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Based on kernel version 3.13. Page generated on 2014-01-20 22:02 EST.

1	
2	The Second Extended Filesystem
3	==============================
4	
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.
10	
11	Options
12	=======
13	
14	Most defaults are determined by the filesystem superblock, and can be
15	set using tune2fs(8). Kernel-determined defaults are indicated by (*).
16	
17	bsddf			(*)	Makes `df' act like BSD.
18	minixdf				Makes `df' act like Minix.
19	
20	check=none, nocheck	(*)	Don't do extra checking of bitmaps on mount
21					(check=normal and check=strict options removed)
22	
23	debug				Extra debugging information is sent to the
24					kernel syslog.  Useful for developers.
25	
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.
29	
30	grpid, bsdgroups		Give objects the same group ID as their parent.
31	nogrpid, sysvgroups		New objects have the group ID of their creator.
32	
33	nouid32				Use 16-bit UIDs and GIDs.
34	
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/.)
41	
42	resuid=n			The user ID which may use the reserved blocks.
43	resgid=n			The group ID which may use the reserved blocks.
44	
45	sb=n				Use alternate superblock at this location.
46	
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.
51	
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.
56	
57	nobh				Do not attach buffer_heads to file pagecache.
58	
59	xip				Use execute in place (no caching) if possible
60	
61	grpquota,noquota,quota,usrquota	Quota options are silently ignored by ext2.
62	
63	
64	Specification
65	=============
66	
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.
74	
75	Blocks
76	------
77	
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.
83	
84	Block Groups
85	------------
86	
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.
95	
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.
100	
101	The Superblock
102	--------------
103	
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.
115	
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.
122	
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.
126	
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.
130	
131	Inodes
132	------
133	
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).
142	
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.
153	
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).
159	
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.
167	
168	Directories
169	-----------
170	
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).
178	
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.
181	
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.
185	
186	The current implementation never removes empty directory blocks once they
187	have been allocated to hold more files.
188	
189	Special files
190	-------------
191	
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.
198	
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.
202	
203	Reserved Space
204	--------------
205	
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-privileged 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.
212	
213	Filesystem check
214	----------------
215	
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.
223	
224	Feature Compatibility
225	---------------------
226	
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.
235	
236	These feature flags have specific meanings for the kernel as follows:
237	
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.
248	
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.
259	
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.
270	
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.
280	
281	Metadata
282	--------
283	
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.
288	
289	If you're exceptionally paranoid, there are 3 ways of making metadata
290	writes synchronous on ext2:
291	
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)
295	
296	the first and last are not ext2 specific but do force the metadata to
297	be written synchronously.  See also Journaling below.
298	
299	Limitations
300	-----------
301	
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.
309	
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).
313	
314	Filesystem block size:     1kB        2kB        4kB        8kB
315	
316	File size limit:          16GB      256GB     2048GB     2048GB
317	Filesystem size limit:  2047GB     8192GB    16384GB    32768GB
318	
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).
324	
325	There is an upper limit of 32000 subdirectories in a single directory.
326	
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).
333	
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.
340	
341	Journaling
342	----------
343	
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.
352	
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.
364	
365	References
366	==========
367	
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/
374	
375	Implementations for:
376	Windows 95/98/NT/2000	http://www.chrysocome.net/explore2fs
377	Windows 95 (*)		http://www.yipton.net/content.html#FSDEXT2
378	DOS client (*)		ftp://metalab.unc.edu/pub/Linux/system/filesystems/ext2/
379	OS/2 (+)		ftp://metalab.unc.edu/pub/Linux/system/filesystems/ext2/
380	RISC 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)
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