view Documentation/filesystems/ext2.txt @ 524:7f8b544237bf

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This is useful if your physical network device is in a utility domain.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
author Keir Fraser <keir.fraser@citrix.com>
date Tue Apr 15 15:18:58 2008 +0100 (2008-04-15)
parents 831230e53067
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2 The Second Extended Filesystem
3 ==============================
5 ext2 was originally released in January 1993. Written by R\'emy Card,
6 Theodore Ts'o and Stephen Tweedie, it was a major rewrite of the
7 Extended Filesystem. It is currently still (April 2001) the predominant
8 filesystem in use by Linux. There are also implementations available
9 for NetBSD, FreeBSD, the GNU HURD, Windows 95/98/NT, OS/2 and RISC OS.
11 Options
12 =======
14 Most defaults are determined by the filesystem superblock, and can be
15 set using tune2fs(8). Kernel-determined defaults are indicated by (*).
17 bsddf (*) Makes `df' act like BSD.
18 minixdf Makes `df' act like Minix.
20 check=none, nocheck (*) Don't do extra checking of bitmaps on mount
21 (check=normal and check=strict options removed)
23 debug Extra debugging information is sent to the
24 kernel syslog. Useful for developers.
26 errors=continue Keep going on a filesystem error.
27 errors=remount-ro Remount the filesystem read-only on an error.
28 errors=panic Panic and halt the machine if an error occurs.
30 grpid, bsdgroups Give objects the same group ID as their parent.
31 nogrpid, sysvgroups New objects have the group ID of their creator.
33 nouid32 Use 16-bit UIDs and GIDs.
35 oldalloc 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.
38 orlov (*) Use the Orlov block allocator.
39 (See http://lwn.net/Articles/14633/ and
40 http://lwn.net/Articles/14446/.)
42 resuid=n The user ID which may use the reserved blocks.
43 resgid=n The group ID which may use the reserved blocks.
45 sb=n Use alternate superblock at this location.
47 user_xattr Enable "user." POSIX Extended Attributes
48 (requires CONFIG_EXT2_FS_XATTR).
49 See also http://acl.bestbits.at
50 nouser_xattr Don't support "user." extended attributes.
52 acl Enable POSIX Access Control Lists support
53 (requires CONFIG_EXT2_FS_POSIX_ACL).
54 See also http://acl.bestbits.at
55 noacl Don't support POSIX ACLs.
57 nobh Do not attach buffer_heads to file pagecache.
59 xip Use execute in place (no caching) if possible
61 grpquota,noquota,quota,usrquota Quota options are silently ignored by ext2.
64 Specification
65 =============
67 ext2 shares many properties with traditional Unix filesystems. It has
68 the concepts of blocks, inodes and directories. It has space in the
69 specification for Access Control Lists (ACLs), fragments, undeletion and
70 compression though these are not yet implemented (some are available as
71 separate patches). There is also a versioning mechanism to allow new
72 features (such as journalling) to be added in a maximally compatible
73 manner.
75 Blocks
76 ------
78 The space in the device or file is split up into blocks. These are
79 a fixed size, of 1024, 2048 or 4096 bytes (8192 bytes on Alpha systems),
80 which is decided when the filesystem is created. Smaller blocks mean
81 less wasted space per file, but require slightly more accounting overhead,
82 and also impose other limits on the size of files and the filesystem.
84 Block Groups
85 ------------
87 Blocks are clustered into block groups in order to reduce fragmentation
88 and minimise the amount of head seeking when reading a large amount
89 of consecutive data. Information about each block group is kept in a
90 descriptor table stored in the block(s) immediately after the superblock.
91 Two blocks near the start of each group are reserved for the block usage
92 bitmap and the inode usage bitmap which show which blocks and inodes
93 are in use. Since each bitmap is limited to a single block, this means
94 that the maximum size of a block group is 8 times the size of a block.
96 The block(s) following the bitmaps in each block group are designated
97 as the inode table for that block group and the remainder are the data
98 blocks. The block allocation algorithm attempts to allocate data blocks
99 in the same block group as the inode which contains them.
101 The Superblock
102 --------------
104 The superblock contains all the information about the configuration of
105 the filing system. The primary copy of the superblock is stored at an
106 offset of 1024 bytes from the start of the device, and it is essential
107 to mounting the filesystem. Since it is so important, backup copies of
108 the superblock are stored in block groups throughout the filesystem.
109 The first version of ext2 (revision 0) stores a copy at the start of
110 every block group, along with backups of the group descriptor block(s).
111 Because this can consume a considerable amount of space for large
112 filesystems, later revisions can optionally reduce the number of backup
113 copies by only putting backups in specific groups (this is the sparse
114 superblock feature). The groups chosen are 0, 1 and powers of 3, 5 and 7.
116 The information in the superblock contains fields such as the total
117 number of inodes and blocks in the filesystem and how many are free,
118 how many inodes and blocks are in each block group, when the filesystem
119 was mounted (and if it was cleanly unmounted), when it was modified,
120 what version of the filesystem it is (see the Revisions section below)
121 and which OS created it.
123 If the filesystem is revision 1 or higher, then there are extra fields,
124 such as a volume name, a unique identification number, the inode size,
125 and space for optional filesystem features to store configuration info.
127 All fields in the superblock (as in all other ext2 structures) are stored
128 on the disc in little endian format, so a filesystem is portable between
129 machines without having to know what machine it was created on.
131 Inodes
132 ------
134 The inode (index node) is a fundamental concept in the ext2 filesystem.
135 Each object in the filesystem is represented by an inode. The inode
136 structure contains pointers to the filesystem blocks which contain the
137 data held in the object and all of the metadata about an object except
138 its name. The metadata about an object includes the permissions, owner,
139 group, flags, size, number of blocks used, access time, change time,
140 modification time, deletion time, number of links, fragments, version
141 (for NFS) and extended attributes (EAs) and/or Access Control Lists (ACLs).
143 There are some reserved fields which are currently unused in the inode
144 structure and several which are overloaded. One field is reserved for the
145 directory ACL if the inode is a directory and alternately for the top 32
146 bits of the file size if the inode is a regular file (allowing file sizes
147 larger than 2GB). The translator field is unused under Linux, but is used
148 by the HURD to reference the inode of a program which will be used to
149 interpret this object. Most of the remaining reserved fields have been
150 used up for both Linux and the HURD for larger owner and group fields,
151 The HURD also has a larger mode field so it uses another of the remaining
152 fields to store the extra more bits.
154 There are pointers to the first 12 blocks which contain the file's data
155 in the inode. There is a pointer to an indirect block (which contains
156 pointers to the next set of blocks), a pointer to a doubly-indirect
157 block (which contains pointers to indirect blocks) and a pointer to a
158 trebly-indirect block (which contains pointers to doubly-indirect blocks).
160 The flags field contains some ext2-specific flags which aren't catered
161 for by the standard chmod flags. These flags can be listed with lsattr
162 and changed with the chattr command, and allow specific filesystem
163 behaviour on a per-file basis. There are flags for secure deletion,
164 undeletable, compression, synchronous updates, immutability, append-only,
165 dumpable, no-atime, indexed directories, and data-journaling. Not all
166 of these are supported yet.
168 Directories
169 -----------
171 A directory is a filesystem object and has an inode just like a file.
172 It is a specially formatted file containing records which associate
173 each name with an inode number. Later revisions of the filesystem also
174 encode the type of the object (file, directory, symlink, device, fifo,
175 socket) 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
177 Glibc 2.2).
179 The inode allocation code tries to assign inodes which are in the same
180 block group as the directory in which they are first created.
182 The current implementation of ext2 uses a singly-linked list to store
183 the filenames in the directory; a pending enhancement uses hashing of the
184 filenames to allow lookup without the need to scan the entire directory.
186 The current implementation never removes empty directory blocks once they
187 have been allocated to hold more files.
189 Special files
190 -------------
192 Symbolic links are also filesystem objects with inodes. They deserve
193 special mention because the data for them is stored within the inode
194 itself if the symlink is less than 60 bytes long. It uses the fields
195 which would normally be used to store the pointers to data blocks.
196 This is a worthwhile optimisation as it we avoid allocating a full
197 block for the symlink, and most symlinks are less than 60 characters long.
199 Character and block special devices never have data blocks assigned to
200 them. Instead, their device number is stored in the inode, again reusing
201 the fields which would be used to point to the data blocks.
203 Reserved Space
204 --------------
206 In ext2, there is a mechanism for reserving a certain number of blocks
207 for a particular user (normally the super-user). This is intended to
208 allow for the system to continue functioning even if non-priveleged users
209 fill up all the space available to them (this is independent of filesystem
210 quotas). It also keeps the filesystem from filling up entirely which
211 helps combat fragmentation.
213 Filesystem check
214 ----------------
216 At boot time, most systems run a consistency check (e2fsck) on their
217 filesystems. The superblock of the ext2 filesystem contains several
218 fields which indicate whether fsck should actually run (since checking
219 the filesystem at boot can take a long time if it is large). fsck will
220 run if the filesystem was not cleanly unmounted, if the maximum mount
221 count has been exceeded or if the maximum time between checks has been
222 exceeded.
224 Feature Compatibility
225 ---------------------
227 The compatibility feature mechanism used in ext2 is sophisticated.
228 It safely allows features to be added to the filesystem, without
229 unnecessarily sacrificing compatibility with older versions of the
230 filesystem code. The feature compatibility mechanism is not supported by
231 the original revision 0 (EXT2_GOOD_OLD_REV) of ext2, but was introduced in
232 revision 1. There are three 32-bit fields, one for compatible features
233 (COMPAT), one for read-only compatible (RO_COMPAT) features and one for
234 incompatible (INCOMPAT) features.
236 These feature flags have specific meanings for the kernel as follows:
238 A COMPAT flag indicates that a feature is present in the filesystem,
239 but the on-disk format is 100% compatible with older on-disk formats, so
240 a kernel which didn't know anything about this feature could read/write
241 the filesystem without any chance of corrupting the filesystem (or even
242 making it inconsistent). This is essentially just a flag which says
243 "this filesystem has a (hidden) feature" that the kernel or e2fsck may
244 want to be aware of (more on e2fsck and feature flags later). The ext3
245 HAS_JOURNAL feature is a COMPAT flag because the ext3 journal is simply
246 a regular file with data blocks in it so the kernel does not need to
247 take any special notice of it if it doesn't understand ext3 journaling.
249 An RO_COMPAT flag indicates that the on-disk format is 100% compatible
250 with older on-disk formats for reading (i.e. the feature does not change
251 the visible on-disk format). However, an old kernel writing to such a
252 filesystem would/could corrupt the filesystem, so this is prevented. The
253 most common such feature, SPARSE_SUPER, is an RO_COMPAT feature because
254 sparse groups allow file data blocks where superblock/group descriptor
255 backups used to live, and ext2_free_blocks() refuses to free these blocks,
256 which would leading to inconsistent bitmaps. An old kernel would also
257 get an error if it tried to free a series of blocks which crossed a group
258 boundary, but this is a legitimate layout in a SPARSE_SUPER filesystem.
260 An INCOMPAT flag indicates the on-disk format has changed in some
261 way that makes it unreadable by older kernels, or would otherwise
262 cause a problem if an old kernel tried to mount it. FILETYPE is an
263 INCOMPAT flag because older kernels would think a filename was longer
264 than 256 characters, which would lead to corrupt directory listings.
265 The COMPRESSION flag is an obvious INCOMPAT flag - if the kernel
266 doesn't understand compression, you would just get garbage back from
267 read() instead of it automatically decompressing your data. The ext3
268 RECOVER flag is needed to prevent a kernel which does not understand the
269 ext3 journal from mounting the filesystem without replaying the journal.
271 For e2fsck, it needs to be more strict with the handling of these
272 flags than the kernel. If it doesn't understand ANY of the COMPAT,
273 RO_COMPAT, or INCOMPAT flags it will refuse to check the filesystem,
274 because it has no way of verifying whether a given feature is valid
275 or not. Allowing e2fsck to succeed on a filesystem with an unknown
276 feature is a false sense of security for the user. Refusing to check
277 a filesystem with unknown features is a good incentive for the user to
278 update to the latest e2fsck. This also means that anyone adding feature
279 flags to ext2 also needs to update e2fsck to verify these features.
281 Metadata
282 --------
284 It is frequently claimed that the ext2 implementation of writing
285 asynchronous metadata is faster than the ffs synchronous metadata
286 scheme but less reliable. Both methods are equally resolvable by their
287 respective fsck programs.
289 If you're exceptionally paranoid, there are 3 ways of making metadata
290 writes synchronous on ext2:
292 per-file if you have the program source: use the O_SYNC flag to open()
293 per-file if you don't have the source: use "chattr +S" on the file
294 per-filesystem: add the "sync" option to mount (or in /etc/fstab)
296 the first and last are not ext2 specific but do force the metadata to
297 be written synchronously. See also Journaling below.
299 Limitations
300 -----------
302 There are various limits imposed by the on-disk layout of ext2. Other
303 limits are imposed by the current implementation of the kernel code.
304 Many of the limits are determined at the time the filesystem is first
305 created, and depend upon the block size chosen. The ratio of inodes to
306 data blocks is fixed at filesystem creation time, so the only way to
307 increase the number of inodes is to increase the size of the filesystem.
308 No tools currently exist which can change the ratio of inodes to blocks.
310 Most of these limits could be overcome with slight changes in the on-disk
311 format and using a compatibility flag to signal the format change (at
312 the expense of some compatibility).
314 Filesystem block size: 1kB 2kB 4kB 8kB
316 File size limit: 16GB 256GB 2048GB 2048GB
317 Filesystem size limit: 2047GB 8192GB 16384GB 32768GB
319 There is a 2.4 kernel limit of 2048GB for a single block device, so no
320 filesystem larger than that can be created at this time. There is also
321 an upper limit on the block size imposed by the page size of the kernel,
322 so 8kB blocks are only allowed on Alpha systems (and other architectures
323 which support larger pages).
325 There is an upper limit of 32768 subdirectories in a single directory.
327 There is a "soft" upper limit of about 10-15k files in a single directory
328 with the current linear linked-list directory implementation. This limit
329 stems from performance problems when creating and deleting (and also
330 finding) files in such large directories. Using a hashed directory index
331 (under development) allows 100k-1M+ files in a single directory without
332 performance problems (although RAM size becomes an issue at this point).
334 The (meaningless) absolute upper limit of files in a single directory
335 (imposed by the file size, the realistic limit is obviously much less)
336 is over 130 trillion files. It would be higher except there are not
337 enough 4-character names to make up unique directory entries, so they
338 have to be 8 character filenames, even then we are fairly close to
339 running out of unique filenames.
341 Journaling
342 ----------
344 A journaling extension to the ext2 code has been developed by Stephen
345 Tweedie. It avoids the risks of metadata corruption and the need to
346 wait for e2fsck to complete after a crash, without requiring a change
347 to the on-disk ext2 layout. In a nutshell, the journal is a regular
348 file which stores whole metadata (and optionally data) blocks that have
349 been modified, prior to writing them into the filesystem. This means
350 it is possible to add a journal to an existing ext2 filesystem without
351 the need for data conversion.
353 When changes to the filesystem (e.g. a file is renamed) they are stored in
354 a transaction in the journal and can either be complete or incomplete at
355 the 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
357 in that transaction are guaranteed to represent a valid filesystem state,
358 and are copied into the filesystem. If a transaction is incomplete at
359 the time of the crash, then there is no guarantee of consistency for
360 the blocks in that transaction so they are discarded (which means any
361 filesystem changes they represent are also lost).
362 Check Documentation/filesystems/ext3.txt if you want to read more about
363 ext3 and journaling.
365 References
366 ==========
368 The kernel source file:/usr/src/linux/fs/ext2/
369 e2fsprogs (e2fsck) http://e2fsprogs.sourceforge.net/
370 Design & Implementation http://e2fsprogs.sourceforge.net/ext2intro.html
371 Journaling (ext3) ftp://ftp.uk.linux.org/pub/linux/sct/fs/jfs/
372 Filesystem Resizing http://ext2resize.sourceforge.net/
373 Compression (*) http://e2compr.sourceforge.net/
375 Implementations for:
376 Windows 95/98/NT/2000 http://uranus.it.swin.edu.au/~jn/linux/Explore2fs.htm
377 Windows 95 (*) http://www.yipton.demon.co.uk/content.html#FSDEXT2
378 DOS client (*) ftp://metalab.unc.edu/pub/Linux/system/filesystems/ext2/
379 OS/2 http://perso.wanadoo.fr/matthieu.willm/ext2-os2/
380 RISC OS client ftp://ftp.barnet.ac.uk/pub/acorn/armlinux/iscafs/
382 (*) no longer actively developed/supported (as of Apr 2001)