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Based on kernel version 4.15. Page generated on 2018-01-29 10:00 EST.

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	dax				Use direct access (no page cache).  See
24					Documentation/filesystems/dax.txt.
26	debug				Extra debugging information is sent to the
27					kernel syslog.  Useful for developers.
29	errors=continue			Keep going on a filesystem error.
30	errors=remount-ro		Remount the filesystem read-only on an error.
31	errors=panic			Panic and halt the machine if an error occurs.
33	grpid, bsdgroups		Give objects the same group ID as their parent.
34	nogrpid, sysvgroups		New objects have the group ID of their creator.
36	nouid32				Use 16-bit UIDs and GIDs.
38	oldalloc			Enable the old block allocator. Orlov should
39					have better performance, we'd like to get some
40					feedback if it's the contrary for you.
41	orlov			(*)	Use the Orlov block allocator.
42					(See http://lwn.net/Articles/14633/ and
43					http://lwn.net/Articles/14446/.)
45	resuid=n			The user ID which may use the reserved blocks.
46	resgid=n			The group ID which may use the reserved blocks.
48	sb=n				Use alternate superblock at this location.
50	user_xattr			Enable "user." POSIX Extended Attributes
51					(requires CONFIG_EXT2_FS_XATTR).
52					See also http://acl.bestbits.at
53	nouser_xattr			Don't support "user." extended attributes.
55	acl				Enable POSIX Access Control Lists support
56					(requires CONFIG_EXT2_FS_POSIX_ACL).
57					See also http://acl.bestbits.at
58	noacl				Don't support POSIX ACLs.
60	nobh				Do not attach buffer_heads to file pagecache.
62	grpquota,noquota,quota,usrquota	Quota options are silently ignored by ext2.
65	Specification
66	=============
68	ext2 shares many properties with traditional Unix filesystems.  It has
69	the concepts of blocks, inodes and directories.  It has space in the
70	specification for Access Control Lists (ACLs), fragments, undeletion and
71	compression though these are not yet implemented (some are available as
72	separate patches).  There is also a versioning mechanism to allow new
73	features (such as journalling) to be added in a maximally compatible
74	manner.
76	Blocks
77	------
79	The space in the device or file is split up into blocks.  These are
80	a fixed size, of 1024, 2048 or 4096 bytes (8192 bytes on Alpha systems),
81	which is decided when the filesystem is created.  Smaller blocks mean
82	less wasted space per file, but require slightly more accounting overhead,
83	and also impose other limits on the size of files and the filesystem.
85	Block Groups
86	------------
88	Blocks are clustered into block groups in order to reduce fragmentation
89	and minimise the amount of head seeking when reading a large amount
90	of consecutive data.  Information about each block group is kept in a
91	descriptor table stored in the block(s) immediately after the superblock.
92	Two blocks near the start of each group are reserved for the block usage
93	bitmap and the inode usage bitmap which show which blocks and inodes
94	are in use.  Since each bitmap is limited to a single block, this means
95	that the maximum size of a block group is 8 times the size of a block.
97	The block(s) following the bitmaps in each block group are designated
98	as the inode table for that block group and the remainder are the data
99	blocks.  The block allocation algorithm attempts to allocate data blocks
100	in the same block group as the inode which contains them.
102	The Superblock
103	--------------
105	The superblock contains all the information about the configuration of
106	the filing system.  The primary copy of the superblock is stored at an
107	offset of 1024 bytes from the start of the device, and it is essential
108	to mounting the filesystem.  Since it is so important, backup copies of
109	the superblock are stored in block groups throughout the filesystem.
110	The first version of ext2 (revision 0) stores a copy at the start of
111	every block group, along with backups of the group descriptor block(s).
112	Because this can consume a considerable amount of space for large
113	filesystems, later revisions can optionally reduce the number of backup
114	copies by only putting backups in specific groups (this is the sparse
115	superblock feature).  The groups chosen are 0, 1 and powers of 3, 5 and 7.
117	The information in the superblock contains fields such as the total
118	number of inodes and blocks in the filesystem and how many are free,
119	how many inodes and blocks are in each block group, when the filesystem
120	was mounted (and if it was cleanly unmounted), when it was modified,
121	what version of the filesystem it is (see the Revisions section below)
122	and which OS created it.
124	If the filesystem is revision 1 or higher, then there are extra fields,
125	such as a volume name, a unique identification number, the inode size,
126	and space for optional filesystem features to store configuration info.
128	All fields in the superblock (as in all other ext2 structures) are stored
129	on the disc in little endian format, so a filesystem is portable between
130	machines without having to know what machine it was created on.
132	Inodes
133	------
135	The inode (index node) is a fundamental concept in the ext2 filesystem.
136	Each object in the filesystem is represented by an inode.  The inode
137	structure contains pointers to the filesystem blocks which contain the
138	data held in the object and all of the metadata about an object except
139	its name.  The metadata about an object includes the permissions, owner,
140	group, flags, size, number of blocks used, access time, change time,
141	modification time, deletion time, number of links, fragments, version
142	(for NFS) and extended attributes (EAs) and/or Access Control Lists (ACLs).
144	There are some reserved fields which are currently unused in the inode
145	structure and several which are overloaded.  One field is reserved for the
146	directory ACL if the inode is a directory and alternately for the top 32
147	bits of the file size if the inode is a regular file (allowing file sizes
148	larger than 2GB).  The translator field is unused under Linux, but is used
149	by the HURD to reference the inode of a program which will be used to
150	interpret this object.  Most of the remaining reserved fields have been
151	used up for both Linux and the HURD for larger owner and group fields,
152	The HURD also has a larger mode field so it uses another of the remaining
153	fields to store the extra more bits.
155	There are pointers to the first 12 blocks which contain the file's data
156	in the inode.  There is a pointer to an indirect block (which contains
157	pointers to the next set of blocks), a pointer to a doubly-indirect
158	block (which contains pointers to indirect blocks) and a pointer to a
159	trebly-indirect block (which contains pointers to doubly-indirect blocks).
161	The flags field contains some ext2-specific flags which aren't catered
162	for by the standard chmod flags.  These flags can be listed with lsattr
163	and changed with the chattr command, and allow specific filesystem
164	behaviour on a per-file basis.  There are flags for secure deletion,
165	undeletable, compression, synchronous updates, immutability, append-only,
166	dumpable, no-atime, indexed directories, and data-journaling.  Not all
167	of these are supported yet.
169	Directories
170	-----------
172	A directory is a filesystem object and has an inode just like a file.
173	It is a specially formatted file containing records which associate
174	each name with an inode number.  Later revisions of the filesystem also
175	encode the type of the object (file, directory, symlink, device, fifo,
176	socket) to avoid the need to check the inode itself for this information
177	(support for taking advantage of this feature does not yet exist in
178	Glibc 2.2).
180	The inode allocation code tries to assign inodes which are in the same
181	block group as the directory in which they are first created.
183	The current implementation of ext2 uses a singly-linked list to store
184	the filenames in the directory; a pending enhancement uses hashing of the
185	filenames to allow lookup without the need to scan the entire directory.
187	The current implementation never removes empty directory blocks once they
188	have been allocated to hold more files.
190	Special files
191	-------------
193	Symbolic links are also filesystem objects with inodes.  They deserve
194	special mention because the data for them is stored within the inode
195	itself if the symlink is less than 60 bytes long.  It uses the fields
196	which would normally be used to store the pointers to data blocks.
197	This is a worthwhile optimisation as it we avoid allocating a full
198	block for the symlink, and most symlinks are less than 60 characters long.
200	Character and block special devices never have data blocks assigned to
201	them.  Instead, their device number is stored in the inode, again reusing
202	the fields which would be used to point to the data blocks.
204	Reserved Space
205	--------------
207	In ext2, there is a mechanism for reserving a certain number of blocks
208	for a particular user (normally the super-user).  This is intended to
209	allow for the system to continue functioning even if non-privileged users
210	fill up all the space available to them (this is independent of filesystem
211	quotas).  It also keeps the filesystem from filling up entirely which
212	helps combat fragmentation.
214	Filesystem check
215	----------------
217	At boot time, most systems run a consistency check (e2fsck) on their
218	filesystems.  The superblock of the ext2 filesystem contains several
219	fields which indicate whether fsck should actually run (since checking
220	the filesystem at boot can take a long time if it is large).  fsck will
221	run if the filesystem was not cleanly unmounted, if the maximum mount
222	count has been exceeded or if the maximum time between checks has been
223	exceeded.
225	Feature Compatibility
226	---------------------
228	The compatibility feature mechanism used in ext2 is sophisticated.
229	It safely allows features to be added to the filesystem, without
230	unnecessarily sacrificing compatibility with older versions of the
231	filesystem code.  The feature compatibility mechanism is not supported by
232	the original revision 0 (EXT2_GOOD_OLD_REV) of ext2, but was introduced in
233	revision 1.  There are three 32-bit fields, one for compatible features
234	(COMPAT), one for read-only compatible (RO_COMPAT) features and one for
235	incompatible (INCOMPAT) features.
237	These feature flags have specific meanings for the kernel as follows:
239	A COMPAT flag indicates that a feature is present in the filesystem,
240	but the on-disk format is 100% compatible with older on-disk formats, so
241	a kernel which didn't know anything about this feature could read/write
242	the filesystem without any chance of corrupting the filesystem (or even
243	making it inconsistent).  This is essentially just a flag which says
244	"this filesystem has a (hidden) feature" that the kernel or e2fsck may
245	want to be aware of (more on e2fsck and feature flags later).  The ext3
246	HAS_JOURNAL feature is a COMPAT flag because the ext3 journal is simply
247	a regular file with data blocks in it so the kernel does not need to
248	take any special notice of it if it doesn't understand ext3 journaling.
250	An RO_COMPAT flag indicates that the on-disk format is 100% compatible
251	with older on-disk formats for reading (i.e. the feature does not change
252	the visible on-disk format).  However, an old kernel writing to such a
253	filesystem would/could corrupt the filesystem, so this is prevented. The
254	most common such feature, SPARSE_SUPER, is an RO_COMPAT feature because
255	sparse groups allow file data blocks where superblock/group descriptor
256	backups used to live, and ext2_free_blocks() refuses to free these blocks,
257	which would leading to inconsistent bitmaps.  An old kernel would also
258	get an error if it tried to free a series of blocks which crossed a group
259	boundary, but this is a legitimate layout in a SPARSE_SUPER filesystem.
261	An INCOMPAT flag indicates the on-disk format has changed in some
262	way that makes it unreadable by older kernels, or would otherwise
263	cause a problem if an old kernel tried to mount it.  FILETYPE is an
264	INCOMPAT flag because older kernels would think a filename was longer
265	than 256 characters, which would lead to corrupt directory listings.
266	The COMPRESSION flag is an obvious INCOMPAT flag - if the kernel
267	doesn't understand compression, you would just get garbage back from
268	read() instead of it automatically decompressing your data.  The ext3
269	RECOVER flag is needed to prevent a kernel which does not understand the
270	ext3 journal from mounting the filesystem without replaying the journal.
272	For e2fsck, it needs to be more strict with the handling of these
273	flags than the kernel.  If it doesn't understand ANY of the COMPAT,
274	RO_COMPAT, or INCOMPAT flags it will refuse to check the filesystem,
275	because it has no way of verifying whether a given feature is valid
276	or not.  Allowing e2fsck to succeed on a filesystem with an unknown
277	feature is a false sense of security for the user.  Refusing to check
278	a filesystem with unknown features is a good incentive for the user to
279	update to the latest e2fsck.  This also means that anyone adding feature
280	flags to ext2 also needs to update e2fsck to verify these features.
282	Metadata
283	--------
285	It is frequently claimed that the ext2 implementation of writing
286	asynchronous metadata is faster than the ffs synchronous metadata
287	scheme but less reliable.  Both methods are equally resolvable by their
288	respective fsck programs.
290	If you're exceptionally paranoid, there are 3 ways of making metadata
291	writes synchronous on ext2:
293	per-file if you have the program source: use the O_SYNC flag to open()
294	per-file if you don't have the source: use "chattr +S" on the file
295	per-filesystem: add the "sync" option to mount (or in /etc/fstab)
297	the first and last are not ext2 specific but do force the metadata to
298	be written synchronously.  See also Journaling below.
300	Limitations
301	-----------
303	There are various limits imposed by the on-disk layout of ext2.  Other
304	limits are imposed by the current implementation of the kernel code.
305	Many of the limits are determined at the time the filesystem is first
306	created, and depend upon the block size chosen.  The ratio of inodes to
307	data blocks is fixed at filesystem creation time, so the only way to
308	increase the number of inodes is to increase the size of the filesystem.
309	No tools currently exist which can change the ratio of inodes to blocks.
311	Most of these limits could be overcome with slight changes in the on-disk
312	format and using a compatibility flag to signal the format change (at
313	the expense of some compatibility).
315	Filesystem block size:     1kB        2kB        4kB        8kB
317	File size limit:          16GB      256GB     2048GB     2048GB
318	Filesystem size limit:  2047GB     8192GB    16384GB    32768GB
320	There is a 2.4 kernel limit of 2048GB for a single block device, so no
321	filesystem larger than that can be created at this time.  There is also
322	an upper limit on the block size imposed by the page size of the kernel,
323	so 8kB blocks are only allowed on Alpha systems (and other architectures
324	which support larger pages).
326	There is an upper limit of 32000 subdirectories in a single directory.
328	There is a "soft" upper limit of about 10-15k files in a single directory
329	with the current linear linked-list directory implementation.  This limit
330	stems from performance problems when creating and deleting (and also
331	finding) files in such large directories.  Using a hashed directory index
332	(under development) allows 100k-1M+ files in a single directory without
333	performance problems (although RAM size becomes an issue at this point).
335	The (meaningless) absolute upper limit of files in a single directory
336	(imposed by the file size, the realistic limit is obviously much less)
337	is over 130 trillion files.  It would be higher except there are not
338	enough 4-character names to make up unique directory entries, so they
339	have to be 8 character filenames, even then we are fairly close to
340	running out of unique filenames.
342	Journaling
343	----------
345	A journaling extension to the ext2 code has been developed by Stephen
346	Tweedie.  It avoids the risks of metadata corruption and the need to
347	wait for e2fsck to complete after a crash, without requiring a change
348	to the on-disk ext2 layout.  In a nutshell, the journal is a regular
349	file which stores whole metadata (and optionally data) blocks that have
350	been modified, prior to writing them into the filesystem.  This means
351	it is possible to add a journal to an existing ext2 filesystem without
352	the need for data conversion.
354	When changes to the filesystem (e.g. a file is renamed) they are stored in
355	a transaction in the journal and can either be complete or incomplete at
356	the time of a crash.  If a transaction is complete at the time of a crash
357	(or in the normal case where the system does not crash), then any blocks
358	in that transaction are guaranteed to represent a valid filesystem state,
359	and are copied into the filesystem.  If a transaction is incomplete at
360	the time of the crash, then there is no guarantee of consistency for
361	the blocks in that transaction so they are discarded (which means any
362	filesystem changes they represent are also lost).
363	Check Documentation/filesystems/ext4.txt if you want to read more about
364	ext4 and journaling.
366	References
367	==========
369	The kernel source	file:/usr/src/linux/fs/ext2/
370	e2fsprogs (e2fsck)	http://e2fsprogs.sourceforge.net/
371	Design & Implementation	http://e2fsprogs.sourceforge.net/ext2intro.html
372	Journaling (ext3)	ftp://ftp.uk.linux.org/pub/linux/sct/fs/jfs/
373	Filesystem Resizing	http://ext2resize.sourceforge.net/
374	Compression (*)		http://e2compr.sourceforge.net/
376	Implementations for:
377	Windows 95/98/NT/2000	http://www.chrysocome.net/explore2fs
378	Windows 95 (*)		http://www.yipton.net/content.html#FSDEXT2
379	DOS client (*)		ftp://metalab.unc.edu/pub/Linux/system/filesystems/ext2/
380	OS/2 (+)		ftp://metalab.unc.edu/pub/Linux/system/filesystems/ext2/
381	RISC OS client		http://www.esw-heim.tu-clausthal.de/~marco/smorbrod/IscaFS/
383	(*) no longer actively developed/supported (as of Apr 2001)
384	(+) no longer actively developed/supported (as of Mar 2009)
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