS_POSIX_ACL). 59noacl Don't support POSIX ACLs. 60 61nobh Do not attach buffer_heads to file pagecache. 62 63quota, usrquota Enable user disk quota support 64 (requires CONFIG_QUOTA). 65 66grpquota Enable group disk quota support 67 (requires CONFIG_QUOTA). 68==================== === ================================================ 69 70noquota option ls silently ignored by ext2. 71 72 73Specification 74============= 75 76ext2 shares many properties with traditional Unix filesystems. It has 77the concepts of blocks, inodes and directories. It has space in the 78specification for Access Control Lists (ACLs), fragments, undeletion and 79compression though these are not yet implemented (some are available as 80separate patches). There is also a versioning mechanism to allow new 81features (such as journalling) to be added in a maximally compatible 82manner. 83 84Blocks 85------ 86 87The space in the device or file is split up into blocks. These are 88a fixed size, of 1024, 2048 or 4096 bytes (8192 bytes on Alpha systems), 89which is decided when the filesystem is created. Smaller blocks mean 90less wasted space per file, but require slightly more accounting overhead, 91and also impose other limits on the size of files and the filesystem. 92 93Block Groups 94------------ 95 96Blocks are clustered into block groups in order to reduce fragmentation 97and minimise the amount of head seeking when reading a large amount 98of consecutive data. Information about each block group is kept in a 99descriptor table stored in the block(s) immediately after the superblock. 100Two blocks near the start of each group are reserved for the block usage 101bitmap and the inode usage bitmap which show which blocks and inodes 102are in use. Since each bitmap is limited to a single block, this means 103that the maximum size of a block group is 8 times the size of a block. 104 105The block(s) following the bitmaps in each block group are designated 106as the inode table for that block group and the remainder are the data 107blocks. The block allocation algorithm attempts to allocate data blocks 108in the same block group as the inode which contains them. 109 110The Superblock 111-------------- 112 113The superblock contains all the information about the configuration of 114the filing system. The primary copy of the superblock is stored at an 115offset of 1024 bytes from the start of the device, and it is essential 116to mounting the filesystem. Since it is so important, backup copies of 117the superblock are stored in block groups throughout the filesystem. 118The first version of ext2 (revision 0) stores a copy at the start of 119every block group, along with backups of the group descriptor block(s). 120Because this can consume a considerable amount of space for large 121filesystems, later revisions can optionally reduce the number of backup 122copies by only putting backups in specific groups (this is the sparse 123superblock feature). The groups chosen are 0, 1 and powers of 3, 5 and 7. 124 125The information in the superblock contains fields such as the total 126number of inodes and blocks in the filesystem and how many are free, 127how many inodes and blocks are in each block group, when the filesystem 128was mounted (and if it was cleanly unmounted), when it was modified, 129what version of the filesystem it is (see the Revisions section below) 130and which OS created it. 131 132If the filesystem is revision 1 or higher, then there are extra fields, 133such as a volume name, a unique identification number, the inode size, 134and space for optional filesystem features to store configuration info. 135 136All fields in the superblock (as in all other ext2 structures) are stored 137on the disc in little endian format, so a filesystem is portable between 138machines without having to know what machine it was created on. 139 140Inodes 141------ 142 143The inode (index node) is a fundamental concept in the ext2 filesystem. 144Each object in the filesystem is represented by an inode. The inode 145structure contains pointers to the filesystem blocks which contain the 146data held in the object and all of the metadata about an object except 147its name. The metadata about an object includes the permissions, owner, 148group, flags, size, number of blocks used, access time, change time, 149modification time, deletion time, number of links, fragments, version 150(for NFS) and extended attributes (EAs) and/or Access Control Lists (ACLs). 151 152There are some reserved fields which are currently unused in the inode 153structure and several which are overloaded. One field is reserved for the 154directory ACL if the inode is a directory and alternately for the top 32 155bits of the file size if the inode is a regular file (allowing file sizes 156larger than 2GB). The translator field is unused under Linux, but is used 157by the HURD to reference the inode of a program which will be used to 158interpret this object. Most of the remaining reserved fields have been 159used up for both Linux and the HURD for larger owner and group fields, 160The HURD also has a larger mode field so it uses another of the remaining 161fields to store the extra more bits. 162 163There are pointers to the first 12 blocks which contain the file's data 164in the inode. There is a pointer to an indirect block (which contains 165pointers to the next set of blocks), a pointer to a doubly-indirect 166block (which contains pointers to indirect blocks) and a pointer to a 167trebly-indirect block (which contains pointers to doubly-indirect blocks). 168 169The flags field contains some ext2-specific flags which aren't catered 170for by the standard chmod flags. These flags can be listed with lsattr 171and changed with the chattr command, and allow specific filesystem 172behaviour on a per-file basis. There are flags for secure deletion, 173undeletable, compression, synchronous updates, immutability, append-only, 174dumpable, no-atime, indexed directories, and data-journaling. Not all 175of these are supported yet. 176 177Directories 178----------- 179 180A directory is a filesystem object and has an inode just like a file. 181It is a specially formatted file containing records which associate 182each name with an inode number. Later revisions of the filesystem also 183encode the type of the object (file, directory, symlink, device, fifo, 184socket) to avoid the need to check the inode itself for this information 185(support for taking advantage of this feature does not yet exist in 186Glibc 2.2). 187 188The inode allocation code tries to assign inodes which are in the same 189block group as the directory in which they are first created. 190 191The current implementation of ext2 uses a singly-linked list to store 192the filenames in the directory; a pending enhancement uses hashing of the 193filenames to allow lookup without the need to scan the entire directory. 194 195The current implementation never removes empty directory blocks once they 196have been allocated to hold more files. 197 198Special files 199------------- 200 201Symbolic links are also filesystem objects with inodes. They deserve 202special mention because the data for them is stored within the inode 203itself if the symlink is less than 60 bytes long. It uses the fields 204which would normally be used to store the pointers to data blocks. 205This is a worthwhile optimisation as it we avoid allocating a full 206block for the symlink, and most symlinks are less than 60 characters long. 207 208Character and block special devices never have data blocks assigned to 209them. Instead, their device number is stored in the inode, again reusing 210the fields which would be used to point to the data blocks. 211 212Reserved Space 213-------------- 214 215In ext2, there is a mechanism for reserving a certain number of blocks 216for a particular user (normally the super-user). This is intended to 217allow for the system to continue functioning even if non-privileged users 218fill up all the space available to them (this is independent of filesystem 219quotas). It also keeps the filesystem from filling up entirely which 220helps combat fragmentation. 221 222Filesystem check 223---------------- 224 225At boot time, most systems run a consistency check (e2fsck) on their 226filesystems. The superblock of the ext2 filesystem contains several 227fields which indicate whether fsck should actually run (since checking 228the filesystem at boot can take a long time if it is large). fsck will 229run if the filesystem was not cleanly unmounted, if the maximum mount 230count has been exceeded or if the maximum time between checks has been 231exceeded. 232 233Feature Compatibility 234--------------------- 235 236The compatibility feature mechanism used in ext2 is sophisticated. 237It safely allows features to be added to the filesystem, without 238unnecessarily sacrificing compatibility with older versions of the 239filesystem code. The feature compatibility mechanism is not supported by 240the original revision 0 (EXT2_GOOD_OLD_REV) of ext2, but was introduced in 241revision 1. There are three 32-bit fields, one for compatible features 242(COMPAT), one for read-only compatible (RO_COMPAT) features and one for 243incompatible (INCOMPAT) features. 244 245These feature flags have specific meanings for the kernel as follows: 246 247A COMPAT flag indicates that a feature is present in the filesystem, 248but the on-disk format is 100% compatible with older on-disk formats, so 249a kernel which didn't know anything about this feature could read/write 250the filesystem without any chance of corrupting the filesystem (or even 251making it inconsistent). This is essentially just a flag which says 252"this filesystem has a (hidden) feature" that the kernel or e2fsck may 253want to be aware of (more on e2fsck and feature flags later). The ext3 254HAS_JOURNAL feature is a COMPAT flag because the ext3 journal is simply 255a regular file with data blocks in it so the kernel does not need to 256take any special notice of it if it doesn't understand ext3 journaling. 257 258An RO_COMPAT flag indicates that the on-disk format is 100% compatible 259with older on-disk formats for reading (i.e. the feature does not change 260the visible on-disk format). However, an old kernel writing to such a 261filesystem would/could corrupt the filesystem, so this is prevented. The 262most common such feature, SPARSE_SUPER, is an RO_COMPAT feature because 263sparse groups allow file data blocks where superblock/group descriptor 264backups used to live, and ext2_free_blocks() refuses to free these blocks, 265which would leading to inconsistent bitmaps. An old kernel would also 266get an error if it tried to free a series of blocks which crossed a group 267boundary, but this is a legitimate layout in a SPARSE_SUPER filesystem. 268 269An INCOMPAT flag indicates the on-disk format has changed in some 270way that makes it unreadable by older kernels, or would otherwise 271cause a problem if an old kernel tried to mount it. FILETYPE is an 272INCOMPAT flag because older kernels would think a filename was longer 273than 256 characters, which would lead to corrupt directory listings. 274The COMPRESSION flag is an obvious INCOMPAT flag - if the kernel 275doesn't understand compression, you would just get garbage back from 276read() instead of it automatically decompressing your data. The ext3 277RECOVER flag is needed to prevent a kernel which does not understand the 278ext3 journal from mounting the filesystem without replaying the journal. 279 280For e2fsck, it needs to be more strict with the handling of these 281flags than the kernel. If it doesn't understand ANY of the COMPAT, 282RO_COMPAT, or INCOMPAT flags it will refuse to check the filesystem, 283because it has no way of verifying whether a given feature is valid 284or not. Allowing e2fsck to succeed on a filesystem with an unknown 285feature is a false sense of security for the user. Refusing to check 286a filesystem with unknown features is a good incentive for the user to 287update to the latest e2fsck. This also means that anyone adding feature 288flags to ext2 also needs to update e2fsck to verify these features. 289 290Metadata 291-------- 292 293It is frequently claimed that the ext2 implementation of writing 294asynchronous metadata is faster than the ffs synchronous metadata 295scheme but less reliable. Both methods are equally resolvable by their 296respective fsck programs. 297 298If you're exceptionally paranoid, there are 3 ways of making metadata 299writes synchronous on ext2: 300 301- per-file if you have the program source: use the O_SYNC flag to open() 302- per-file if you don't have the source: use "chattr +S" on the file 303- per-filesystem: add the "sync" option to mount (or in /etc/fstab) 304 305the first and last are not ext2 specific but do force the metadata to 306be written synchronously. See also Journaling below. 307 308Limitations 309----------- 310 311There are various limits imposed by the on-disk layout of ext2. Other 312limits are imposed by the current implementation of the kernel code. 313Many of the limits are determined at the time the filesystem is first 314created, and depend upon the block size chosen. The ratio of inodes to 315data blocks is fixed at filesystem creation time, so the only way to 316increase the number of inodes is to increase the size of the filesystem. 317No tools currently exist which can change the ratio of inodes to blocks. 318 319Most of these limits could be overcome with slight changes in the on-disk 320format and using a compatibility flag to signal the format change (at 321the expense of some compatibility). 322 323===================== ======= ======= ======= ======== 324Filesystem block size 1kB 2kB 4kB 8kB 325===================== ======= ======= ======= ======== 326File size limit 16GB 256GB 2048GB 2048GB 327Filesystem size limit 2047GB 8192GB 16384GB 32768GB 328===================== ======= ======= ======= ======== 329 330There is a 2.4 kernel limit of 2048GB for a single block device, so no 331filesystem larger than that can be created at this time. There is also 332an upper limit on the block size imposed by the page size of the kernel, 333so 8kB blocks are only allowed on Alpha systems (and other architectures 334which support larger pages). 335 336There is an upper limit of 32000 subdirectories in a single directory. 337 338There is a "soft" upper limit of about 10-15k files in a single directory 339with the current linear linked-list directory implementation. This limit 340stems from performance problems when creating and deleting (and also 341finding) files in such large directories. Using a hashed directory index 342(under development) allows 100k-1M+ files in a single directory without 343performance problems (although RAM size becomes an issue at this point). 344 345The (meaningless) absolute upper limit of files in a single directory 346(imposed by the file size, the realistic limit is obviously much less) 347is over 130 trillion files. It would be higher except there are not 348enough 4-character names to make up unique directory entries, so they 349have to be 8 character filenames, even then we are fairly close to 350running out of unique filenames. 351 352Journaling 353---------- 354 355A journaling extension to the ext2 code has been developed by Stephen 356Tweedie. It avoids the risks of metadata corruption and the need to 357wait for e2fsck to complete after a crash, without requiring a change 358to the on-disk ext2 layout. In a nutshell, the journal is a regular 359file which stores whole metadata (and optionally data) blocks that have 360been modified, prior to writing them into the filesystem. This means 361it is possible to add a journal to an existing ext2 filesystem without 362the need for data conversion. 363 364When changes to the filesystem (e.g. a file is renamed) they are stored in 365a transaction in the journal and can either be complete or incomplete at 366the time of a crash. If a transaction is complete at the time of a crash 367(or in the normal case where the system does not crash), then any blocks 368in that transaction are guaranteed to represent a valid filesystem state, 369and are copied into the filesystem. If a transaction is incomplete at 370the time of the crash, then there is no guarantee of consistency for 371the blocks in that transaction so they are discarded (which means any 372filesystem changes they represent are also lost). 373Check Documentation/filesystems/ext4/ if you want to read more about 374ext4 and journaling. 375 376References 377========== 378 379======================= =============================================== 380The kernel source file:/usr/src/linux/fs/ext2/ 381e2fsprogs (e2fsck) http://e2fsprogs.sourceforge.net/ 382Design & Implementation http://e2fsprogs.sourceforge.net/ext2intro.html 383Journaling (ext3) ftp://ftp.uk.linux.org/pub/linux/sct/fs/jfs/ 384Filesystem Resizing http://ext2resize.sourceforge.net/ 385Compression [1]_ http://e2compr.sourceforge.net/ 386======================= =============================================== 387 388Implementations for: 389 390======================= =========================================================== 391Windows 95/98/NT/2000 http://www.chrysocome.net/explore2fs 392Windows 95 [1]_ http://www.yipton.net/content.html#FSDEXT2 393DOS client [1]_ ftp://metalab.unc.edu/pub/Linux/system/filesystems/ext2/ 394OS/2 [2]_ ftp://metalab.unc.edu/pub/Linux/system/filesystems/ext2/ 395RISC OS client http://www.esw-heim.tu-clausthal.de/~marco/smorbrod/IscaFS/ 396======================= =========================================================== 397 398.. [1] no longer actively developed/supported (as of Apr 2001) 399.. [2] no longer actively developed/supported (as of Mar 2009) 400