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 noload                  Don't load the journal on mounting. Note that this forces
  36                         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=1               This enables/disables barriers.  barrier=0 disables
  63                         it, barrier=1 enables it.
  64
  65 orlov           (*)     This enables the new Orlov block allocator. It is
  66                         enabled by default.
  67
  68 oldalloc                This disables the Orlov block allocator and enables
  69                         the old block allocator.  Orlov should have better
  70                         performance - we'd like to get some feedback if it's
  71                         the contrary for you.
  72
  73 user_xattr              Enables Extended User Attributes.  Additionally, you
  74                         need to have extended attribute support enabled in the
  75                         kernel configuration (CONFIG_EXT3_FS_XATTR).  See the
  76                         attr(5) manual page and http://acl.bestbits.at/ to
  77                         learn more about extended attributes.
  78
  79 nouser_xattr            Disables Extended User Attributes.
  80
  81 acl                     Enables POSIX Access Control Lists support.
  82                         Additionally, you need to have ACL support enabled in
  83                         the kernel configuration (CONFIG_EXT3_FS_POSIX_ACL).
  84                         See the acl(5) manual page and http://acl.bestbits.at/
  85                         for more information.
  86
  87 noacl                   This option disables POSIX Access Control List
  88                         support.
  89
  90 reservation
  91
  92 noreservation
  93
  94 bsddf           (*)     Make 'df' act like BSD.
  95 minixdf                 Make 'df' act like Minix.
  96
  97 check=none              Don't do extra checking of bitmaps on mount.
  98 nocheck
  99
 100 debug                   Extra debugging information is sent to syslog.
 101
 102 errors=remount-ro       Remount the filesystem read-only on an error.
 103 errors=continue         Keep going on a filesystem error.
 104 errors=panic            Panic and halt the machine if an error occurs.
 105                         (These mount options override the errors behavior
 106                         specified in the superblock, which can be
 107                         configured using tune2fs.)
 108
 109 data_err=ignore(*)      Just print an error message if an error occurs
 110                         in a file data buffer in ordered mode.
 111 data_err=abort          Abort the journal if an error occurs in a file
 112                         data buffer in ordered mode.
 113
 114 grpid                   Give objects the same group ID as their creator.
 115 bsdgroups
 116
 117 nogrpid         (*)     New objects have the group ID of their creator.
 118 sysvgroups
 119
 120 resgid=n                The group ID which may use the reserved blocks.
 121
 122 resuid=n                The user ID which may use the reserved blocks.
 123
 124 sb=n                    Use alternate superblock at this location.
 125
 126 quota                   These options are ignored by the filesystem. They
 127 noquota                 are used only by quota tools to recognize volumes
 128 grpquota                where quota should be turned on. See documentation
 129 usrquota                in the quota-tools package for more details
 130                         (http://sourceforge.net/projects/linuxquota).
 131
 132 jqfmt=<quota type>      These options tell filesystem details about quota
 133 usrjquota=<file>        so that quota information can be properly updated
 134 grpjquota=<file>        during journal replay. They replace the above
 135                         quota options. See documentation in the quota-tools
 136                         package for more details
 137                         (http://sourceforge.net/projects/linuxquota).
 138
 139 bh              (*)     ext3 associates buffer heads to data pages to
 140 nobh                    (a) cache disk block mapping information
 141                         (b) link pages into transaction to provide
 142                             ordering guarantees.
 143                         "bh" option forces use of buffer heads.
 144                         "nobh" option tries to avoid associating buffer
 145                         heads (supported only for "writeback" mode).
 146
 147
 148 Specification
 149 =============
 150 Ext3 shares all disk implementation with the ext2 filesystem, and adds
 151 transactions capabilities to ext2.  Journaling is done by the Journaling Block
 152 Device layer.
 153
 154 Journaling Block Device layer
 155 -----------------------------
 156 The Journaling Block Device layer (JBD) isn't ext3 specific.  It was designed
 157 to add journaling capabilities to a block device.  The ext3 filesystem code
 158 will inform the JBD of modifications it is performing (called a transaction).
 159 The journal supports the transactions start and stop, and in case of a crash,
 160 the journal can replay the transactions to quickly put the partition back into
 161 a consistent state.
 162
 163 Handles represent a single atomic update to a filesystem.  JBD can handle an
 164 external journal on a block device.
 165
 166 Data Mode
 167 ---------
 168 There are 3 different data modes:
 169
 170 * writeback mode
 171 In data=writeback mode, ext3 does not journal data at all.  This mode provides
 172 a similar level of journaling as that of XFS, JFS, and ReiserFS in its default
 173 mode - metadata journaling.  A crash+recovery can cause incorrect data to
 174 appear in files which were written shortly before the crash.  This mode will
 175 typically provide the best ext3 performance.
 176
 177 * ordered mode
 178 In data=ordered mode, ext3 only officially journals metadata, but it logically
 179 groups metadata and data blocks into a single unit called a transaction.  When
 180 it's time to write the new metadata out to disk, the associated data blocks
 181 are written first.  In general, this mode performs slightly slower than
 182 writeback but significantly faster than journal mode.
 183
 184 * journal mode
 185 data=journal mode provides full data and metadata journaling.  All new data is
 186 written to the journal first, and then to its final location.
 187 In the event of a crash, the journal can be replayed, bringing both data and
 188 metadata into a consistent state.  This mode is the slowest except when data
 189 needs to be read from and written to disk at the same time where it
 190 outperforms all other modes.
 191
 192 Compatibility
 193 -------------
 194
 195 Ext2 partitions can be easily convert to ext3, with `tune2fs -j <dev>`.
 196 Ext3 is fully compatible with Ext2.  Ext3 partitions can easily be mounted as
 197 Ext2.
 198
 199
 200 External Tools
 201 ==============
 202 See manual pages to learn more.
 203
 204 tune2fs:        create a ext3 journal on a ext2 partition with the -j flag.
 205 mke2fs:         create a ext3 partition with the -j flag.
 206 debugfs:        ext2 and ext3 file system debugger.
 207 ext2online:     online (mounted) ext2 and ext3 filesystem resizer
 208
 209
 210 References
 211 ==========
 212
 213 kernel source:  <file:fs/ext3/>
 214                 <file:fs/jbd/>
 215
 216 programs:       http://e2fsprogs.sourceforge.net/
 217                 http://ext2resize.sourceforge.net
 218
 219 useful links:   http://www.ibm.com/developerworks/library/l-fs7.html
 220                 http://www.ibm.com/developerworks/library/l-fs8.html