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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) |
384 |
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