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 .. SPDX-License-Identifier: GPL-2.0
 
 
 ==============================
 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.rst.
 
 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.
 
 quota, usrquota                 Enable user disk quota support
                                 (requires CONFIG_QUOTA).
 
 grpquota                        Enable group disk quota support
                                 (requires CONFIG_QUOTA).
 ====================    ===     ================================================
 
 noquota option ls 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/ 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 [1]_        http://e2compr.sourceforge.net/
 ======================= ===============================================
 
 Implementations for:
 
 ======================= ===========================================================
 Windows 95/98/NT/2000   http://www.chrysocome.net/explore2fs
 Windows 95 [1]_         http://www.yipton.net/content.html#FSDEXT2
 DOS client [1]_         ftp://metalab.unc.edu/pub/Linux/system/filesystems/ext2/
 OS/2 [2]_               ftp://metalab.unc.edu/pub/Linux/system/filesystems/ext2/
 RISC OS client          http://www.esw-heim.tu-clausthal.de/~marco/smorbrod/IscaFS/
 ======================= ===========================================================
 
 .. [1] no longer actively developed/supported (as of Apr 2001)
 .. [2] no longer actively developed/supported (as of Mar 2009)

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