linux/Documentation/filesystems/ext2.txt
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   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
  23debug                           Extra debugging information is sent to the
  24                                kernel syslog.  Useful for developers.
  25
  26errors=continue                 Keep going on a filesystem error.
  27errors=remount-ro               Remount the filesystem read-only on an error.
  28errors=panic                    Panic and halt the machine if an error occurs.
  29
  30grpid, bsdgroups                Give objects the same group ID as their parent.
  31nogrpid, sysvgroups             New objects have the group ID of their creator.
  32
  33nouid32                         Use 16-bit UIDs and GIDs.
  34
  35oldalloc                        Enable the old block allocator. Orlov should
  36                                have better performance, we'd like to get some
  37                                feedback if it's the contrary for you.
  38orlov                   (*)     Use the Orlov block allocator.
  39                                (See http://lwn.net/Articles/14633/ and
  40                                http://lwn.net/Articles/14446/.)
  41
  42resuid=n                        The user ID which may use the reserved blocks.
  43resgid=n                        The group ID which may use the reserved blocks.
  44
  45sb=n                            Use alternate superblock at this location.
  46
  47user_xattr                      Enable "user." POSIX Extended Attributes
  48                                (requires CONFIG_EXT2_FS_XATTR).
  49                                See also http://acl.bestbits.at
  50nouser_xattr                    Don't support "user." extended attributes.
  51
  52acl                             Enable POSIX Access Control Lists support
  53                                (requires CONFIG_EXT2_FS_POSIX_ACL).
  54                                See also http://acl.bestbits.at
  55noacl                           Don't support POSIX ACLs.
  56
  57nobh                            Do not attach buffer_heads to file pagecache.
  58
  59xip                             Use execute in place (no caching) if possible
  60
  61grpquota,noquota,quota,usrquota Quota options are silently ignored by ext2.
  62
  63
  64Specification
  65=============
  66
  67ext2 shares many properties with traditional Unix filesystems.  It has
  68the concepts of blocks, inodes and directories.  It has space in the
  69specification for Access Control Lists (ACLs), fragments, undeletion and
  70compression though these are not yet implemented (some are available as
  71separate patches).  There is also a versioning mechanism to allow new
  72features (such as journalling) to be added in a maximally compatible
  73manner.
  74
  75Blocks
  76------
  77
  78The space in the device or file is split up into blocks.  These are
  79a fixed size, of 1024, 2048 or 4096 bytes (8192 bytes on Alpha systems),
  80which is decided when the filesystem is created.  Smaller blocks mean
  81less wasted space per file, but require slightly more accounting overhead,
  82and also impose other limits on the size of files and the filesystem.
  83
  84Block Groups
  85------------
  86
  87Blocks are clustered into block groups in order to reduce fragmentation
  88and minimise the amount of head seeking when reading a large amount
  89of consecutive data.  Information about each block group is kept in a
  90descriptor table stored in the block(s) immediately after the superblock.
  91Two blocks near the start of each group are reserved for the block usage
  92bitmap and the inode usage bitmap which show which blocks and inodes
  93are in use.  Since each bitmap is limited to a single block, this means
  94that the maximum size of a block group is 8 times the size of a block.
  95
  96The block(s) following the bitmaps in each block group are designated
  97as the inode table for that block group and the remainder are the data
  98blocks.  The block allocation algorithm attempts to allocate data blocks
  99in the same block group as the inode which contains them.
 100
 101The Superblock
 102--------------
 103
 104The superblock contains all the information about the configuration of
 105the filing system.  The primary copy of the superblock is stored at an
 106offset of 1024 bytes from the start of the device, and it is essential
 107to mounting the filesystem.  Since it is so important, backup copies of
 108the superblock are stored in block groups throughout the filesystem.
 109The first version of ext2 (revision 0) stores a copy at the start of
 110every block group, along with backups of the group descriptor block(s).
 111Because this can consume a considerable amount of space for large
 112filesystems, later revisions can optionally reduce the number of backup
 113copies by only putting backups in specific groups (this is the sparse
 114superblock feature).  The groups chosen are 0, 1 and powers of 3, 5 and 7.
 115
 116The information in the superblock contains fields such as the total
 117number of inodes and blocks in the filesystem and how many are free,
 118how many inodes and blocks are in each block group, when the filesystem
 119was mounted (and if it was cleanly unmounted), when it was modified,
 120what version of the filesystem it is (see the Revisions section below)
 121and which OS created it.
 122
 123If the filesystem is revision 1 or higher, then there are extra fields,
 124such as a volume name, a unique identification number, the inode size,
 125and space for optional filesystem features to store configuration info.
 126
 127All fields in the superblock (as in all other ext2 structures) are stored
 128on the disc in little endian format, so a filesystem is portable between
 129machines without having to know what machine it was created on.
 130
 131Inodes
 132------
 133
 134The inode (index node) is a fundamental concept in the ext2 filesystem.
 135Each object in the filesystem is represented by an inode.  The inode
 136structure contains pointers to the filesystem blocks which contain the
 137data held in the object and all of the metadata about an object except
 138its name.  The metadata about an object includes the permissions, owner,
 139group, flags, size, number of blocks used, access time, change time,
 140modification time, deletion time, number of links, fragments, version
 141(for NFS) and extended attributes (EAs) and/or Access Control Lists (ACLs).
 142
 143There are some reserved fields which are currently unused in the inode
 144structure and several which are overloaded.  One field is reserved for the
 145directory ACL if the inode is a directory and alternately for the top 32
 146bits of the file size if the inode is a regular file (allowing file sizes
 147larger than 2GB).  The translator field is unused under Linux, but is used
 148by the HURD to reference the inode of a program which will be used to
 149interpret this object.  Most of the remaining reserved fields have been
 150used up for both Linux and the HURD for larger owner and group fields,
 151The HURD also has a larger mode field so it uses another of the remaining
 152fields to store the extra more bits.
 153
 154There are pointers to the first 12 blocks which contain the file's data
 155in the inode.  There is a pointer to an indirect block (which contains
 156pointers to the next set of blocks), a pointer to a doubly-indirect
 157block (which contains pointers to indirect blocks) and a pointer to a
 158trebly-indirect block (which contains pointers to doubly-indirect blocks).
 159
 160The flags field contains some ext2-specific flags which aren't catered
 161for by the standard chmod flags.  These flags can be listed with lsattr
 162and changed with the chattr command, and allow specific filesystem
 163behaviour on a per-file basis.  There are flags for secure deletion,
 164undeletable, compression, synchronous updates, immutability, append-only,
 165dumpable, no-atime, indexed directories, and data-journaling.  Not all
 166of these are supported yet.
 167
 168Directories
 169-----------
 170
 171A directory is a filesystem object and has an inode just like a file.
 172It is a specially formatted file containing records which associate
 173each name with an inode number.  Later revisions of the filesystem also
 174encode the type of the object (file, directory, symlink, device, fifo,
 175socket) to avoid the need to check the inode itself for this information
 176(support for taking advantage of this feature does not yet exist in
 177Glibc 2.2).
 178
 179The inode allocation code tries to assign inodes which are in the same
 180block group as the directory in which they are first created.
 181
 182The current implementation of ext2 uses a singly-linked list to store
 183the filenames in the directory; a pending enhancement uses hashing of the
 184filenames to allow lookup without the need to scan the entire directory.
 185
 186The current implementation never removes empty directory blocks once they
 187have been allocated to hold more files.
 188
 189Special files
 190-------------
 191
 192Symbolic links are also filesystem objects with inodes.  They deserve
 193special mention because the data for them is stored within the inode
 194itself if the symlink is less than 60 bytes long.  It uses the fields
 195which would normally be used to store the pointers to data blocks.
 196This is a worthwhile optimisation as it we avoid allocating a full
 197block for the symlink, and most symlinks are less than 60 characters long.
 198
 199Character and block special devices never have data blocks assigned to
 200them.  Instead, their device number is stored in the inode, again reusing
 201the fields which would be used to point to the data blocks.
 202
 203Reserved Space
 204--------------
 205
 206In ext2, there is a mechanism for reserving a certain number of blocks
 207for a particular user (normally the super-user).  This is intended to
 208allow for the system to continue functioning even if non-privileged users
 209fill up all the space available to them (this is independent of filesystem
 210quotas).  It also keeps the filesystem from filling up entirely which
 211helps combat fragmentation.
 212
 213Filesystem check
 214----------------
 215
 216At boot time, most systems run a consistency check (e2fsck) on their
 217filesystems.  The superblock of the ext2 filesystem contains several
 218fields which indicate whether fsck should actually run (since checking
 219the filesystem at boot can take a long time if it is large).  fsck will
 220run if the filesystem was not cleanly unmounted, if the maximum mount
 221count has been exceeded or if the maximum time between checks has been
 222exceeded.
 223
 224Feature Compatibility
 225---------------------
 226
 227The compatibility feature mechanism used in ext2 is sophisticated.
 228It safely allows features to be added to the filesystem, without
 229unnecessarily sacrificing compatibility with older versions of the
 230filesystem code.  The feature compatibility mechanism is not supported by
 231the original revision 0 (EXT2_GOOD_OLD_REV) of ext2, but was introduced in
 232revision 1.  There are three 32-bit fields, one for compatible features
 233(COMPAT), one for read-only compatible (RO_COMPAT) features and one for
 234incompatible (INCOMPAT) features.
 235
 236These feature flags have specific meanings for the kernel as follows:
 237
 238A COMPAT flag indicates that a feature is present in the filesystem,
 239but the on-disk format is 100% compatible with older on-disk formats, so
 240a kernel which didn't know anything about this feature could read/write
 241the filesystem without any chance of corrupting the filesystem (or even
 242making it inconsistent).  This is essentially just a flag which says
 243"this filesystem has a (hidden) feature" that the kernel or e2fsck may
 244want to be aware of (more on e2fsck and feature flags later).  The ext3
 245HAS_JOURNAL feature is a COMPAT flag because the ext3 journal is simply
 246a regular file with data blocks in it so the kernel does not need to
 247take any special notice of it if it doesn't understand ext3 journaling.
 248
 249An RO_COMPAT flag indicates that the on-disk format is 100% compatible
 250with older on-disk formats for reading (i.e. the feature does not change
 251the visible on-disk format).  However, an old kernel writing to such a
 252filesystem would/could corrupt the filesystem, so this is prevented. The
 253most common such feature, SPARSE_SUPER, is an RO_COMPAT feature because
 254sparse groups allow file data blocks where superblock/group descriptor
 255backups used to live, and ext2_free_blocks() refuses to free these blocks,
 256which would leading to inconsistent bitmaps.  An old kernel would also
 257get an error if it tried to free a series of blocks which crossed a group
 258boundary, but this is a legitimate layout in a SPARSE_SUPER filesystem.
 259
 260An INCOMPAT flag indicates the on-disk format has changed in some
 261way that makes it unreadable by older kernels, or would otherwise
 262cause a problem if an old kernel tried to mount it.  FILETYPE is an
 263INCOMPAT flag because older kernels would think a filename was longer
 264than 256 characters, which would lead to corrupt directory listings.
 265The COMPRESSION flag is an obvious INCOMPAT flag - if the kernel
 266doesn't understand compression, you would just get garbage back from
 267read() instead of it automatically decompressing your data.  The ext3
 268RECOVER flag is needed to prevent a kernel which does not understand the
 269ext3 journal from mounting the filesystem without replaying the journal.
 270
 271For e2fsck, it needs to be more strict with the handling of these
 272flags than the kernel.  If it doesn't understand ANY of the COMPAT,
 273RO_COMPAT, or INCOMPAT flags it will refuse to check the filesystem,
 274because it has no way of verifying whether a given feature is valid
 275or not.  Allowing e2fsck to succeed on a filesystem with an unknown
 276feature is a false sense of security for the user.  Refusing to check
 277a filesystem with unknown features is a good incentive for the user to
 278update to the latest e2fsck.  This also means that anyone adding feature
 279flags to ext2 also needs to update e2fsck to verify these features.
 280
 281Metadata
 282--------
 283
 284It is frequently claimed that the ext2 implementation of writing
 285asynchronous metadata is faster than the ffs synchronous metadata
 286scheme but less reliable.  Both methods are equally resolvable by their
 287respective fsck programs.
 288
 289If you're exceptionally paranoid, there are 3 ways of making metadata
 290writes synchronous on ext2:
 291
 292per-file if you have the program source: use the O_SYNC flag to open()
 293per-file if you don't have the source: use "chattr +S" on the file
 294per-filesystem: add the "sync" option to mount (or in /etc/fstab)
 295
 296the first and last are not ext2 specific but do force the metadata to
 297be written synchronously.  See also Journaling below.
 298
 299Limitations
 300-----------
 301
 302There are various limits imposed by the on-disk layout of ext2.  Other
 303limits are imposed by the current implementation of the kernel code.
 304Many of the limits are determined at the time the filesystem is first
 305created, and depend upon the block size chosen.  The ratio of inodes to
 306data blocks is fixed at filesystem creation time, so the only way to
 307increase the number of inodes is to increase the size of the filesystem.
 308No tools currently exist which can change the ratio of inodes to blocks.
 309
 310Most of these limits could be overcome with slight changes in the on-disk
 311format and using a compatibility flag to signal the format change (at
 312the expense of some compatibility).
 313
 314Filesystem block size:     1kB        2kB        4kB        8kB
 315
 316File size limit:          16GB      256GB     2048GB     2048GB
 317Filesystem size limit:  2047GB     8192GB    16384GB    32768GB
 318
 319There is a 2.4 kernel limit of 2048GB for a single block device, so no
 320filesystem larger than that can be created at this time.  There is also
 321an upper limit on the block size imposed by the page size of the kernel,
 322so 8kB blocks are only allowed on Alpha systems (and other architectures
 323which support larger pages).
 324
 325There is an upper limit of 32000 subdirectories in a single directory.
 326
 327There is a "soft" upper limit of about 10-15k files in a single directory
 328with the current linear linked-list directory implementation.  This limit
 329stems from performance problems when creating and deleting (and also
 330finding) files in such large directories.  Using a hashed directory index
 331(under development) allows 100k-1M+ files in a single directory without
 332performance problems (although RAM size becomes an issue at this point).
 333
 334The (meaningless) absolute upper limit of files in a single directory
 335(imposed by the file size, the realistic limit is obviously much less)
 336is over 130 trillion files.  It would be higher except there are not
 337enough 4-character names to make up unique directory entries, so they
 338have to be 8 character filenames, even then we are fairly close to
 339running out of unique filenames.
 340
 341Journaling
 342----------
 343
 344A journaling extension to the ext2 code has been developed by Stephen
 345Tweedie.  It avoids the risks of metadata corruption and the need to
 346wait for e2fsck to complete after a crash, without requiring a change
 347to the on-disk ext2 layout.  In a nutshell, the journal is a regular
 348file which stores whole metadata (and optionally data) blocks that have
 349been modified, prior to writing them into the filesystem.  This means
 350it is possible to add a journal to an existing ext2 filesystem without
 351the need for data conversion.
 352
 353When changes to the filesystem (e.g. a file is renamed) they are stored in
 354a transaction in the journal and can either be complete or incomplete at
 355the time of a crash.  If a transaction is complete at the time of a crash
 356(or in the normal case where the system does not crash), then any blocks
 357in that transaction are guaranteed to represent a valid filesystem state,
 358and are copied into the filesystem.  If a transaction is incomplete at
 359the time of the crash, then there is no guarantee of consistency for
 360the blocks in that transaction so they are discarded (which means any
 361filesystem changes they represent are also lost).
 362Check Documentation/filesystems/ext3.txt if you want to read more about
 363ext3 and journaling.
 364
 365References
 366==========
 367
 368The kernel source       file:/usr/src/linux/fs/ext2/
 369e2fsprogs (e2fsck)      http://e2fsprogs.sourceforge.net/
 370Design & Implementation http://e2fsprogs.sourceforge.net/ext2intro.html
 371Journaling (ext3)       ftp://ftp.uk.linux.org/pub/linux/sct/fs/jfs/
 372Filesystem Resizing     http://ext2resize.sourceforge.net/
 373Compression (*)         http://e2compr.sourceforge.net/
 374
 375Implementations for:
 376Windows 95/98/NT/2000   http://www.chrysocome.net/explore2fs
 377Windows 95 (*)          http://www.yipton.net/content.html#FSDEXT2
 378DOS client (*)          ftp://metalab.unc.edu/pub/Linux/system/filesystems/ext2/
 379OS/2 (+)                ftp://metalab.unc.edu/pub/Linux/system/filesystems/ext2/
 380RISC OS client          http://www.esw-heim.tu-clausthal.de/~marco/smorbrod/IscaFS/
 381
 382(*) no longer actively developed/supported (as of Apr 2001)
 383(+) no longer actively developed/supported (as of Mar 2009)
 384
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