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 user_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
  82 nouser_xattr            Disables Extended User Attributes.
  83
  84 acl                     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
  90 noacl                   This option disables POSIX Access Control List
  91                         support.
  92
  93 reservation
  94
  95 noreservation
  96
  97 bsddf           (*)     Make 'df' act like BSD.
  98 minixdf                 Make 'df' act like Minix.
  99
 100 check=none              Don't do extra checking of bitmaps on mount.
 101 nocheck
 102
 103 debug                   Extra debugging information is sent to syslog.
 104
 105 errors=remount-ro       Remount the filesystem read-only on an error.
 106 errors=continue         Keep going on a filesystem error.
 107 errors=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
 112 data_err=ignore(*)      Just print an error message if an error occurs
 113                         in a file data buffer in ordered mode.
 114 data_err=abort          Abort the journal if an error occurs in a file
 115                         data buffer in ordered mode.
 116
 117 grpid                   Give objects the same group ID as their creator.
 118 bsdgroups
 119
 120 nogrpid         (*)     New objects have the group ID of their creator.
 121 sysvgroups
 122
 123 resgid=n                The group ID which may use the reserved blocks.
 124
 125 resuid=n                The user ID which may use the reserved blocks.
 126
 127 sb=n                    Use alternate superblock at this location.
 128
 129 quota                   These options are ignored by the filesystem. They
 130 noquota                 are used only by quota tools to recognize volumes
 131 grpquota                where quota should be turned on. See documentation
 132 usrquota                in the quota-tools package for more details
 133                         (http://sourceforge.net/projects/linuxquota).
 134
 135 jqfmt=<quota type>      These options tell filesystem details about quota
 136 usrjquota=<file>        so that quota information can be properly updated
 137 grpjquota=<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
 142 Specification
 143 =============
 144 Ext3 shares all disk implementation with the ext2 filesystem, and adds
 145 transactions capabilities to ext2.  Journaling is done by the Journaling Block
 146 Device layer.
 147
 148 Journaling Block Device layer
 149 -----------------------------
 150 The Journaling Block Device layer (JBD) isn't ext3 specific.  It was designed
 151 to add journaling capabilities to a block device.  The ext3 filesystem code
 152 will inform the JBD of modifications it is performing (called a transaction).
 153 The journal supports the transactions start and stop, and in case of a crash,
 154 the journal can replay the transactions to quickly put the partition back into
 155 a consistent state.
 156
 157 Handles represent a single atomic update to a filesystem.  JBD can handle an
 158 external journal on a block device.
 159
 160 Data Mode
 161 ---------
 162 There are 3 different data modes:
 163
 164 * writeback mode
 165 In data=writeback mode, ext3 does not journal data at all.  This mode provides
 166 a similar level of journaling as that of XFS, JFS, and ReiserFS in its default
 167 mode - metadata journaling.  A crash+recovery can cause incorrect data to
 168 appear in files which were written shortly before the crash.  This mode will
 169 typically provide the best ext3 performance.
 170
 171 * ordered mode
 172 In data=ordered mode, ext3 only officially journals metadata, but it logically
 173 groups metadata and data blocks into a single unit called a transaction.  When
 174 it's time to write the new metadata out to disk, the associated data blocks
 175 are written first.  In general, this mode performs slightly slower than
 176 writeback but significantly faster than journal mode.
 177
 178 * journal mode
 179 data=journal mode provides full data and metadata journaling.  All new data is
 180 written to the journal first, and then to its final location.
 181 In the event of a crash, the journal can be replayed, bringing both data and
 182 metadata into a consistent state.  This mode is the slowest except when data
 183 needs to be read from and written to disk at the same time where it
 184 outperforms all other modes.
 185
 186 Compatibility
 187 -------------
 188
 189 Ext2 partitions can be easily convert to ext3, with `tune2fs -j <dev>`.
 190 Ext3 is fully compatible with Ext2.  Ext3 partitions can easily be mounted as
 191 Ext2.
 192
 193
 194 External Tools
 195 ==============
 196 See manual pages to learn more.
 197
 198 tune2fs:        create a ext3 journal on a ext2 partition with the -j flag.
 199 mke2fs:         create a ext3 partition with the -j flag.
 200 debugfs:        ext2 and ext3 file system debugger.
 201 ext2online:     online (mounted) ext2 and ext3 filesystem resizer
 202
 203
 204 References
 205 ==========
 206
 207 kernel source:  <file:fs/ext3/>
 208                 <file:fs/jbd/>
 209
 210 programs:       http://e2fsprogs.sourceforge.net/
 211                 http://ext2resize.sourceforge.net
 212
 213 useful links:   http://www.ibm.com/developerworks/library/l-fs7/index.html
 214         http://www.ibm.com/developerworks/library/l-fs8/index.html