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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=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.
-
-xip Use execute in place (no caching) if possible
-
-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/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/
-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)