Based on kernel version 2.6.26. Page generated on 2008-07-16 21:12 EST.
1 2 Ext4 Filesystem 3 =============== 4 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. 9 10 Mailing list: linux-ext4[AT]vger.kernel[DOT]org 11 12 13 1. 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 42 2. Features 43 =========== 44 45 2.1 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 52 2.1 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 57 2.2 Candidate features for future inclusion 58 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: 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 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 73 roadmap. 74 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. 82 83 3. Options 84 ========== 85 86 When mounting an ext4 filesystem, the following option are accepted: 87 (*) == default 88 89 extents (*) ext4 will use extents to address file data. The 90 file system will no longer be mountable by ext3. 91 92 noextents ext4 will not use extents for newly created files 93 94 journal_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 99 journal_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. 103 104 journal=update Update the ext4 file system's journal to the current 105 format. 106 107 journal=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. 110 111 journal_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. 116 117 noload Don't load the journal on mounting. 118 119 data=journal All data are committed into the journal prior to being 120 written into the main file system. 121 122 data=ordered (*) All data are forced directly out to the main file 123 system prior to its metadata being committed to the 124 journal. 125 126 data=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. 129 130 commit=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. 141 142 barrier=1 This enables/disables barriers. barrier=0 disables 143 it, barrier=1 enables it. 144 145 orlov (*) This enables the new Orlov block allocator. It is 146 enabled by default. 147 148 oldalloc 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. 152 153 user_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. 158 159 nouser_xattr Disables Extended User Attributes. 160 161 acl 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. 166 167 noacl This option disables POSIX Access Control List 168 support. 169 170 reservation 171 172 noreservation 173 174 bsddf (*) Make 'df' act like BSD. 175 minixdf Make 'df' act like Minix. 176 177 check=none Don't do extra checking of bitmaps on mount. 178 nocheck 179 180 debug Extra debugging information is sent to syslog. 181 182 errors=remount-ro(*) Remount the filesystem read-only on an error. 183 errors=continue Keep going on a filesystem error. 184 errors=panic Panic and halt the machine if an error occurs. 185 186 grpid Give objects the same group ID as their creator. 187 bsdgroups 188 189 nogrpid (*) New objects have the group ID of their creator. 190 sysvgroups 191 192 resgid=n The group ID which may use the reserved blocks. 193 194 resuid=n The user ID which may use the reserved blocks. 195 196 sb=n Use alternate superblock at this location. 197 198 quota 199 noquota 200 grpquota 201 usrquota 202 203 bh (*) ext4 associates buffer heads to data pages to 204 nobh (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). 210 211 mballoc (*) Use the multiple block allocator for block allocation 212 nomballoc disabled multiple block allocator for block allocation. 213 stripe=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. 217 218 Data Mode 219 --------- 220 There are 3 different data modes: 221 222 * writeback mode 223 In data=writeback mode, ext4 does not journal data at all. This mode provides 224 a similar level of journaling as that of XFS, JFS, and ReiserFS in its default 225 mode - metadata journaling. A crash+recovery can cause incorrect data to 226 appear in files which were written shortly before the crash. This mode will 227 typically provide the best ext4 performance. 228 229 * ordered mode 230 In data=ordered mode, ext4 only officially journals metadata, but it logically 231 groups metadata and data blocks into a single unit called a transaction. When 232 it's time to write the new metadata out to disk, the associated data blocks 233 are written first. In general, this mode performs slightly slower than 234 writeback but significantly faster than journal mode. 235 236 * journal mode 237 data=journal mode provides full data and metadata journaling. All new data is 238 written to the journal first, and then to its final location. 239 In the event of a crash, the journal can be replayed, bringing both data and 240 metadata into a consistent state. This mode is the slowest except when data 241 needs to be read from and written to disk at the same time where it 242 outperforms all others modes. 243 244 References 245 ========== 246 247 kernel source: <file:fs/ext4/> 248 <file:fs/jbd2/> 249 250 programs: http://e2fsprogs.sourceforge.net/ 251 http://ext2resize.sourceforge.net 252 253 useful links: http://fedoraproject.org/wiki/ext3-devel 254 http://www.bullopensource.org/ext4/