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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)
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
23dax Use direct access (no page cache). See
24 Documentation/filesystems/dax.txt.
25
26debug Extra debugging information is sent to the
27 kernel syslog. Useful for developers.
28
29errors=continue Keep going on a filesystem error.
30errors=remount-ro Remount the filesystem read-only on an error.
31errors=panic Panic and halt the machine if an error occurs.
32
33grpid, bsdgroups Give objects the same group ID as their parent.
34nogrpid, sysvgroups New objects have the group ID of their creator.
35
36nouid32 Use 16-bit UIDs and GIDs.
37
38oldalloc 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.
41orlov (*) Use the Orlov block allocator.
42 (See http://lwn.net/Articles/14633/ and
43 http://lwn.net/Articles/14446/.)
44
45resuid=n The user ID which may use the reserved blocks.
46resgid=n The group ID which may use the reserved blocks.
47
48sb=n Use alternate superblock at this location.
49
50user_xattr Enable "user." POSIX Extended Attributes
51 (requires CONFIG_EXT2_FS_XATTR).
52nouser_xattr Don't support "user." extended attributes.
53
54acl Enable POSIX Access Control Lists support
55 (requires CONFIG_EXT2_FS_POSIX_ACL).
56noacl Don't support POSIX ACLs.
57
58nobh Do not attach buffer_heads to file pagecache.
59
60grpquota,noquota,quota,usrquota Quota options are silently ignored by ext2.
61
62
63Specification
64=============
65
66ext2 shares many properties with traditional Unix filesystems. It has
67the concepts of blocks, inodes and directories. It has space in the
68specification for Access Control Lists (ACLs), fragments, undeletion and
69compression though these are not yet implemented (some are available as
70separate patches). There is also a versioning mechanism to allow new
71features (such as journalling) to be added in a maximally compatible
72manner.
73
74Blocks
75------
76
77The space in the device or file is split up into blocks. These are
78a fixed size, of 1024, 2048 or 4096 bytes (8192 bytes on Alpha systems),
79which is decided when the filesystem is created. Smaller blocks mean
80less wasted space per file, but require slightly more accounting overhead,
81and also impose other limits on the size of files and the filesystem.
82
83Block Groups
84------------
85
86Blocks are clustered into block groups in order to reduce fragmentation
87and minimise the amount of head seeking when reading a large amount
88of consecutive data. Information about each block group is kept in a
89descriptor table stored in the block(s) immediately after the superblock.
90Two blocks near the start of each group are reserved for the block usage
91bitmap and the inode usage bitmap which show which blocks and inodes
92are in use. Since each bitmap is limited to a single block, this means
93that the maximum size of a block group is 8 times the size of a block.
94
95The block(s) following the bitmaps in each block group are designated
96as the inode table for that block group and the remainder are the data
97blocks. The block allocation algorithm attempts to allocate data blocks
98in the same block group as the inode which contains them.
99
100The Superblock
101--------------
102
103The superblock contains all the information about the configuration of
104the filing system. The primary copy of the superblock is stored at an
105offset of 1024 bytes from the start of the device, and it is essential
106to mounting the filesystem. Since it is so important, backup copies of
107the superblock are stored in block groups throughout the filesystem.
108The first version of ext2 (revision 0) stores a copy at the start of
109every block group, along with backups of the group descriptor block(s).
110Because this can consume a considerable amount of space for large
111filesystems, later revisions can optionally reduce the number of backup
112copies by only putting backups in specific groups (this is the sparse
113superblock feature). The groups chosen are 0, 1 and powers of 3, 5 and 7.
114
115The information in the superblock contains fields such as the total
116number of inodes and blocks in the filesystem and how many are free,
117how many inodes and blocks are in each block group, when the filesystem
118was mounted (and if it was cleanly unmounted), when it was modified,
119what version of the filesystem it is (see the Revisions section below)
120and which OS created it.
121
122If the filesystem is revision 1 or higher, then there are extra fields,
123such as a volume name, a unique identification number, the inode size,
124and space for optional filesystem features to store configuration info.
125
126All fields in the superblock (as in all other ext2 structures) are stored
127on the disc in little endian format, so a filesystem is portable between
128machines without having to know what machine it was created on.
129
130Inodes
131------
132
133The inode (index node) is a fundamental concept in the ext2 filesystem.
134Each object in the filesystem is represented by an inode. The inode
135structure contains pointers to the filesystem blocks which contain the
136data held in the object and all of the metadata about an object except
137its name. The metadata about an object includes the permissions, owner,
138group, flags, size, number of blocks used, access time, change time,
139modification time, deletion time, number of links, fragments, version
140(for NFS) and extended attributes (EAs) and/or Access Control Lists (ACLs).
141
142There are some reserved fields which are currently unused in the inode
143structure and several which are overloaded. One field is reserved for the
144directory ACL if the inode is a directory and alternately for the top 32
145bits of the file size if the inode is a regular file (allowing file sizes
146larger than 2GB). The translator field is unused under Linux, but is used
147by the HURD to reference the inode of a program which will be used to
148interpret this object. Most of the remaining reserved fields have been
149used up for both Linux and the HURD for larger owner and group fields,
150The HURD also has a larger mode field so it uses another of the remaining
151fields to store the extra more bits.
152
153There are pointers to the first 12 blocks which contain the file's data
154in the inode. There is a pointer to an indirect block (which contains
155pointers to the next set of blocks), a pointer to a doubly-indirect
156block (which contains pointers to indirect blocks) and a pointer to a
157trebly-indirect block (which contains pointers to doubly-indirect blocks).
158
159The flags field contains some ext2-specific flags which aren't catered
160for by the standard chmod flags. These flags can be listed with lsattr
161and changed with the chattr command, and allow specific filesystem
162behaviour on a per-file basis. There are flags for secure deletion,
163undeletable, compression, synchronous updates, immutability, append-only,
164dumpable, no-atime, indexed directories, and data-journaling. Not all
165of these are supported yet.
166
167Directories
168-----------
169
170A directory is a filesystem object and has an inode just like a file.
171It is a specially formatted file containing records which associate
172each name with an inode number. Later revisions of the filesystem also
173encode the type of the object (file, directory, symlink, device, fifo,
174socket) 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
176Glibc 2.2).
177
178The inode allocation code tries to assign inodes which are in the same
179block group as the directory in which they are first created.
180
181The current implementation of ext2 uses a singly-linked list to store
182the filenames in the directory; a pending enhancement uses hashing of the
183filenames to allow lookup without the need to scan the entire directory.
184
185The current implementation never removes empty directory blocks once they
186have been allocated to hold more files.
187
188Special files
189-------------
190
191Symbolic links are also filesystem objects with inodes. They deserve
192special mention because the data for them is stored within the inode
193itself if the symlink is less than 60 bytes long. It uses the fields
194which would normally be used to store the pointers to data blocks.
195This is a worthwhile optimisation as it we avoid allocating a full
196block for the symlink, and most symlinks are less than 60 characters long.
197
198Character and block special devices never have data blocks assigned to
199them. Instead, their device number is stored in the inode, again reusing
200the fields which would be used to point to the data blocks.
201
202Reserved Space
203--------------
204
205In ext2, there is a mechanism for reserving a certain number of blocks
206for a particular user (normally the super-user). This is intended to
207allow for the system to continue functioning even if non-privileged users
208fill up all the space available to them (this is independent of filesystem
209quotas). It also keeps the filesystem from filling up entirely which
210helps combat fragmentation.
211
212Filesystem check
213----------------
214
215At boot time, most systems run a consistency check (e2fsck) on their
216filesystems. The superblock of the ext2 filesystem contains several
217fields which indicate whether fsck should actually run (since checking
218the filesystem at boot can take a long time if it is large). fsck will
219run if the filesystem was not cleanly unmounted, if the maximum mount
220count has been exceeded or if the maximum time between checks has been
221exceeded.
222
223Feature Compatibility
224---------------------
225
226The compatibility feature mechanism used in ext2 is sophisticated.
227It safely allows features to be added to the filesystem, without
228unnecessarily sacrificing compatibility with older versions of the
229filesystem code. The feature compatibility mechanism is not supported by
230the original revision 0 (EXT2_GOOD_OLD_REV) of ext2, but was introduced in
231revision 1. There are three 32-bit fields, one for compatible features
232(COMPAT), one for read-only compatible (RO_COMPAT) features and one for
233incompatible (INCOMPAT) features.
234
235These feature flags have specific meanings for the kernel as follows:
236
237A COMPAT flag indicates that a feature is present in the filesystem,
238but the on-disk format is 100% compatible with older on-disk formats, so
239a kernel which didn't know anything about this feature could read/write
240the filesystem without any chance of corrupting the filesystem (or even
241making it inconsistent). This is essentially just a flag which says
242"this filesystem has a (hidden) feature" that the kernel or e2fsck may
243want to be aware of (more on e2fsck and feature flags later). The ext3
244HAS_JOURNAL feature is a COMPAT flag because the ext3 journal is simply
245a regular file with data blocks in it so the kernel does not need to
246take any special notice of it if it doesn't understand ext3 journaling.
247
248An RO_COMPAT flag indicates that the on-disk format is 100% compatible
249with older on-disk formats for reading (i.e. the feature does not change
250the visible on-disk format). However, an old kernel writing to such a
251filesystem would/could corrupt the filesystem, so this is prevented. The
252most common such feature, SPARSE_SUPER, is an RO_COMPAT feature because
253sparse groups allow file data blocks where superblock/group descriptor
254backups used to live, and ext2_free_blocks() refuses to free these blocks,
255which would leading to inconsistent bitmaps. An old kernel would also
256get an error if it tried to free a series of blocks which crossed a group
257boundary, but this is a legitimate layout in a SPARSE_SUPER filesystem.
258
259An INCOMPAT flag indicates the on-disk format has changed in some
260way that makes it unreadable by older kernels, or would otherwise
261cause a problem if an old kernel tried to mount it. FILETYPE is an
262INCOMPAT flag because older kernels would think a filename was longer
263than 256 characters, which would lead to corrupt directory listings.
264The COMPRESSION flag is an obvious INCOMPAT flag - if the kernel
265doesn't understand compression, you would just get garbage back from
266read() instead of it automatically decompressing your data. The ext3
267RECOVER flag is needed to prevent a kernel which does not understand the
268ext3 journal from mounting the filesystem without replaying the journal.
269
270For e2fsck, it needs to be more strict with the handling of these
271flags than the kernel. If it doesn't understand ANY of the COMPAT,
272RO_COMPAT, or INCOMPAT flags it will refuse to check the filesystem,
273because it has no way of verifying whether a given feature is valid
274or not. Allowing e2fsck to succeed on a filesystem with an unknown
275feature is a false sense of security for the user. Refusing to check
276a filesystem with unknown features is a good incentive for the user to
277update to the latest e2fsck. This also means that anyone adding feature
278flags to ext2 also needs to update e2fsck to verify these features.
279
280Metadata
281--------
282
283It is frequently claimed that the ext2 implementation of writing
284asynchronous metadata is faster than the ffs synchronous metadata
285scheme but less reliable. Both methods are equally resolvable by their
286respective fsck programs.
287
288If you're exceptionally paranoid, there are 3 ways of making metadata
289writes synchronous on ext2:
290
291per-file if you have the program source: use the O_SYNC flag to open()
292per-file if you don't have the source: use "chattr +S" on the file
293per-filesystem: add the "sync" option to mount (or in /etc/fstab)
294
295the first and last are not ext2 specific but do force the metadata to
296be written synchronously. See also Journaling below.
297
298Limitations
299-----------
300
301There are various limits imposed by the on-disk layout of ext2. Other
302limits are imposed by the current implementation of the kernel code.
303Many of the limits are determined at the time the filesystem is first
304created, and depend upon the block size chosen. The ratio of inodes to
305data blocks is fixed at filesystem creation time, so the only way to
306increase the number of inodes is to increase the size of the filesystem.
307No tools currently exist which can change the ratio of inodes to blocks.
308
309Most of these limits could be overcome with slight changes in the on-disk
310format and using a compatibility flag to signal the format change (at
311the expense of some compatibility).
312
313Filesystem block size: 1kB 2kB 4kB 8kB
314
315File size limit: 16GB 256GB 2048GB 2048GB
316Filesystem size limit: 2047GB 8192GB 16384GB 32768GB
317
318There is a 2.4 kernel limit of 2048GB for a single block device, so no
319filesystem larger than that can be created at this time. There is also
320an upper limit on the block size imposed by the page size of the kernel,
321so 8kB blocks are only allowed on Alpha systems (and other architectures
322which support larger pages).
323
324There is an upper limit of 32000 subdirectories in a single directory.
325
326There is a "soft" upper limit of about 10-15k files in a single directory
327with the current linear linked-list directory implementation. This limit
328stems from performance problems when creating and deleting (and also
329finding) files in such large directories. Using a hashed directory index
330(under development) allows 100k-1M+ files in a single directory without
331performance problems (although RAM size becomes an issue at this point).
332
333The (meaningless) absolute upper limit of files in a single directory
334(imposed by the file size, the realistic limit is obviously much less)
335is over 130 trillion files. It would be higher except there are not
336enough 4-character names to make up unique directory entries, so they
337have to be 8 character filenames, even then we are fairly close to
338running out of unique filenames.
339
340Journaling
341----------
342
343A journaling extension to the ext2 code has been developed by Stephen
344Tweedie. It avoids the risks of metadata corruption and the need to
345wait for e2fsck to complete after a crash, without requiring a change
346to the on-disk ext2 layout. In a nutshell, the journal is a regular
347file which stores whole metadata (and optionally data) blocks that have
348been modified, prior to writing them into the filesystem. This means
349it is possible to add a journal to an existing ext2 filesystem without
350the need for data conversion.
351
352When changes to the filesystem (e.g. a file is renamed) they are stored in
353a transaction in the journal and can either be complete or incomplete at
354the 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
356in that transaction are guaranteed to represent a valid filesystem state,
357and are copied into the filesystem. If a transaction is incomplete at
358the time of the crash, then there is no guarantee of consistency for
359the blocks in that transaction so they are discarded (which means any
360filesystem changes they represent are also lost).
361Check Documentation/filesystems/ext4.txt if you want to read more about
362ext4 and journaling.
363
364References
365==========
366
367The kernel source file:/usr/src/linux/fs/ext2/
368e2fsprogs (e2fsck) http://e2fsprogs.sourceforge.net/
369Design & Implementation http://e2fsprogs.sourceforge.net/ext2intro.html
370Journaling (ext3) ftp://ftp.uk.linux.org/pub/linux/sct/fs/jfs/
371Filesystem Resizing http://ext2resize.sourceforge.net/
372Compression (*) http://e2compr.sourceforge.net/
373
374Implementations for:
375Windows 95/98/NT/2000 http://www.chrysocome.net/explore2fs
376Windows 95 (*) http://www.yipton.net/content.html#FSDEXT2
377DOS client (*) ftp://metalab.unc.edu/pub/Linux/system/filesystems/ext2/
378OS/2 (+) ftp://metalab.unc.edu/pub/Linux/system/filesystems/ext2/
379RISC 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)