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authorLinus Torvalds <torvalds@ppc970.osdl.org>2005-04-16 15:20:36 -0700
committerLinus Torvalds <torvalds@ppc970.osdl.org>2005-04-16 15:20:36 -0700
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tree0bba044c4ce775e45a88a51686b5d9f90697ea9d /Documentation/filesystems/ext2.txt
downloadlinux-stericsson-2.6.12-rc2.tar.gz
Linux-2.6.12-rc2v2.6.12-rc2
Initial git repository build. I'm not bothering with the full history, even though we have it. We can create a separate "historical" git archive of that later if we want to, and in the meantime it's about 3.2GB when imported into git - space that would just make the early git days unnecessarily complicated, when we don't have a lot of good infrastructure for it. Let it rip!
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+
+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 Check block and inode bitmaps at mount time
+ (requires CONFIG_EXT2_CHECK).
+check=none, nocheck (*) Don't do extra checking of bitmaps on mount
+ (check=normal and check=strict options removed)
+
+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).
+ See also http://acl.bestbits.at
+nouser_xattr Don't support "user." extended attributes.
+
+acl Enable POSIX Access Control Lists support
+ (requires CONFIG_EXT2_FS_POSIX_ACL).
+ See also http://acl.bestbits.at
+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-priveleged 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 32768 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/ext3.txt if you want to read more about
+ext3 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/
+Hashed Directories http://kernelnewbies.org/~phillips/htree/
+Filesystem Resizing http://ext2resize.sourceforge.net/
+Compression (*) http://www.netspace.net.au/~reiter/e2compr/
+
+Implementations for:
+Windows 95/98/NT/2000 http://uranus.it.swin.edu.au/~jn/linux/Explore2fs.htm
+Windows 95 (*) http://www.yipton.demon.co.uk/content.html#FSDEXT2
+DOS client (*) ftp://metalab.unc.edu/pub/Linux/system/filesystems/ext2/
+OS/2 http://perso.wanadoo.fr/matthieu.willm/ext2-os2/
+RISC OS client ftp://ftp.barnet.ac.uk/pub/acorn/armlinux/iscafs/
+
+(*) no longer actively developed/supported (as of Apr 2001)