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diff --git a/Documentation/filesystems/ext2.txt b/Documentation/filesystems/ext2.txt new file mode 100644 index 000000000..81c0becab --- /dev/null +++ b/Documentation/filesystems/ext2.txt @@ -0,0 +1,382 @@ + +The Second Extended Filesystem +============================== + +ext2 was originally released in January 1993. Written by R\'emy Card, +Theodore Ts'o and Stephen Tweedie, it was a major rewrite of the +Extended Filesystem. It is currently still (April 2001) the predominant +filesystem in use by Linux. There are also implementations available +for NetBSD, FreeBSD, the GNU HURD, Windows 95/98/NT, OS/2 and RISC OS. + +Options +======= + +Most defaults are determined by the filesystem superblock, and can be +set using tune2fs(8). Kernel-determined defaults are indicated by (*). + +bsddf (*) Makes `df' act like BSD. +minixdf Makes `df' act like Minix. + +check=none, nocheck (*) Don't do extra checking of bitmaps on mount + (check=normal and check=strict options removed) + +dax Use direct access (no page cache). See + Documentation/filesystems/dax.txt. + +debug Extra debugging information is sent to the + kernel syslog. Useful for developers. + +errors=continue Keep going on a filesystem error. +errors=remount-ro Remount the filesystem read-only on an error. +errors=panic Panic and halt the machine if an error occurs. + +grpid, bsdgroups Give objects the same group ID as their parent. +nogrpid, sysvgroups New objects have the group ID of their creator. + +nouid32 Use 16-bit UIDs and GIDs. + +oldalloc Enable the old block allocator. Orlov should + have better performance, we'd like to get some + feedback if it's the contrary for you. +orlov (*) Use the Orlov block allocator. + (See http://lwn.net/Articles/14633/ and + http://lwn.net/Articles/14446/.) + +resuid=n The user ID which may use the reserved blocks. +resgid=n The group ID which may use the reserved blocks. + +sb=n Use alternate superblock at this location. + +user_xattr Enable "user." POSIX Extended Attributes + (requires CONFIG_EXT2_FS_XATTR). +nouser_xattr Don't support "user." extended attributes. + +acl Enable POSIX Access Control Lists support + (requires CONFIG_EXT2_FS_POSIX_ACL). +noacl Don't support POSIX ACLs. + +nobh Do not attach buffer_heads to file pagecache. + +grpquota,noquota,quota,usrquota Quota options are silently ignored by ext2. + + +Specification +============= + +ext2 shares many properties with traditional Unix filesystems. It has +the concepts of blocks, inodes and directories. It has space in the +specification for Access Control Lists (ACLs), fragments, undeletion and +compression though these are not yet implemented (some are available as +separate patches). There is also a versioning mechanism to allow new +features (such as journalling) to be added in a maximally compatible +manner. + +Blocks +------ + +The space in the device or file is split up into blocks. These are +a fixed size, of 1024, 2048 or 4096 bytes (8192 bytes on Alpha systems), +which is decided when the filesystem is created. Smaller blocks mean +less wasted space per file, but require slightly more accounting overhead, +and also impose other limits on the size of files and the filesystem. + +Block Groups +------------ + +Blocks are clustered into block groups in order to reduce fragmentation +and minimise the amount of head seeking when reading a large amount +of consecutive data. Information about each block group is kept in a +descriptor table stored in the block(s) immediately after the superblock. +Two blocks near the start of each group are reserved for the block usage +bitmap and the inode usage bitmap which show which blocks and inodes +are in use. Since each bitmap is limited to a single block, this means +that the maximum size of a block group is 8 times the size of a block. + +The block(s) following the bitmaps in each block group are designated +as the inode table for that block group and the remainder are the data +blocks. The block allocation algorithm attempts to allocate data blocks +in the same block group as the inode which contains them. + +The Superblock +-------------- + +The superblock contains all the information about the configuration of +the filing system. The primary copy of the superblock is stored at an +offset of 1024 bytes from the start of the device, and it is essential +to mounting the filesystem. Since it is so important, backup copies of +the superblock are stored in block groups throughout the filesystem. +The first version of ext2 (revision 0) stores a copy at the start of +every block group, along with backups of the group descriptor block(s). +Because this can consume a considerable amount of space for large +filesystems, later revisions can optionally reduce the number of backup +copies by only putting backups in specific groups (this is the sparse +superblock feature). The groups chosen are 0, 1 and powers of 3, 5 and 7. + +The information in the superblock contains fields such as the total +number of inodes and blocks in the filesystem and how many are free, +how many inodes and blocks are in each block group, when the filesystem +was mounted (and if it was cleanly unmounted), when it was modified, +what version of the filesystem it is (see the Revisions section below) +and which OS created it. + +If the filesystem is revision 1 or higher, then there are extra fields, +such as a volume name, a unique identification number, the inode size, +and space for optional filesystem features to store configuration info. + +All fields in the superblock (as in all other ext2 structures) are stored +on the disc in little endian format, so a filesystem is portable between +machines without having to know what machine it was created on. + +Inodes +------ + +The inode (index node) is a fundamental concept in the ext2 filesystem. +Each object in the filesystem is represented by an inode. The inode +structure contains pointers to the filesystem blocks which contain the +data held in the object and all of the metadata about an object except +its name. The metadata about an object includes the permissions, owner, +group, flags, size, number of blocks used, access time, change time, +modification time, deletion time, number of links, fragments, version +(for NFS) and extended attributes (EAs) and/or Access Control Lists (ACLs). + +There are some reserved fields which are currently unused in the inode +structure and several which are overloaded. One field is reserved for the +directory ACL if the inode is a directory and alternately for the top 32 +bits of the file size if the inode is a regular file (allowing file sizes +larger than 2GB). The translator field is unused under Linux, but is used +by the HURD to reference the inode of a program which will be used to +interpret this object. Most of the remaining reserved fields have been +used up for both Linux and the HURD for larger owner and group fields, +The HURD also has a larger mode field so it uses another of the remaining +fields to store the extra more bits. + +There are pointers to the first 12 blocks which contain the file's data +in the inode. There is a pointer to an indirect block (which contains +pointers to the next set of blocks), a pointer to a doubly-indirect +block (which contains pointers to indirect blocks) and a pointer to a +trebly-indirect block (which contains pointers to doubly-indirect blocks). + +The flags field contains some ext2-specific flags which aren't catered +for by the standard chmod flags. These flags can be listed with lsattr +and changed with the chattr command, and allow specific filesystem +behaviour on a per-file basis. There are flags for secure deletion, +undeletable, compression, synchronous updates, immutability, append-only, +dumpable, no-atime, indexed directories, and data-journaling. Not all +of these are supported yet. + +Directories +----------- + +A directory is a filesystem object and has an inode just like a file. +It is a specially formatted file containing records which associate +each name with an inode number. Later revisions of the filesystem also +encode the type of the object (file, directory, symlink, device, fifo, +socket) to avoid the need to check the inode itself for this information +(support for taking advantage of this feature does not yet exist in +Glibc 2.2). + +The inode allocation code tries to assign inodes which are in the same +block group as the directory in which they are first created. + +The current implementation of ext2 uses a singly-linked list to store +the filenames in the directory; a pending enhancement uses hashing of the +filenames to allow lookup without the need to scan the entire directory. + +The current implementation never removes empty directory blocks once they +have been allocated to hold more files. + +Special files +------------- + +Symbolic links are also filesystem objects with inodes. They deserve +special mention because the data for them is stored within the inode +itself if the symlink is less than 60 bytes long. It uses the fields +which would normally be used to store the pointers to data blocks. +This is a worthwhile optimisation as it we avoid allocating a full +block for the symlink, and most symlinks are less than 60 characters long. + +Character and block special devices never have data blocks assigned to +them. Instead, their device number is stored in the inode, again reusing +the fields which would be used to point to the data blocks. + +Reserved Space +-------------- + +In ext2, there is a mechanism for reserving a certain number of blocks +for a particular user (normally the super-user). This is intended to +allow for the system to continue functioning even if non-privileged users +fill up all the space available to them (this is independent of filesystem +quotas). It also keeps the filesystem from filling up entirely which +helps combat fragmentation. + +Filesystem check +---------------- + +At boot time, most systems run a consistency check (e2fsck) on their +filesystems. The superblock of the ext2 filesystem contains several +fields which indicate whether fsck should actually run (since checking +the filesystem at boot can take a long time if it is large). fsck will +run if the filesystem was not cleanly unmounted, if the maximum mount +count has been exceeded or if the maximum time between checks has been +exceeded. + +Feature Compatibility +--------------------- + +The compatibility feature mechanism used in ext2 is sophisticated. +It safely allows features to be added to the filesystem, without +unnecessarily sacrificing compatibility with older versions of the +filesystem code. The feature compatibility mechanism is not supported by +the original revision 0 (EXT2_GOOD_OLD_REV) of ext2, but was introduced in +revision 1. There are three 32-bit fields, one for compatible features +(COMPAT), one for read-only compatible (RO_COMPAT) features and one for +incompatible (INCOMPAT) features. + +These feature flags have specific meanings for the kernel as follows: + +A COMPAT flag indicates that a feature is present in the filesystem, +but the on-disk format is 100% compatible with older on-disk formats, so +a kernel which didn't know anything about this feature could read/write +the filesystem without any chance of corrupting the filesystem (or even +making it inconsistent). This is essentially just a flag which says +"this filesystem has a (hidden) feature" that the kernel or e2fsck may +want to be aware of (more on e2fsck and feature flags later). The ext3 +HAS_JOURNAL feature is a COMPAT flag because the ext3 journal is simply +a regular file with data blocks in it so the kernel does not need to +take any special notice of it if it doesn't understand ext3 journaling. + +An RO_COMPAT flag indicates that the on-disk format is 100% compatible +with older on-disk formats for reading (i.e. the feature does not change +the visible on-disk format). However, an old kernel writing to such a +filesystem would/could corrupt the filesystem, so this is prevented. The +most common such feature, SPARSE_SUPER, is an RO_COMPAT feature because +sparse groups allow file data blocks where superblock/group descriptor +backups used to live, and ext2_free_blocks() refuses to free these blocks, +which would leading to inconsistent bitmaps. An old kernel would also +get an error if it tried to free a series of blocks which crossed a group +boundary, but this is a legitimate layout in a SPARSE_SUPER filesystem. + +An INCOMPAT flag indicates the on-disk format has changed in some +way that makes it unreadable by older kernels, or would otherwise +cause a problem if an old kernel tried to mount it. FILETYPE is an +INCOMPAT flag because older kernels would think a filename was longer +than 256 characters, which would lead to corrupt directory listings. +The COMPRESSION flag is an obvious INCOMPAT flag - if the kernel +doesn't understand compression, you would just get garbage back from +read() instead of it automatically decompressing your data. The ext3 +RECOVER flag is needed to prevent a kernel which does not understand the +ext3 journal from mounting the filesystem without replaying the journal. + +For e2fsck, it needs to be more strict with the handling of these +flags than the kernel. If it doesn't understand ANY of the COMPAT, +RO_COMPAT, or INCOMPAT flags it will refuse to check the filesystem, +because it has no way of verifying whether a given feature is valid +or not. Allowing e2fsck to succeed on a filesystem with an unknown +feature is a false sense of security for the user. Refusing to check +a filesystem with unknown features is a good incentive for the user to +update to the latest e2fsck. This also means that anyone adding feature +flags to ext2 also needs to update e2fsck to verify these features. + +Metadata +-------- + +It is frequently claimed that the ext2 implementation of writing +asynchronous metadata is faster than the ffs synchronous metadata +scheme but less reliable. Both methods are equally resolvable by their +respective fsck programs. + +If you're exceptionally paranoid, there are 3 ways of making metadata +writes synchronous on ext2: + +per-file if you have the program source: use the O_SYNC flag to open() +per-file if you don't have the source: use "chattr +S" on the file +per-filesystem: add the "sync" option to mount (or in /etc/fstab) + +the first and last are not ext2 specific but do force the metadata to +be written synchronously. See also Journaling below. + +Limitations +----------- + +There are various limits imposed by the on-disk layout of ext2. Other +limits are imposed by the current implementation of the kernel code. +Many of the limits are determined at the time the filesystem is first +created, and depend upon the block size chosen. The ratio of inodes to +data blocks is fixed at filesystem creation time, so the only way to +increase the number of inodes is to increase the size of the filesystem. +No tools currently exist which can change the ratio of inodes to blocks. + +Most of these limits could be overcome with slight changes in the on-disk +format and using a compatibility flag to signal the format change (at +the expense of some compatibility). + +Filesystem block size: 1kB 2kB 4kB 8kB + +File size limit: 16GB 256GB 2048GB 2048GB +Filesystem size limit: 2047GB 8192GB 16384GB 32768GB + +There is a 2.4 kernel limit of 2048GB for a single block device, so no +filesystem larger than that can be created at this time. There is also +an upper limit on the block size imposed by the page size of the kernel, +so 8kB blocks are only allowed on Alpha systems (and other architectures +which support larger pages). + +There is an upper limit of 32000 subdirectories in a single directory. + +There is a "soft" upper limit of about 10-15k files in a single directory +with the current linear linked-list directory implementation. This limit +stems from performance problems when creating and deleting (and also +finding) files in such large directories. Using a hashed directory index +(under development) allows 100k-1M+ files in a single directory without +performance problems (although RAM size becomes an issue at this point). + +The (meaningless) absolute upper limit of files in a single directory +(imposed by the file size, the realistic limit is obviously much less) +is over 130 trillion files. It would be higher except there are not +enough 4-character names to make up unique directory entries, so they +have to be 8 character filenames, even then we are fairly close to +running out of unique filenames. + +Journaling +---------- + +A journaling extension to the ext2 code has been developed by Stephen +Tweedie. It avoids the risks of metadata corruption and the need to +wait for e2fsck to complete after a crash, without requiring a change +to the on-disk ext2 layout. In a nutshell, the journal is a regular +file which stores whole metadata (and optionally data) blocks that have +been modified, prior to writing them into the filesystem. This means +it is possible to add a journal to an existing ext2 filesystem without +the need for data conversion. + +When changes to the filesystem (e.g. a file is renamed) they are stored in +a transaction in the journal and can either be complete or incomplete at +the time of a crash. If a transaction is complete at the time of a crash +(or in the normal case where the system does not crash), then any blocks +in that transaction are guaranteed to represent a valid filesystem state, +and are copied into the filesystem. If a transaction is incomplete at +the time of the crash, then there is no guarantee of consistency for +the blocks in that transaction so they are discarded (which means any +filesystem changes they represent are also lost). +Check Documentation/filesystems/ext4.txt if you want to read more about +ext4 and journaling. + +References +========== + +The kernel source file:/usr/src/linux/fs/ext2/ +e2fsprogs (e2fsck) http://e2fsprogs.sourceforge.net/ +Design & Implementation http://e2fsprogs.sourceforge.net/ext2intro.html +Journaling (ext3) ftp://ftp.uk.linux.org/pub/linux/sct/fs/jfs/ +Filesystem Resizing http://ext2resize.sourceforge.net/ +Compression (*) http://e2compr.sourceforge.net/ + +Implementations for: +Windows 95/98/NT/2000 http://www.chrysocome.net/explore2fs +Windows 95 (*) http://www.yipton.net/content.html#FSDEXT2 +DOS client (*) ftp://metalab.unc.edu/pub/Linux/system/filesystems/ext2/ +OS/2 (+) ftp://metalab.unc.edu/pub/Linux/system/filesystems/ext2/ +RISC OS client http://www.esw-heim.tu-clausthal.de/~marco/smorbrod/IscaFS/ + +(*) no longer actively developed/supported (as of Apr 2001) +(+) no longer actively developed/supported (as of Mar 2009) |