linux/Documentation/filesystems/ext3.txt
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   1
   2Ext3 Filesystem
   3===============
   4
   5Ext3 was originally released in September 1999. Written by Stephen Tweedie
   6for the 2.2 branch, and ported to 2.4 kernels by Peter Braam, Andreas Dilger,
   7Andrew Morton, Alexander Viro, Ted Ts'o and Stephen Tweedie.
   8
   9Ext3 is the ext2 filesystem enhanced with journalling capabilities.
  10
  11Options
  12=======
  13
  14When mounting an ext3 filesystem, the following option are accepted:
  15(*) == default
  16
  17ro                      Mount filesystem read only. Note that ext3 will replay
  18                        the journal (and thus write to the partition) even when
  19                        mounted "read only". Mount options "ro,noload" can be
  20                        used to prevent writes to the filesystem.
  21
  22journal=update          Update the ext3 file system's journal to the current
  23                        format.
  24
  25journal=inum            When a journal already exists, this option is ignored.
  26                        Otherwise, it specifies the number of the inode which
  27                        will represent the ext3 file system's journal file.
  28
  29journal_dev=devnum      When the external journal device's major/minor numbers
  30                        have changed, this option allows the user to specify
  31                        the new journal location.  The journal device is
  32                        identified through its new major/minor numbers encoded
  33                        in devnum.
  34
  35norecovery              Don't load the journal on mounting. Note that this forces
  36noload                  mount of inconsistent filesystem, which can lead to
  37                        various problems.
  38
  39data=journal            All data are committed into the journal prior to being
  40                        written into the main file system.
  41
  42data=ordered    (*)     All data are forced directly out to the main file
  43                        system prior to its metadata being committed to the
  44                        journal.
  45
  46data=writeback          Data ordering is not preserved, data may be written
  47                        into the main file system after its metadata has been
  48                        committed to the journal.
  49
  50commit=nrsec    (*)     Ext3 can be told to sync all its data and metadata
  51                        every 'nrsec' seconds. The default value is 5 seconds.
  52                        This means that if you lose your power, you will lose
  53                        as much as the latest 5 seconds of work (your
  54                        filesystem will not be damaged though, thanks to the
  55                        journaling).  This default value (or any low value)
  56                        will hurt performance, but it's good for data-safety.
  57                        Setting it to 0 will have the same effect as leaving
  58                        it at the default (5 seconds).
  59                        Setting it to very large values will improve
  60                        performance.
  61
  62barrier=<0|1(*)>        This enables/disables the use of write barriers in
  63barrier (*)             the jbd code.  barrier=0 disables, barrier=1 enables.
  64nobarrier               This also requires an IO stack which can support
  65                        barriers, and if jbd gets an error on a barrier
  66                        write, it will disable again with a warning.
  67                        Write barriers enforce proper on-disk ordering
  68                        of journal commits, making volatile disk write caches
  69                        safe to use, at some performance penalty.  If
  70                        your disks are battery-backed in one way or another,
  71                        disabling barriers may safely improve performance.
  72                        The mount options "barrier" and "nobarrier" can
  73                        also be used to enable or disable barriers, for
  74                        consistency with other ext3 mount options.
  75
  76user_xattr              Enables Extended User Attributes.  Additionally, you
  77                        need to have extended attribute support enabled in the
  78                        kernel configuration (CONFIG_EXT3_FS_XATTR).  See the
  79                        attr(5) manual page and http://acl.bestbits.at/ to
  80                        learn more about extended attributes.
  81
  82nouser_xattr            Disables Extended User Attributes.
  83
  84acl                     Enables POSIX Access Control Lists support.
  85                        Additionally, you need to have ACL support enabled in
  86                        the kernel configuration (CONFIG_EXT3_FS_POSIX_ACL).
  87                        See the acl(5) manual page and http://acl.bestbits.at/
  88                        for more information.
  89
  90noacl                   This option disables POSIX Access Control List
  91                        support.
  92
  93reservation
  94
  95noreservation
  96
  97bsddf           (*)     Make 'df' act like BSD.
  98minixdf                 Make 'df' act like Minix.
  99
 100check=none              Don't do extra checking of bitmaps on mount.
 101nocheck
 102
 103debug                   Extra debugging information is sent to syslog.
 104
 105errors=remount-ro       Remount the filesystem read-only on an error.
 106errors=continue         Keep going on a filesystem error.
 107errors=panic            Panic and halt the machine if an error occurs.
 108                        (These mount options override the errors behavior
 109                        specified in the superblock, which can be
 110                        configured using tune2fs.)
 111
 112data_err=ignore(*)      Just print an error message if an error occurs
 113                        in a file data buffer in ordered mode.
 114data_err=abort          Abort the journal if an error occurs in a file
 115                        data buffer in ordered mode.
 116
 117grpid                   Give objects the same group ID as their creator.
 118bsdgroups
 119
 120nogrpid         (*)     New objects have the group ID of their creator.
 121sysvgroups
 122
 123resgid=n                The group ID which may use the reserved blocks.
 124
 125resuid=n                The user ID which may use the reserved blocks.
 126
 127sb=n                    Use alternate superblock at this location.
 128
 129quota                   These options are ignored by the filesystem. They
 130noquota                 are used only by quota tools to recognize volumes
 131grpquota                where quota should be turned on. See documentation
 132usrquota                in the quota-tools package for more details
 133                        (http://sourceforge.net/projects/linuxquota).
 134
 135jqfmt=<quota type>      These options tell filesystem details about quota
 136usrjquota=<file>        so that quota information can be properly updated
 137grpjquota=<file>        during journal replay. They replace the above
 138                        quota options. See documentation in the quota-tools
 139                        package for more details
 140                        (http://sourceforge.net/projects/linuxquota).
 141
 142Specification
 143=============
 144Ext3 shares all disk implementation with the ext2 filesystem, and adds
 145transactions capabilities to ext2.  Journaling is done by the Journaling Block
 146Device layer.
 147
 148Journaling Block Device layer
 149-----------------------------
 150The Journaling Block Device layer (JBD) isn't ext3 specific.  It was designed
 151to add journaling capabilities to a block device.  The ext3 filesystem code
 152will inform the JBD of modifications it is performing (called a transaction).
 153The journal supports the transactions start and stop, and in case of a crash,
 154the journal can replay the transactions to quickly put the partition back into
 155a consistent state.
 156
 157Handles represent a single atomic update to a filesystem.  JBD can handle an
 158external journal on a block device.
 159
 160Data Mode
 161---------
 162There are 3 different data modes:
 163
 164* writeback mode
 165In data=writeback mode, ext3 does not journal data at all.  This mode provides
 166a similar level of journaling as that of XFS, JFS, and ReiserFS in its default
 167mode - metadata journaling.  A crash+recovery can cause incorrect data to
 168appear in files which were written shortly before the crash.  This mode will
 169typically provide the best ext3 performance.
 170
 171* ordered mode
 172In data=ordered mode, ext3 only officially journals metadata, but it logically
 173groups metadata and data blocks into a single unit called a transaction.  When
 174it's time to write the new metadata out to disk, the associated data blocks
 175are written first.  In general, this mode performs slightly slower than
 176writeback but significantly faster than journal mode.
 177
 178* journal mode
 179data=journal mode provides full data and metadata journaling.  All new data is
 180written to the journal first, and then to its final location.
 181In the event of a crash, the journal can be replayed, bringing both data and
 182metadata into a consistent state.  This mode is the slowest except when data
 183needs to be read from and written to disk at the same time where it
 184outperforms all other modes.
 185
 186Compatibility
 187-------------
 188
 189Ext2 partitions can be easily convert to ext3, with `tune2fs -j <dev>`.
 190Ext3 is fully compatible with Ext2.  Ext3 partitions can easily be mounted as
 191Ext2.
 192
 193
 194External Tools
 195==============
 196See manual pages to learn more.
 197
 198tune2fs:        create a ext3 journal on a ext2 partition with the -j flag.
 199mke2fs:         create a ext3 partition with the -j flag.
 200debugfs:        ext2 and ext3 file system debugger.
 201ext2online:     online (mounted) ext2 and ext3 filesystem resizer
 202
 203
 204References
 205==========
 206
 207kernel source:  <file:fs/ext3/>
 208                <file:fs/jbd/>
 209
 210programs:       http://e2fsprogs.sourceforge.net/
 211                http://ext2resize.sourceforge.net
 212
 213useful links:   http://www.ibm.com/developerworks/library/l-fs7/index.html
 214        http://www.ibm.com/developerworks/library/l-fs8/index.html
 215
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