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