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Dave Kleikampfc513a32006-10-11 01:21:25 -07001
2Ext4 Filesystem
3===============
4
5This is a development version of the ext4 filesystem, an advanced level
6of the ext3 filesystem which incorporates scalability and reliability
7enhancements for supporting large filesystems (64 bit) in keeping with
8increasing disk capacities and state-of-the-art feature requirements.
9
10Mailing list: linux-ext4@vger.kernel.org
11
12
131. Quick usage instructions:
14===========================
15
16 - Grab updated e2fsprogs from
17 ftp://ftp.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs-interim/
18 This is a patchset on top of e2fsprogs-1.39, which can be found at
19 ftp://ftp.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs/
20
21 - It's still mke2fs -j /dev/hda1
22
23 - mount /dev/hda1 /wherever -t ext4dev
24
25 - To enable extents,
26
27 mount /dev/hda1 /wherever -t ext4dev -o extents
28
29 - The filesystem is compatible with the ext3 driver until you add a file
30 which has extents (ie: `mount -o extents', then create a file).
31
32 NOTE: The "extents" mount flag is temporary. It will soon go away and
33 extents will be enabled by the "-o extents" flag to mke2fs or tune2fs
34
35 - When comparing performance with other filesystems, remember that
36 ext3/4 by default offers higher data integrity guarantees than most. So
37 when comparing with a metadata-only journalling filesystem, use `mount -o
38 data=writeback'. And you might as well use `mount -o nobh' too along
39 with it. Making the journal larger than the mke2fs default often helps
40 performance with metadata-intensive workloads.
41
422. Features
43===========
44
452.1 Currently available
46
47* ability to use filesystems > 16TB
48* extent format reduces metadata overhead (RAM, IO for access, transactions)
49* extent format more robust in face of on-disk corruption due to magics,
50* internal redunancy in tree
51
522.1 Previously available, soon to be enabled by default by "mkefs.ext4":
53
54* dir_index and resize inode will be on by default
55* large inodes will be used by default for fast EAs, nsec timestamps, etc
56
572.2 Candidate features for future inclusion
58
59There are several under discussion, whether they all make it in is
60partly a function of how much time everyone has to work on them:
61
62* improved file allocation (multi-block alloc, delayed alloc; basically done)
63* fix 32000 subdirectory limit (patch exists, needs some e2fsck work)
64* nsec timestamps for mtime, atime, ctime, create time (patch exists,
65 needs some e2fsck work)
66* inode version field on disk (NFSv4, Lustre; prototype exists)
67* reduced mke2fs/e2fsck time via uninitialized groups (prototype exists)
68* journal checksumming for robustness, performance (prototype exists)
69* persistent file preallocation (e.g for streaming media, databases)
70
71Features like metadata checksumming have been discussed and planned for
72a bit but no patches exist yet so I'm not sure they're in the near-term
73roadmap.
74
75The big performance win will come with mballoc and delalloc. CFS has
76been using mballoc for a few years already with Lustre, and IBM + Bull
77did a lot of benchmarking on it. The reason it isn't in the first set of
78patches is partly a manageability issue, and partly because it doesn't
79directly affect the on-disk format (outside of much better allocation)
80so it isn't critical to get into the first round of changes. I believe
81Alex is working on a new set of patches right now.
82
833. Options
84==========
85
86When mounting an ext4 filesystem, the following option are accepted:
87(*) == default
88
Alex Tomasc9de5602008-01-29 00:19:52 -050089extents (*) ext4 will use extents to address file data. The
Dave Kleikampfc513a32006-10-11 01:21:25 -070090 file system will no longer be mountable by ext3.
91
Alex Tomasc9de5602008-01-29 00:19:52 -050092noextents ext4 will not use extents for newly created files
93
Girish Shilamkar818d2762008-01-28 23:58:27 -050094journal_checksum Enable checksumming of the journal transactions.
95 This will allow the recovery code in e2fsck and the
96 kernel to detect corruption in the kernel. It is a
97 compatible change and will be ignored by older kernels.
98
99journal_async_commit Commit block can be written to disk without waiting
100 for descriptor blocks. If enabled older kernels cannot
101 mount the device. This will enable 'journal_checksum'
102 internally.
103
Dave Kleikampfc513a32006-10-11 01:21:25 -0700104journal=update Update the ext4 file system's journal to the current
105 format.
106
107journal=inum When a journal already exists, this option is ignored.
108 Otherwise, it specifies the number of the inode which
109 will represent the ext4 file system's journal file.
110
111journal_dev=devnum When the external journal device's major/minor numbers
112 have changed, this option allows the user to specify
113 the new journal location. The journal device is
114 identified through its new major/minor numbers encoded
115 in devnum.
116
117noload Don't load the journal on mounting.
118
119data=journal All data are committed into the journal prior to being
120 written into the main file system.
121
122data=ordered (*) All data are forced directly out to the main file
123 system prior to its metadata being committed to the
124 journal.
125
126data=writeback Data ordering is not preserved, data may be written
127 into the main file system after its metadata has been
128 committed to the journal.
129
130commit=nrsec (*) Ext4 can be told to sync all its data and metadata
131 every 'nrsec' seconds. The default value is 5 seconds.
132 This means that if you lose your power, you will lose
133 as much as the latest 5 seconds of work (your
134 filesystem will not be damaged though, thanks to the
135 journaling). This default value (or any low value)
136 will hurt performance, but it's good for data-safety.
137 Setting it to 0 will have the same effect as leaving
138 it at the default (5 seconds).
139 Setting it to very large values will improve
140 performance.
141
142barrier=1 This enables/disables barriers. barrier=0 disables
143 it, barrier=1 enables it.
144
145orlov (*) This enables the new Orlov block allocator. It is
146 enabled by default.
147
148oldalloc This disables the Orlov block allocator and enables
149 the old block allocator. Orlov should have better
150 performance - we'd like to get some feedback if it's
151 the contrary for you.
152
153user_xattr Enables Extended User Attributes. Additionally, you
154 need to have extended attribute support enabled in the
155 kernel configuration (CONFIG_EXT4_FS_XATTR). See the
156 attr(5) manual page and http://acl.bestbits.at/ to
157 learn more about extended attributes.
158
159nouser_xattr Disables Extended User Attributes.
160
161acl Enables POSIX Access Control Lists support.
162 Additionally, you need to have ACL support enabled in
163 the kernel configuration (CONFIG_EXT4_FS_POSIX_ACL).
164 See the acl(5) manual page and http://acl.bestbits.at/
165 for more information.
166
167noacl This option disables POSIX Access Control List
168 support.
169
170reservation
171
172noreservation
173
174bsddf (*) Make 'df' act like BSD.
175minixdf Make 'df' act like Minix.
176
177check=none Don't do extra checking of bitmaps on mount.
178nocheck
179
180debug Extra debugging information is sent to syslog.
181
182errors=remount-ro(*) Remount the filesystem read-only on an error.
183errors=continue Keep going on a filesystem error.
184errors=panic Panic and halt the machine if an error occurs.
185
186grpid Give objects the same group ID as their creator.
187bsdgroups
188
189nogrpid (*) New objects have the group ID of their creator.
190sysvgroups
191
192resgid=n The group ID which may use the reserved blocks.
193
194resuid=n The user ID which may use the reserved blocks.
195
196sb=n Use alternate superblock at this location.
197
198quota
199noquota
200grpquota
201usrquota
202
203bh (*) ext4 associates buffer heads to data pages to
204nobh (a) cache disk block mapping information
205 (b) link pages into transaction to provide
206 ordering guarantees.
207 "bh" option forces use of buffer heads.
208 "nobh" option tries to avoid associating buffer
209 heads (supported only for "writeback" mode).
210
Alex Tomasc9de5602008-01-29 00:19:52 -0500211mballoc (*) Use the multiple block allocator for block allocation
212nomballoc disabled multiple block allocator for block allocation.
213stripe=n Number of filesystem blocks that mballoc will try
214 to use for allocation size and alignment. For RAID5/6
215 systems this should be the number of data
216 disks * RAID chunk size in file system blocks.
Dave Kleikampfc513a32006-10-11 01:21:25 -0700217
218Data Mode
219---------
220There are 3 different data modes:
221
222* writeback mode
223In data=writeback mode, ext4 does not journal data at all. This mode provides
224a similar level of journaling as that of XFS, JFS, and ReiserFS in its default
225mode - metadata journaling. A crash+recovery can cause incorrect data to
226appear in files which were written shortly before the crash. This mode will
227typically provide the best ext4 performance.
228
229* ordered mode
230In data=ordered mode, ext4 only officially journals metadata, but it logically
231groups metadata and data blocks into a single unit called a transaction. When
232it's time to write the new metadata out to disk, the associated data blocks
233are written first. In general, this mode performs slightly slower than
234writeback but significantly faster than journal mode.
235
236* journal mode
237data=journal mode provides full data and metadata journaling. All new data is
238written to the journal first, and then to its final location.
239In the event of a crash, the journal can be replayed, bringing both data and
240metadata into a consistent state. This mode is the slowest except when data
241needs to be read from and written to disk at the same time where it
242outperforms all others modes.
243
244References
245==========
246
247kernel source: <file:fs/ext4/>
248 <file:fs/jbd2/>
249
250programs: http://e2fsprogs.sourceforge.net/
251 http://ext2resize.sourceforge.net
252
253useful links: http://fedoraproject.org/wiki/ext3-devel
254 http://www.bullopensource.org/ext4/