Documentation/filesystems/ext3.txt

   1
   2 Ext3 Filesystem
   3 ===============
   4
   5 Ext3 was originally released in September 1999. Written by Stephen Tweedie
   6 for the 2.2 branch, and ported to 2.4 kernels by Peter Braam, Andreas Dilger,
   7 Andrew Morton, Alexander Viro, Ted Ts'o and Stephen Tweedie.
   8
   9 Ext3 is the ext2 filesystem enhanced with journalling capabilities.
  10
  11 Options
  12 =======
  13
  14 When mounting an ext3 filesystem, the following option are accepted:
  15 (*) == default
  16
  17 ro                      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
  22 journal=update          Update the ext3 file system's journal to the current
  23                         format.
  24
  25 journal=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
  29 journal_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
  35 norecovery              Don't load the journal on mounting. Note that this forces
  36 noload                  mount of inconsistent filesystem, which can lead to
  37                         various problems.
  38
  39 data=journal            All data are committed into the journal prior to being
  40                         written into the main file system.
  41
  42 data=ordered    (*)     All data are forced directly out to the main file
  43                         system prior to its metadata being committed to the
  44                         journal.
  45
  46 data=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
  50 commit=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
  62 barrier=<0(*)|1>        This enables/disables the use of write barriers in
  63 barrier                 the jbd code.  barrier=0 disables, barrier=1 enables.
  64 nobarrier       (*)     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
  76 orlov           (*)     This enables the new Orlov block allocator. It is
  77                         enabled by default.
  78
  79 oldalloc                This disables the Orlov block allocator and enables
  80                         the old block allocator.  Orlov should have better
  81                         performance - we'd like to get some feedback if it's
  82                         the contrary for you.
  83
  84 user_xattr              Enables Extended User Attributes.  Additionally, you
  85                         need to have extended attribute support enabled in the
  86                         kernel configuration (CONFIG_EXT3_FS_XATTR).  See the
  87                         attr(5) manual page and http://acl.bestbits.at/ to
  88                         learn more about extended attributes.
  89
  90 nouser_xattr            Disables Extended User Attributes.
  91
  92 acl                     Enables POSIX Access Control Lists support.
  93                         Additionally, you need to have ACL support enabled in
  94                         the kernel configuration (CONFIG_EXT3_FS_POSIX_ACL).
  95                         See the acl(5) manual page and http://acl.bestbits.at/
  96                         for more information.
  97
  98 noacl                   This option disables POSIX Access Control List
  99                         support.
 100
 101 reservation
 102
 103 noreservation
 104
 105 bsddf           (*)     Make 'df' act like BSD.
 106 minixdf                 Make 'df' act like Minix.
 107
 108 check=none              Don't do extra checking of bitmaps on mount.
 109 nocheck
 110
 111 debug                   Extra debugging information is sent to syslog.
 112
 113 errors=remount-ro       Remount the filesystem read-only on an error.
 114 errors=continue         Keep going on a filesystem error.
 115 errors=panic            Panic and halt the machine if an error occurs.
 116                         (These mount options override the errors behavior
 117                         specified in the superblock, which can be
 118                         configured using tune2fs.)
 119
 120 data_err=ignore(*)      Just print an error message if an error occurs
 121                         in a file data buffer in ordered mode.
 122 data_err=abort          Abort the journal if an error occurs in a file
 123                         data buffer in ordered mode.
 124
 125 grpid                   Give objects the same group ID as their creator.
 126 bsdgroups
 127
 128 nogrpid         (*)     New objects have the group ID of their creator.
 129 sysvgroups
 130
 131 resgid=n                The group ID which may use the reserved blocks.
 132
 133 resuid=n                The user ID which may use the reserved blocks.
 134
 135 sb=n                    Use alternate superblock at this location.
 136
 137 quota                   These options are ignored by the filesystem. They
 138 noquota                 are used only by quota tools to recognize volumes
 139 grpquota                where quota should be turned on. See documentation
 140 usrquota                in the quota-tools package for more details
 141                         (http://sourceforge.net/projects/linuxquota).
 142
 143 jqfmt=<quota type>      These options tell filesystem details about quota
 144 usrjquota=<file>        so that quota information can be properly updated
 145 grpjquota=<file>        during journal replay. They replace the above
 146                         quota options. See documentation in the quota-tools
 147                         package for more details
 148                         (http://sourceforge.net/projects/linuxquota).
 149
 150 Specification
 151 =============
 152 Ext3 shares all disk implementation with the ext2 filesystem, and adds
 153 transactions capabilities to ext2.  Journaling is done by the Journaling Block
 154 Device layer.
 155
 156 Journaling Block Device layer
 157 -----------------------------
 158 The Journaling Block Device layer (JBD) isn't ext3 specific.  It was designed
 159 to add journaling capabilities to a block device.  The ext3 filesystem code
 160 will inform the JBD of modifications it is performing (called a transaction).
 161 The journal supports the transactions start and stop, and in case of a crash,
 162 the journal can replay the transactions to quickly put the partition back into
 163 a consistent state.
 164
 165 Handles represent a single atomic update to a filesystem.  JBD can handle an
 166 external journal on a block device.
 167
 168 Data Mode
 169 ---------
 170 There are 3 different data modes:
 171
 172 * writeback mode
 173 In data=writeback mode, ext3 does not journal data at all.  This mode provides
 174 a similar level of journaling as that of XFS, JFS, and ReiserFS in its default
 175 mode - metadata journaling.  A crash+recovery can cause incorrect data to
 176 appear in files which were written shortly before the crash.  This mode will
 177 typically provide the best ext3 performance.
 178
 179 * ordered mode
 180 In data=ordered mode, ext3 only officially journals metadata, but it logically
 181 groups metadata and data blocks into a single unit called a transaction.  When
 182 it's time to write the new metadata out to disk, the associated data blocks
 183 are written first.  In general, this mode performs slightly slower than
 184 writeback but significantly faster than journal mode.
 185
 186 * journal mode
 187 data=journal mode provides full data and metadata journaling.  All new data is
 188 written to the journal first, and then to its final location.
 189 In the event of a crash, the journal can be replayed, bringing both data and
 190 metadata into a consistent state.  This mode is the slowest except when data
 191 needs to be read from and written to disk at the same time where it
 192 outperforms all other modes.
 193
 194 Compatibility
 195 -------------
 196
 197 Ext2 partitions can be easily convert to ext3, with `tune2fs -j <dev>`.
 198 Ext3 is fully compatible with Ext2.  Ext3 partitions can easily be mounted as
 199 Ext2.
 200
 201
 202 External Tools
 203 ==============
 204 See manual pages to learn more.
 205
 206 tune2fs:        create a ext3 journal on a ext2 partition with the -j flag.
 207 mke2fs:         create a ext3 partition with the -j flag.
 208 debugfs:        ext2 and ext3 file system debugger.
 209 ext2online:     online (mounted) ext2 and ext3 filesystem resizer
 210
 211
 212 References
 213 ==========
 214
 215 kernel source:  <file:fs/ext3/>
 216                 <file:fs/jbd/>
 217
 218 programs:       http://e2fsprogs.sourceforge.net/
 219                 http://ext2resize.sourceforge.net
 220
 221 useful links:   http://www.ibm.com/developerworks/library/l-fs7.html
 222                 http://www.ibm.com/developerworks/library/l-fs8.html