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