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Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 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	dax				Use direct access (no page cache).  See
24					Documentation/filesystems/dax.txt.
25	
26	debug				Extra debugging information is sent to the
27					kernel syslog.  Useful for developers.
28	
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.
32	
33	grpid, bsdgroups		Give objects the same group ID as their parent.
34	nogrpid, sysvgroups		New objects have the group ID of their creator.
35	
36	nouid32				Use 16-bit UIDs and GIDs.
37	
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/.)
44	
45	resuid=n			The user ID which may use the reserved blocks.
46	resgid=n			The group ID which may use the reserved blocks.
47	
48	sb=n				Use alternate superblock at this location.
49	
50	user_xattr			Enable "user." POSIX Extended Attributes
51					(requires CONFIG_EXT2_FS_XATTR).
52	nouser_xattr			Don't support "user." extended attributes.
53	
54	acl				Enable POSIX Access Control Lists support
55					(requires CONFIG_EXT2_FS_POSIX_ACL).
56	noacl				Don't support POSIX ACLs.
57	
58	nobh				Do not attach buffer_heads to file pagecache.
59	
60	grpquota,noquota,quota,usrquota	Quota options are silently ignored by ext2.
61	
62	
63	Specification
64	=============
65	
66	ext2 shares many properties with traditional Unix filesystems.  It has
67	the concepts of blocks, inodes and directories.  It has space in the
68	specification for Access Control Lists (ACLs), fragments, undeletion and
69	compression though these are not yet implemented (some are available as
70	separate patches).  There is also a versioning mechanism to allow new
71	features (such as journalling) to be added in a maximally compatible
72	manner.
73	
74	Blocks
75	------
76	
77	The space in the device or file is split up into blocks.  These are
78	a fixed size, of 1024, 2048 or 4096 bytes (8192 bytes on Alpha systems),
79	which is decided when the filesystem is created.  Smaller blocks mean
80	less wasted space per file, but require slightly more accounting overhead,
81	and also impose other limits on the size of files and the filesystem.
82	
83	Block Groups
84	------------
85	
86	Blocks are clustered into block groups in order to reduce fragmentation
87	and minimise the amount of head seeking when reading a large amount
88	of consecutive data.  Information about each block group is kept in a
89	descriptor table stored in the block(s) immediately after the superblock.
90	Two blocks near the start of each group are reserved for the block usage
91	bitmap and the inode usage bitmap which show which blocks and inodes
92	are in use.  Since each bitmap is limited to a single block, this means
93	that the maximum size of a block group is 8 times the size of a block.
94	
95	The block(s) following the bitmaps in each block group are designated
96	as the inode table for that block group and the remainder are the data
97	blocks.  The block allocation algorithm attempts to allocate data blocks
98	in the same block group as the inode which contains them.
99	
100	The Superblock
101	--------------
102	
103	The superblock contains all the information about the configuration of
104	the filing system.  The primary copy of the superblock is stored at an
105	offset of 1024 bytes from the start of the device, and it is essential
106	to mounting the filesystem.  Since it is so important, backup copies of
107	the superblock are stored in block groups throughout the filesystem.
108	The first version of ext2 (revision 0) stores a copy at the start of
109	every block group, along with backups of the group descriptor block(s).
110	Because this can consume a considerable amount of space for large
111	filesystems, later revisions can optionally reduce the number of backup
112	copies by only putting backups in specific groups (this is the sparse
113	superblock feature).  The groups chosen are 0, 1 and powers of 3, 5 and 7.
114	
115	The information in the superblock contains fields such as the total
116	number of inodes and blocks in the filesystem and how many are free,
117	how many inodes and blocks are in each block group, when the filesystem
118	was mounted (and if it was cleanly unmounted), when it was modified,
119	what version of the filesystem it is (see the Revisions section below)
120	and which OS created it.
121	
122	If the filesystem is revision 1 or higher, then there are extra fields,
123	such as a volume name, a unique identification number, the inode size,
124	and space for optional filesystem features to store configuration info.
125	
126	All fields in the superblock (as in all other ext2 structures) are stored
127	on the disc in little endian format, so a filesystem is portable between
128	machines without having to know what machine it was created on.
129	
130	Inodes
131	------
132	
133	The inode (index node) is a fundamental concept in the ext2 filesystem.
134	Each object in the filesystem is represented by an inode.  The inode
135	structure contains pointers to the filesystem blocks which contain the
136	data held in the object and all of the metadata about an object except
137	its name.  The metadata about an object includes the permissions, owner,
138	group, flags, size, number of blocks used, access time, change time,
139	modification time, deletion time, number of links, fragments, version
140	(for NFS) and extended attributes (EAs) and/or Access Control Lists (ACLs).
141	
142	There are some reserved fields which are currently unused in the inode
143	structure and several which are overloaded.  One field is reserved for the
144	directory ACL if the inode is a directory and alternately for the top 32
145	bits of the file size if the inode is a regular file (allowing file sizes
146	larger than 2GB).  The translator field is unused under Linux, but is used
147	by the HURD to reference the inode of a program which will be used to
148	interpret this object.  Most of the remaining reserved fields have been
149	used up for both Linux and the HURD for larger owner and group fields,
150	The HURD also has a larger mode field so it uses another of the remaining
151	fields to store the extra more bits.
152	
153	There are pointers to the first 12 blocks which contain the file's data
154	in the inode.  There is a pointer to an indirect block (which contains
155	pointers to the next set of blocks), a pointer to a doubly-indirect
156	block (which contains pointers to indirect blocks) and a pointer to a
157	trebly-indirect block (which contains pointers to doubly-indirect blocks).
158	
159	The flags field contains some ext2-specific flags which aren't catered
160	for by the standard chmod flags.  These flags can be listed with lsattr
161	and changed with the chattr command, and allow specific filesystem
162	behaviour on a per-file basis.  There are flags for secure deletion,
163	undeletable, compression, synchronous updates, immutability, append-only,
164	dumpable, no-atime, indexed directories, and data-journaling.  Not all
165	of these are supported yet.
166	
167	Directories
168	-----------
169	
170	A directory is a filesystem object and has an inode just like a file.
171	It is a specially formatted file containing records which associate
172	each name with an inode number.  Later revisions of the filesystem also
173	encode the type of the object (file, directory, symlink, device, fifo,
174	socket) to avoid the need to check the inode itself for this information
175	(support for taking advantage of this feature does not yet exist in
176	Glibc 2.2).
177	
178	The inode allocation code tries to assign inodes which are in the same
179	block group as the directory in which they are first created.
180	
181	The current implementation of ext2 uses a singly-linked list to store
182	the filenames in the directory; a pending enhancement uses hashing of the
183	filenames to allow lookup without the need to scan the entire directory.
184	
185	The current implementation never removes empty directory blocks once they
186	have been allocated to hold more files.
187	
188	Special files
189	-------------
190	
191	Symbolic links are also filesystem objects with inodes.  They deserve
192	special mention because the data for them is stored within the inode
193	itself if the symlink is less than 60 bytes long.  It uses the fields
194	which would normally be used to store the pointers to data blocks.
195	This is a worthwhile optimisation as it we avoid allocating a full
196	block for the symlink, and most symlinks are less than 60 characters long.
197	
198	Character and block special devices never have data blocks assigned to
199	them.  Instead, their device number is stored in the inode, again reusing
200	the fields which would be used to point to the data blocks.
201	
202	Reserved Space
203	--------------
204	
205	In ext2, there is a mechanism for reserving a certain number of blocks
206	for a particular user (normally the super-user).  This is intended to
207	allow for the system to continue functioning even if non-privileged users
208	fill up all the space available to them (this is independent of filesystem
209	quotas).  It also keeps the filesystem from filling up entirely which
210	helps combat fragmentation.
211	
212	Filesystem check
213	----------------
214	
215	At boot time, most systems run a consistency check (e2fsck) on their
216	filesystems.  The superblock of the ext2 filesystem contains several
217	fields which indicate whether fsck should actually run (since checking
218	the filesystem at boot can take a long time if it is large).  fsck will
219	run if the filesystem was not cleanly unmounted, if the maximum mount
220	count has been exceeded or if the maximum time between checks has been
221	exceeded.
222	
223	Feature Compatibility
224	---------------------
225	
226	The compatibility feature mechanism used in ext2 is sophisticated.
227	It safely allows features to be added to the filesystem, without
228	unnecessarily sacrificing compatibility with older versions of the
229	filesystem code.  The feature compatibility mechanism is not supported by
230	the original revision 0 (EXT2_GOOD_OLD_REV) of ext2, but was introduced in
231	revision 1.  There are three 32-bit fields, one for compatible features
232	(COMPAT), one for read-only compatible (RO_COMPAT) features and one for
233	incompatible (INCOMPAT) features.
234	
235	These feature flags have specific meanings for the kernel as follows:
236	
237	A COMPAT flag indicates that a feature is present in the filesystem,
238	but the on-disk format is 100% compatible with older on-disk formats, so
239	a kernel which didn't know anything about this feature could read/write
240	the filesystem without any chance of corrupting the filesystem (or even
241	making it inconsistent).  This is essentially just a flag which says
242	"this filesystem has a (hidden) feature" that the kernel or e2fsck may
243	want to be aware of (more on e2fsck and feature flags later).  The ext3
244	HAS_JOURNAL feature is a COMPAT flag because the ext3 journal is simply
245	a regular file with data blocks in it so the kernel does not need to
246	take any special notice of it if it doesn't understand ext3 journaling.
247	
248	An RO_COMPAT flag indicates that the on-disk format is 100% compatible
249	with older on-disk formats for reading (i.e. the feature does not change
250	the visible on-disk format).  However, an old kernel writing to such a
251	filesystem would/could corrupt the filesystem, so this is prevented. The
252	most common such feature, SPARSE_SUPER, is an RO_COMPAT feature because
253	sparse groups allow file data blocks where superblock/group descriptor
254	backups used to live, and ext2_free_blocks() refuses to free these blocks,
255	which would leading to inconsistent bitmaps.  An old kernel would also
256	get an error if it tried to free a series of blocks which crossed a group
257	boundary, but this is a legitimate layout in a SPARSE_SUPER filesystem.
258	
259	An INCOMPAT flag indicates the on-disk format has changed in some
260	way that makes it unreadable by older kernels, or would otherwise
261	cause a problem if an old kernel tried to mount it.  FILETYPE is an
262	INCOMPAT flag because older kernels would think a filename was longer
263	than 256 characters, which would lead to corrupt directory listings.
264	The COMPRESSION flag is an obvious INCOMPAT flag - if the kernel
265	doesn't understand compression, you would just get garbage back from
266	read() instead of it automatically decompressing your data.  The ext3
267	RECOVER flag is needed to prevent a kernel which does not understand the
268	ext3 journal from mounting the filesystem without replaying the journal.
269	
270	For e2fsck, it needs to be more strict with the handling of these
271	flags than the kernel.  If it doesn't understand ANY of the COMPAT,
272	RO_COMPAT, or INCOMPAT flags it will refuse to check the filesystem,
273	because it has no way of verifying whether a given feature is valid
274	or not.  Allowing e2fsck to succeed on a filesystem with an unknown
275	feature is a false sense of security for the user.  Refusing to check
276	a filesystem with unknown features is a good incentive for the user to
277	update to the latest e2fsck.  This also means that anyone adding feature
278	flags to ext2 also needs to update e2fsck to verify these features.
279	
280	Metadata
281	--------
282	
283	It is frequently claimed that the ext2 implementation of writing
284	asynchronous metadata is faster than the ffs synchronous metadata
285	scheme but less reliable.  Both methods are equally resolvable by their
286	respective fsck programs.
287	
288	If you're exceptionally paranoid, there are 3 ways of making metadata
289	writes synchronous on ext2:
290	
291	per-file if you have the program source: use the O_SYNC flag to open()
292	per-file if you don't have the source: use "chattr +S" on the file
293	per-filesystem: add the "sync" option to mount (or in /etc/fstab)
294	
295	the first and last are not ext2 specific but do force the metadata to
296	be written synchronously.  See also Journaling below.
297	
298	Limitations
299	-----------
300	
301	There are various limits imposed by the on-disk layout of ext2.  Other
302	limits are imposed by the current implementation of the kernel code.
303	Many of the limits are determined at the time the filesystem is first
304	created, and depend upon the block size chosen.  The ratio of inodes to
305	data blocks is fixed at filesystem creation time, so the only way to
306	increase the number of inodes is to increase the size of the filesystem.
307	No tools currently exist which can change the ratio of inodes to blocks.
308	
309	Most of these limits could be overcome with slight changes in the on-disk
310	format and using a compatibility flag to signal the format change (at
311	the expense of some compatibility).
312	
313	Filesystem block size:     1kB        2kB        4kB        8kB
314	
315	File size limit:          16GB      256GB     2048GB     2048GB
316	Filesystem size limit:  2047GB     8192GB    16384GB    32768GB
317	
318	There is a 2.4 kernel limit of 2048GB for a single block device, so no
319	filesystem larger than that can be created at this time.  There is also
320	an upper limit on the block size imposed by the page size of the kernel,
321	so 8kB blocks are only allowed on Alpha systems (and other architectures
322	which support larger pages).
323	
324	There is an upper limit of 32000 subdirectories in a single directory.
325	
326	There is a "soft" upper limit of about 10-15k files in a single directory
327	with the current linear linked-list directory implementation.  This limit
328	stems from performance problems when creating and deleting (and also
329	finding) files in such large directories.  Using a hashed directory index
330	(under development) allows 100k-1M+ files in a single directory without
331	performance problems (although RAM size becomes an issue at this point).
332	
333	The (meaningless) absolute upper limit of files in a single directory
334	(imposed by the file size, the realistic limit is obviously much less)
335	is over 130 trillion files.  It would be higher except there are not
336	enough 4-character names to make up unique directory entries, so they
337	have to be 8 character filenames, even then we are fairly close to
338	running out of unique filenames.
339	
340	Journaling
341	----------
342	
343	A journaling extension to the ext2 code has been developed by Stephen
344	Tweedie.  It avoids the risks of metadata corruption and the need to
345	wait for e2fsck to complete after a crash, without requiring a change
346	to the on-disk ext2 layout.  In a nutshell, the journal is a regular
347	file which stores whole metadata (and optionally data) blocks that have
348	been modified, prior to writing them into the filesystem.  This means
349	it is possible to add a journal to an existing ext2 filesystem without
350	the need for data conversion.
351	
352	When changes to the filesystem (e.g. a file is renamed) they are stored in
353	a transaction in the journal and can either be complete or incomplete at
354	the time of a crash.  If a transaction is complete at the time of a crash
355	(or in the normal case where the system does not crash), then any blocks
356	in that transaction are guaranteed to represent a valid filesystem state,
357	and are copied into the filesystem.  If a transaction is incomplete at
358	the time of the crash, then there is no guarantee of consistency for
359	the blocks in that transaction so they are discarded (which means any
360	filesystem changes they represent are also lost).
361	Check Documentation/filesystems/ext4.txt if you want to read more about
362	ext4 and journaling.
363	
364	References
365	==========
366	
367	The kernel source	file:/usr/src/linux/fs/ext2/
368	e2fsprogs (e2fsck)	http://e2fsprogs.sourceforge.net/
369	Design & Implementation	http://e2fsprogs.sourceforge.net/ext2intro.html
370	Journaling (ext3)	ftp://ftp.uk.linux.org/pub/linux/sct/fs/jfs/
371	Filesystem Resizing	http://ext2resize.sourceforge.net/
372	Compression (*)		http://e2compr.sourceforge.net/
373	
374	Implementations for:
375	Windows 95/98/NT/2000	http://www.chrysocome.net/explore2fs
376	Windows 95 (*)		http://www.yipton.net/content.html#FSDEXT2
377	DOS client (*)		ftp://metalab.unc.edu/pub/Linux/system/filesystems/ext2/
378	OS/2 (+)		ftp://metalab.unc.edu/pub/Linux/system/filesystems/ext2/
379	RISC OS client		http://www.esw-heim.tu-clausthal.de/~marco/smorbrod/IscaFS/
380	
381	(*) no longer actively developed/supported (as of Apr 2001)
382	(+) no longer actively developed/supported (as of Mar 2009)
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