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 - 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/
21 - It's still mke2fs -j /dev/hda1
23 - mount /dev/hda1 /wherever -t ext4dev
27 mount /dev/hda1 /wherever -t ext4dev -o extents
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).
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
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.
45 2.1 Currently available
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
52 2.1 Previously available, soon to be enabled by default by "mkefs.ext4":
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
57 2.2 Candidate features for future inclusion
59 There are several under discussion, whether they all make it in is
60 partly a function of how much time everyone has to work on them:
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)
71 Features like metadata checksumming have been discussed and planned for
72 a bit but no patches exist yet so I'm not sure they're in the near-term
75 The big performance win will come with mballoc and delalloc. CFS has
76 been using mballoc for a few years already with Lustre, and IBM + Bull
77 did a lot of benchmarking on it. The reason it isn't in the first set of
78 patches is partly a manageability issue, and partly because it doesn't
79 directly affect the on-disk format (outside of much better allocation)
80 so it isn't critical to get into the first round of changes. I believe
81 Alex is working on a new set of patches right now.
86 When mounting an ext4 filesystem, the following option are accepted:
89 extents ext4 will use extents to address file data. The
90 file system will no longer be mountable by ext3.
92 journal_checksum Enable checksumming of the journal transactions.
93 This will allow the recovery code in e2fsck and the
94 kernel to detect corruption in the kernel. It is a
95 compatible change and will be ignored by older kernels.
97 journal_async_commit Commit block can be written to disk without waiting
98 for descriptor blocks. If enabled older kernels cannot
99 mount the device. This will enable 'journal_checksum'
102 journal=update Update the ext4 file system's journal to the current
105 journal=inum When a journal already exists, this option is ignored.
106 Otherwise, it specifies the number of the inode which
107 will represent the ext4 file system's journal file.
109 journal_dev=devnum When the external journal device's major/minor numbers
110 have changed, this option allows the user to specify
111 the new journal location. The journal device is
112 identified through its new major/minor numbers encoded
115 noload Don't load the journal on mounting.
117 data=journal All data are committed into the journal prior to being
118 written into the main file system.
120 data=ordered (*) All data are forced directly out to the main file
121 system prior to its metadata being committed to the
124 data=writeback Data ordering is not preserved, data may be written
125 into the main file system after its metadata has been
126 committed to the journal.
128 commit=nrsec (*) Ext4 can be told to sync all its data and metadata
129 every 'nrsec' seconds. The default value is 5 seconds.
130 This means that if you lose your power, you will lose
131 as much as the latest 5 seconds of work (your
132 filesystem will not be damaged though, thanks to the
133 journaling). This default value (or any low value)
134 will hurt performance, but it's good for data-safety.
135 Setting it to 0 will have the same effect as leaving
136 it at the default (5 seconds).
137 Setting it to very large values will improve
140 barrier=1 This enables/disables barriers. barrier=0 disables
141 it, barrier=1 enables it.
143 orlov (*) This enables the new Orlov block allocator. It is
146 oldalloc This disables the Orlov block allocator and enables
147 the old block allocator. Orlov should have better
148 performance - we'd like to get some feedback if it's
149 the contrary for you.
151 user_xattr Enables Extended User Attributes. Additionally, you
152 need to have extended attribute support enabled in the
153 kernel configuration (CONFIG_EXT4_FS_XATTR). See the
154 attr(5) manual page and http://acl.bestbits.at/ to
155 learn more about extended attributes.
157 nouser_xattr Disables Extended User Attributes.
159 acl Enables POSIX Access Control Lists support.
160 Additionally, you need to have ACL support enabled in
161 the kernel configuration (CONFIG_EXT4_FS_POSIX_ACL).
162 See the acl(5) manual page and http://acl.bestbits.at/
163 for more information.
165 noacl This option disables POSIX Access Control List
172 bsddf (*) Make 'df' act like BSD.
173 minixdf Make 'df' act like Minix.
175 check=none Don't do extra checking of bitmaps on mount.
178 debug Extra debugging information is sent to syslog.
180 errors=remount-ro(*) Remount the filesystem read-only on an error.
181 errors=continue Keep going on a filesystem error.
182 errors=panic Panic and halt the machine if an error occurs.
184 grpid Give objects the same group ID as their creator.
187 nogrpid (*) New objects have the group ID of their creator.
190 resgid=n The group ID which may use the reserved blocks.
192 resuid=n The user ID which may use the reserved blocks.
194 sb=n Use alternate superblock at this location.
201 bh (*) ext4 associates buffer heads to data pages to
202 nobh (a) cache disk block mapping information
203 (b) link pages into transaction to provide
205 "bh" option forces use of buffer heads.
206 "nobh" option tries to avoid associating buffer
207 heads (supported only for "writeback" mode).
212 There are 3 different data modes:
215 In data=writeback mode, ext4 does not journal data at all. This mode provides
216 a similar level of journaling as that of XFS, JFS, and ReiserFS in its default
217 mode - metadata journaling. A crash+recovery can cause incorrect data to
218 appear in files which were written shortly before the crash. This mode will
219 typically provide the best ext4 performance.
222 In data=ordered mode, ext4 only officially journals metadata, but it logically
223 groups metadata and data blocks into a single unit called a transaction. When
224 it's time to write the new metadata out to disk, the associated data blocks
225 are written first. In general, this mode performs slightly slower than
226 writeback but significantly faster than journal mode.
229 data=journal mode provides full data and metadata journaling. All new data is
230 written to the journal first, and then to its final location.
231 In the event of a crash, the journal can be replayed, bringing both data and
232 metadata into a consistent state. This mode is the slowest except when data
233 needs to be read from and written to disk at the same time where it
234 outperforms all others modes.
239 kernel source: <file:fs/ext4/>
242 programs: http://e2fsprogs.sourceforge.net/
243 http://ext2resize.sourceforge.net
245 useful links: http://fedoraproject.org/wiki/ext3-devel
246 http://www.bullopensource.org/ext4/