Documentation/filesystems/ext4.txt

   1
   2 Ext4 Filesystem
   3 ===============
   4
   5 Ext4 is an an advanced level of the ext3 filesystem which incorporates
   6 scalability and reliability enhancements for supporting large filesystems
   7 (64 bit) in keeping with increasing disk capacities and state-of-the-art
   8 feature requirements.
   9
  10 Mailing list:   linux-ext4@vger.kernel.org
  11 Web site:       http://ext4.wiki.kernel.org
  12
  13
  14 1. Quick usage instructions:
  15 ===========================
  16
  17 Note: More extensive information for getting started with ext4 can be
  18       found at the ext4 wiki site at the URL:
  19       http://ext4.wiki.kernel.org/index.php/Ext4_Howto
  20
  21   - Compile and install the latest version of e2fsprogs (as of this
  22     writing version 1.41.3) from:
  23
  24     http://sourceforge.net/project/showfiles.php?group_id=2406
  25
  26         or
  27
  28     ftp://ftp.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs/
  29
  30         or grab the latest git repository from:
  31
  32     git://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git
  33
  34   - Note that it is highly important to install the mke2fs.conf file
  35     that comes with the e2fsprogs 1.41.x sources in /etc/mke2fs.conf. If
  36     you have edited the /etc/mke2fs.conf file installed on your system,
  37     you will need to merge your changes with the version from e2fsprogs
  38     1.41.x.
  39
  40   - Create a new filesystem using the ext4 filesystem type:
  41
  42         # mke2fs -t ext4 /dev/hda1
  43
  44     Or to configure an existing ext3 filesystem to support extents:
  45
  46         # tune2fs -O extents /dev/hda1
  47
  48     If the filesystem was created with 128 byte inodes, it can be
  49     converted to use 256 byte for greater efficiency via:
  50
  51         # tune2fs -I 256 /dev/hda1
  52
  53     (Note: we currently do not have tools to convert an ext4
  54     filesystem back to ext3; so please do not do try this on production
  55     filesystems.)
  56
  57   - Mounting:
  58
  59         # mount -t ext4 /dev/hda1 /wherever
  60
  61   - When comparing performance with other filesystems, remember that
  62     ext3/4 by default offers higher data integrity guarantees than most.
  63     So when comparing with a metadata-only journalling filesystem, such
  64     as ext3, use `mount -o data=writeback'.  And you might as well use
  65     `mount -o nobh' too along with it.  Making the journal larger than
  66     the mke2fs default often helps performance with metadata-intensive
  67     workloads.
  68
  69 2. Features
  70 ===========
  71
  72 2.1 Currently available
  73
  74 * ability to use filesystems > 16TB (e2fsprogs support not available yet)
  75 * extent format reduces metadata overhead (RAM, IO for access, transactions)
  76 * extent format more robust in face of on-disk corruption due to magics,
  77 * internal redunancy in tree
  78 * improved file allocation (multi-block alloc)
  79 * fix 32000 subdirectory limit
  80 * nsec timestamps for mtime, atime, ctime, create time
  81 * inode version field on disk (NFSv4, Lustre)
  82 * reduced e2fsck time via uninit_bg feature
  83 * journal checksumming for robustness, performance
  84 * persistent file preallocation (e.g for streaming media, databases)
  85 * ability to pack bitmaps and inode tables into larger virtual groups via the
  86   flex_bg feature
  87 * large file support
  88 * Inode allocation using large virtual block groups via flex_bg
  89 * delayed allocation
  90 * large block (up to pagesize) support
  91 * efficent new ordered mode in JBD2 and ext4(avoid using buffer head to force
  92   the ordering)
  93
  94 2.2 Candidate features for future inclusion
  95
  96 * Online defrag (patches available but not well tested)
  97 * reduced mke2fs time via lazy itable initialization in conjuction with
  98   the uninit_bg feature (capability to do this is available in e2fsprogs
  99   but a kernel thread to do lazy zeroing of unused inode table blocks
 100   after filesystem is first mounted is required for safety)
 101
 102 There are several others under discussion, whether they all make it in is
 103 partly a function of how much time everyone has to work on them. Features like
 104 metadata checksumming have been discussed and planned for a bit but no patches
 105 exist yet so I'm not sure they're in the near-term roadmap.
 106
 107 The big performance win will come with mballoc, delalloc and flex_bg
 108 grouping of bitmaps and inode tables.  Some test results available here:
 109
 110  - http://www.bullopensource.org/ext4/20080818-ffsb/ffsb-write-2.6.27-rc1.html
 111  - http://www.bullopensource.org/ext4/20080818-ffsb/ffsb-readwrite-2.6.27-rc1.html
 112
 113 3. Options
 114 ==========
 115
 116 When mounting an ext4 filesystem, the following option are accepted:
 117 (*) == default
 118
 119 extents         (*)     ext4 will use extents to address file data.  The
 120                         file system will no longer be mountable by ext3.
 121
 122 noextents               ext4 will not use extents for newly created files
 123
 124 journal_checksum        Enable checksumming of the journal transactions.
 125                         This will allow the recovery code in e2fsck and the
 126                         kernel to detect corruption in the kernel.  It is a
 127                         compatible change and will be ignored by older kernels.
 128
 129 journal_async_commit    Commit block can be written to disk without waiting
 130                         for descriptor blocks. If enabled older kernels cannot
 131                         mount the device. This will enable 'journal_checksum'
 132                         internally.
 133
 134 journal=update          Update the ext4 file system's journal to the current
 135                         format.
 136
 137 journal=inum            When a journal already exists, this option is ignored.
 138                         Otherwise, it specifies the number of the inode which
 139                         will represent the ext4 file system's journal file.
 140
 141 journal_dev=devnum      When the external journal device's major/minor numbers
 142                         have changed, this option allows the user to specify
 143                         the new journal location.  The journal device is
 144                         identified through its new major/minor numbers encoded
 145                         in devnum.
 146
 147 noload                  Don't load the journal on mounting.
 148
 149 data=journal            All data are committed into the journal prior to being
 150                         written into the main file system.
 151
 152 data=ordered    (*)     All data are forced directly out to the main file
 153                         system prior to its metadata being committed to the
 154                         journal.
 155
 156 data=writeback          Data ordering is not preserved, data may be written
 157                         into the main file system after its metadata has been
 158                         committed to the journal.
 159
 160 commit=nrsec    (*)     Ext4 can be told to sync all its data and metadata
 161                         every 'nrsec' seconds. The default value is 5 seconds.
 162                         This means that if you lose your power, you will lose
 163                         as much as the latest 5 seconds of work (your
 164                         filesystem will not be damaged though, thanks to the
 165                         journaling).  This default value (or any low value)
 166                         will hurt performance, but it's good for data-safety.
 167                         Setting it to 0 will have the same effect as leaving
 168                         it at the default (5 seconds).
 169                         Setting it to very large values will improve
 170                         performance.
 171
 172 barrier=<0|1(*)>        This enables/disables the use of write barriers in
 173                         the jbd code.  barrier=0 disables, barrier=1 enables.
 174                         This also requires an IO stack which can support
 175                         barriers, and if jbd gets an error on a barrier
 176                         write, it will disable again with a warning.
 177                         Write barriers enforce proper on-disk ordering
 178                         of journal commits, making volatile disk write caches
 179                         safe to use, at some performance penalty.  If
 180                         your disks are battery-backed in one way or another,
 181                         disabling barriers may safely improve performance.
 182
 183 inode_readahead=n       This tuning parameter controls the maximum
 184                         number of inode table blocks that ext4's inode
 185                         table readahead algorithm will pre-read into
 186                         the buffer cache.  The default value is 32 blocks.
 187
 188 orlov           (*)     This enables the new Orlov block allocator. It is
 189                         enabled by default.
 190
 191 oldalloc                This disables the Orlov block allocator and enables
 192                         the old block allocator.  Orlov should have better
 193                         performance - we'd like to get some feedback if it's
 194                         the contrary for you.
 195
 196 user_xattr              Enables Extended User Attributes.  Additionally, you
 197                         need to have extended attribute support enabled in the
 198                         kernel configuration (CONFIG_EXT4_FS_XATTR).  See the
 199                         attr(5) manual page and http://acl.bestbits.at/ to
 200                         learn more about extended attributes.
 201
 202 nouser_xattr            Disables Extended User Attributes.
 203
 204 acl                     Enables POSIX Access Control Lists support.
 205                         Additionally, you need to have ACL support enabled in
 206                         the kernel configuration (CONFIG_EXT4_FS_POSIX_ACL).
 207                         See the acl(5) manual page and http://acl.bestbits.at/
 208                         for more information.
 209
 210 noacl                   This option disables POSIX Access Control List
 211                         support.
 212
 213 reservation
 214
 215 noreservation
 216
 217 bsddf           (*)     Make 'df' act like BSD.
 218 minixdf                 Make 'df' act like Minix.
 219
 220 debug                   Extra debugging information is sent to syslog.
 221
 222 errors=remount-ro(*)    Remount the filesystem read-only on an error.
 223 errors=continue         Keep going on a filesystem error.
 224 errors=panic            Panic and halt the machine if an error occurs.
 225
 226 data_err=ignore(*)      Just print an error message if an error occurs
 227                         in a file data buffer in ordered mode.
 228 data_err=abort          Abort the journal if an error occurs in a file
 229                         data buffer in ordered mode.
 230
 231 grpid                   Give objects the same group ID as their creator.
 232 bsdgroups
 233
 234 nogrpid         (*)     New objects have the group ID of their creator.
 235 sysvgroups
 236
 237 resgid=n                The group ID which may use the reserved blocks.
 238
 239 resuid=n                The user ID which may use the reserved blocks.
 240
 241 sb=n                    Use alternate superblock at this location.
 242
 243 quota
 244 noquota
 245 grpquota
 246 usrquota
 247
 248 bh              (*)     ext4 associates buffer heads to data pages to
 249 nobh                    (a) cache disk block mapping information
 250                         (b) link pages into transaction to provide
 251                             ordering guarantees.
 252                         "bh" option forces use of buffer heads.
 253                         "nobh" option tries to avoid associating buffer
 254                         heads (supported only for "writeback" mode).
 255
 256 stripe=n                Number of filesystem blocks that mballoc will try
 257                         to use for allocation size and alignment. For RAID5/6
 258                         systems this should be the number of data
 259                         disks *  RAID chunk size in file system blocks.
 260 delalloc        (*)     Deferring block allocation until write-out time.
 261 nodelalloc              Disable delayed allocation. Blocks are allocation
 262                         when data is copied from user to page cache.
 263
 264 Data Mode
 265 =========
 266 There are 3 different data modes:
 267
 268 * writeback mode
 269 In data=writeback mode, ext4 does not journal data at all.  This mode provides
 270 a similar level of journaling as that of XFS, JFS, and ReiserFS in its default
 271 mode - metadata journaling.  A crash+recovery can cause incorrect data to
 272 appear in files which were written shortly before the crash.  This mode will
 273 typically provide the best ext4 performance.
 274
 275 * ordered mode
 276 In data=ordered mode, ext4 only officially journals metadata, but it logically
 277 groups metadata information related to data changes with the data blocks into a
 278 single unit called a transaction.  When it's time to write the new metadata
 279 out to disk, the associated data blocks are written first.  In general,
 280 this mode performs slightly slower than writeback but significantly faster than journal mode.
 281
 282 * journal mode
 283 data=journal mode provides full data and metadata journaling.  All new data is
 284 written to the journal first, and then to its final location.
 285 In the event of a crash, the journal can be replayed, bringing both data and
 286 metadata into a consistent state.  This mode is the slowest except when data
 287 needs to be read from and written to disk at the same time where it
 288 outperforms all others modes.  Curently ext4 does not have delayed
 289 allocation support if this data journalling mode is selected.
 290
 291 References
 292 ==========
 293
 294 kernel source:  <file:fs/ext4/>
 295                 <file:fs/jbd2/>
 296
 297 programs:       http://e2fsprogs.sourceforge.net/
 298
 299 useful links:   http://fedoraproject.org/wiki/ext3-devel
 300                 http://www.bullopensource.org/ext4/
 301                 http://ext4.wiki.kernel.org/index.php/Main_Page
 302                 http://fedoraproject.org/wiki/Features/Ext4