5 This is a development version of the ext4 filesystem, an advanced level
6 of the ext3 filesystem which incorporates scalability and reliability
7 enhancements for supporting large filesystems (64 bit) in keeping with
8 increasing disk capacities and state-of-the-art feature requirements.
10 Mailing list: linux-ext4@vger.kernel.org
13 1. Quick usage instructions:
14 ===========================
16 - Compile and install the latest version of e2fsprogs (as of this
17 writing version 1.41) from:
19 http://sourceforge.net/project/showfiles.php?group_id=2406
23 ftp://ftp.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs/
25 or grab the latest git repository from:
27 git://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git
29 - Create a new filesystem using the ext4dev filesystem type:
31 # mke2fs -t ext4dev /dev/hda1
33 Or configure an existing ext3 filesystem to support extents and set
34 the test_fs flag to indicate that it's ok for an in-development
35 filesystem to touch this filesystem:
37 # tune2fs -O extents -E test_fs /dev/hda1
39 If the filesystem was created with 128 byte inodes, it can be
40 converted to use 256 byte for greater efficiency via:
42 # tune2fs -I 256 /dev/hda1
44 (Note: we currently do not have tools to convert an ext4dev
45 filesystem back to ext3; so please do not do try this on production
50 # mount -t ext4dev /dev/hda1 /wherever
52 - When comparing performance with other filesystems, remember that
53 ext3/4 by default offers higher data integrity guarantees than most.
54 So when comparing with a metadata-only journalling filesystem, such
55 as ext3, use `mount -o data=writeback'. And you might as well use
56 `mount -o nobh' too along with it. Making the journal larger than
57 the mke2fs default often helps performance with metadata-intensive
63 2.1 Currently available
65 * ability to use filesystems > 16TB (e2fsprogs support not available yet)
66 * extent format reduces metadata overhead (RAM, IO for access, transactions)
67 * extent format more robust in face of on-disk corruption due to magics,
68 * internal redunancy in tree
69 * improved file allocation (multi-block alloc, delayed alloc)
70 * fix 32000 subdirectory limit
71 * nsec timestamps for mtime, atime, ctime, create time
72 * inode version field on disk (NFSv4, Lustre)
73 * reduced e2fsck time via uninit_bg feature
74 * journal checksumming for robustness, performance
75 * persistent file preallocation (e.g for streaming media, databases)
76 * ability to pack bitmaps and inode tables into larger virtual groups via the
79 * Inode allocation using large virtual block groups via flex_bg
81 2.2 Candidate features for future inclusion
83 * Online defrag (patches available but not well tested)
84 * reduced mke2fs time via lazy itable initialization in conjuction with
85 the uninit_bg feature (capability to do this is available in e2fsprogs
86 but a kernel thread to do lazy zeroing of unused inode table blocks
87 after filesystem is first mounted is required for safety)
89 There are several others under discussion, whether they all make it in is
90 partly a function of how much time everyone has to work on them. Features like
91 metadata checksumming have been discussed and planned for a bit but no patches
92 exist yet so I'm not sure they're in the near-term roadmap.
94 The big performance win will come with mballoc, delalloc and flex_bg
95 grouping of bitmaps and inode tables. Some test results available here:
97 - http://www.bullopensource.org/ext4/20080530/ffsb-write-2.6.26-rc2.html
98 - http://www.bullopensource.org/ext4/20080530/ffsb-readwrite-2.6.26-rc2.html
103 When mounting an ext4 filesystem, the following option are accepted:
106 extents (*) ext4 will use extents to address file data. The
107 file system will no longer be mountable by ext3.
109 noextents ext4 will not use extents for newly created files
111 journal_checksum Enable checksumming of the journal transactions.
112 This will allow the recovery code in e2fsck and the
113 kernel to detect corruption in the kernel. It is a
114 compatible change and will be ignored by older kernels.
116 journal_async_commit Commit block can be written to disk without waiting
117 for descriptor blocks. If enabled older kernels cannot
118 mount the device. This will enable 'journal_checksum'
121 journal=update Update the ext4 file system's journal to the current
124 journal=inum When a journal already exists, this option is ignored.
125 Otherwise, it specifies the number of the inode which
126 will represent the ext4 file system's journal file.
128 journal_dev=devnum When the external journal device's major/minor numbers
129 have changed, this option allows the user to specify
130 the new journal location. The journal device is
131 identified through its new major/minor numbers encoded
134 noload Don't load the journal on mounting.
136 data=journal All data are committed into the journal prior to being
137 written into the main file system.
139 data=ordered (*) All data are forced directly out to the main file
140 system prior to its metadata being committed to the
143 data=writeback Data ordering is not preserved, data may be written
144 into the main file system after its metadata has been
145 committed to the journal.
147 commit=nrsec (*) Ext4 can be told to sync all its data and metadata
148 every 'nrsec' seconds. The default value is 5 seconds.
149 This means that if you lose your power, you will lose
150 as much as the latest 5 seconds of work (your
151 filesystem will not be damaged though, thanks to the
152 journaling). This default value (or any low value)
153 will hurt performance, but it's good for data-safety.
154 Setting it to 0 will have the same effect as leaving
155 it at the default (5 seconds).
156 Setting it to very large values will improve
159 barrier=<0|1(*)> This enables/disables the use of write barriers in
160 the jbd code. barrier=0 disables, barrier=1 enables.
161 This also requires an IO stack which can support
162 barriers, and if jbd gets an error on a barrier
163 write, it will disable again with a warning.
164 Write barriers enforce proper on-disk ordering
165 of journal commits, making volatile disk write caches
166 safe to use, at some performance penalty. If
167 your disks are battery-backed in one way or another,
168 disabling barriers may safely improve performance.
170 orlov (*) This enables the new Orlov block allocator. It is
173 oldalloc This disables the Orlov block allocator and enables
174 the old block allocator. Orlov should have better
175 performance - we'd like to get some feedback if it's
176 the contrary for you.
178 user_xattr Enables Extended User Attributes. Additionally, you
179 need to have extended attribute support enabled in the
180 kernel configuration (CONFIG_EXT4_FS_XATTR). See the
181 attr(5) manual page and http://acl.bestbits.at/ to
182 learn more about extended attributes.
184 nouser_xattr Disables Extended User Attributes.
186 acl Enables POSIX Access Control Lists support.
187 Additionally, you need to have ACL support enabled in
188 the kernel configuration (CONFIG_EXT4_FS_POSIX_ACL).
189 See the acl(5) manual page and http://acl.bestbits.at/
190 for more information.
192 noacl This option disables POSIX Access Control List
199 bsddf (*) Make 'df' act like BSD.
200 minixdf Make 'df' act like Minix.
202 check=none Don't do extra checking of bitmaps on mount.
205 debug Extra debugging information is sent to syslog.
207 errors=remount-ro(*) Remount the filesystem read-only on an error.
208 errors=continue Keep going on a filesystem error.
209 errors=panic Panic and halt the machine if an error occurs.
211 grpid Give objects the same group ID as their creator.
214 nogrpid (*) New objects have the group ID of their creator.
217 resgid=n The group ID which may use the reserved blocks.
219 resuid=n The user ID which may use the reserved blocks.
221 sb=n Use alternate superblock at this location.
228 bh (*) ext4 associates buffer heads to data pages to
229 nobh (a) cache disk block mapping information
230 (b) link pages into transaction to provide
232 "bh" option forces use of buffer heads.
233 "nobh" option tries to avoid associating buffer
234 heads (supported only for "writeback" mode).
236 mballoc (*) Use the multiple block allocator for block allocation
237 nomballoc disabled multiple block allocator for block allocation.
238 stripe=n Number of filesystem blocks that mballoc will try
239 to use for allocation size and alignment. For RAID5/6
240 systems this should be the number of data
241 disks * RAID chunk size in file system blocks.
245 There are 3 different data modes:
248 In data=writeback mode, ext4 does not journal data at all. This mode provides
249 a similar level of journaling as that of XFS, JFS, and ReiserFS in its default
250 mode - metadata journaling. A crash+recovery can cause incorrect data to
251 appear in files which were written shortly before the crash. This mode will
252 typically provide the best ext4 performance.
255 In data=ordered mode, ext4 only officially journals metadata, but it logically
256 groups metadata and data blocks into a single unit called a transaction. When
257 it's time to write the new metadata out to disk, the associated data blocks
258 are written first. In general, this mode performs slightly slower than
259 writeback but significantly faster than journal mode.
262 data=journal mode provides full data and metadata journaling. All new data is
263 written to the journal first, and then to its final location.
264 In the event of a crash, the journal can be replayed, bringing both data and
265 metadata into a consistent state. This mode is the slowest except when data
266 needs to be read from and written to disk at the same time where it
267 outperforms all others modes.
272 kernel source: <file:fs/ext4/>
275 programs: http://e2fsprogs.sourceforge.net/
277 useful links: http://fedoraproject.org/wiki/ext3-devel
278 http://www.bullopensource.org/ext4/
279 http://ext4.wiki.kernel.org/index.php/Main_Page
280 http://fedoraproject.org/wiki/Features/Ext4