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