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 journal=update          Update the ext3 file system's journal to the current
  18                         format.
  19
  20 journal=inum            When a journal already exists, this option is ignored.
  21                         Otherwise, it specifies the number of the inode which
  22                         will represent the ext3 file system's journal file.
  23
  24 journal_dev=devnum      When the external journal device's major/minor numbers
  25                         have changed, this option allows the user to specify
  26                         the new journal location.  The journal device is
  27                         identified through its new major/minor numbers encoded
  28                         in devnum.
  29
  30 noload                  Don't load the journal on mounting.
  31
  32 data=journal            All data are committed into the journal prior to being
  33                         written into the main file system.
  34
  35 data=ordered    (*)     All data are forced directly out to the main file
  36                         system prior to its metadata being committed to the
  37                         journal.
  38
  39 data=writeback          Data ordering is not preserved, data may be written
  40                         into the main file system after its metadata has been
  41                         committed to the journal.
  42
  43 commit=nrsec    (*)     Ext3 can be told to sync all its data and metadata
  44                         every 'nrsec' seconds. The default value is 5 seconds.
  45                         This means that if you lose your power, you will lose
  46                         as much as the latest 5 seconds of work (your
  47                         filesystem will not be damaged though, thanks to the
  48                         journaling).  This default value (or any low value)
  49                         will hurt performance, but it's good for data-safety.
  50                         Setting it to 0 will have the same effect as leaving
  51                         it at the default (5 seconds).
  52                         Setting it to very large values will improve
  53                         performance.
  54
  55 barrier=1               This enables/disables barriers.  barrier=0 disables
  56                         it, barrier=1 enables it.
  57
  58 orlov           (*)     This enables the new Orlov block allocator. It is
  59                         enabled by default.
  60
  61 oldalloc                This disables the Orlov block allocator and enables
  62                         the old block allocator.  Orlov should have better
  63                         performance - we'd like to get some feedback if it's
  64                         the contrary for you.
  65
  66 user_xattr              Enables Extended User Attributes.  Additionally, you
  67                         need to have extended attribute support enabled in the
  68                         kernel configuration (CONFIG_EXT3_FS_XATTR).  See the
  69                         attr(5) manual page and http://acl.bestbits.at/ to
  70                         learn more about extended attributes.
  71
  72 nouser_xattr            Disables Extended User Attributes.
  73
  74 acl                     Enables POSIX Access Control Lists support.
  75                         Additionally, you need to have ACL support enabled in
  76                         the kernel configuration (CONFIG_EXT3_FS_POSIX_ACL).
  77                         See the acl(5) manual page and http://acl.bestbits.at/
  78                         for more information.
  79
  80 noacl                   This option disables POSIX Access Control List
  81                         support.
  82
  83 reservation
  84
  85 noreservation
  86
  87 bsddf           (*)     Make 'df' act like BSD.
  88 minixdf                 Make 'df' act like Minix.
  89
  90 check=none              Don't do extra checking of bitmaps on mount.
  91 nocheck
  92
  93 debug                   Extra debugging information is sent to syslog.
  94
  95 errors=remount-ro(*)    Remount the filesystem read-only on an error.
  96 errors=continue         Keep going on a filesystem error.
  97 errors=panic            Panic and halt the machine if an error occurs.
  98
  99 data_err=ignore(*)      Just print an error message if an error occurs
 100                         in a file data buffer in ordered mode.
 101 data_err=abort          Abort the journal if an error occurs in a file
 102                         data buffer in ordered mode.
 103
 104 grpid                   Give objects the same group ID as their creator.
 105 bsdgroups
 106
 107 nogrpid         (*)     New objects have the group ID of their creator.
 108 sysvgroups
 109
 110 resgid=n                The group ID which may use the reserved blocks.
 111
 112 resuid=n                The user ID which may use the reserved blocks.
 113
 114 sb=n                    Use alternate superblock at this location.
 115
 116 quota
 117 noquota
 118 grpquota
 119 usrquota
 120
 121 bh              (*)     ext3 associates buffer heads to data pages to
 122 nobh                    (a) cache disk block mapping information
 123                         (b) link pages into transaction to provide
 124                             ordering guarantees.
 125                         "bh" option forces use of buffer heads.
 126                         "nobh" option tries to avoid associating buffer
 127                         heads (supported only for "writeback" mode).
 128
 129
 130 Specification
 131 =============
 132 Ext3 shares all disk implementation with the ext2 filesystem, and adds
 133 transactions capabilities to ext2.  Journaling is done by the Journaling Block
 134 Device layer.
 135
 136 Journaling Block Device layer
 137 -----------------------------
 138 The Journaling Block Device layer (JBD) isn't ext3 specific.  It was designed
 139 to add journaling capabilities to a block device.  The ext3 filesystem code
 140 will inform the JBD of modifications it is performing (called a transaction).
 141 The journal supports the transactions start and stop, and in case of a crash,
 142 the journal can replay the transactions to quickly put the partition back into
 143 a consistent state.
 144
 145 Handles represent a single atomic update to a filesystem.  JBD can handle an
 146 external journal on a block device.
 147
 148 Data Mode
 149 ---------
 150 There are 3 different data modes:
 151
 152 * writeback mode
 153 In data=writeback mode, ext3 does not journal data at all.  This mode provides
 154 a similar level of journaling as that of XFS, JFS, and ReiserFS in its default
 155 mode - metadata journaling.  A crash+recovery can cause incorrect data to
 156 appear in files which were written shortly before the crash.  This mode will
 157 typically provide the best ext3 performance.
 158
 159 * ordered mode
 160 In data=ordered mode, ext3 only officially journals metadata, but it logically
 161 groups metadata and data blocks into a single unit called a transaction.  When
 162 it's time to write the new metadata out to disk, the associated data blocks
 163 are written first.  In general, this mode performs slightly slower than
 164 writeback but significantly faster than journal mode.
 165
 166 * journal mode
 167 data=journal mode provides full data and metadata journaling.  All new data is
 168 written to the journal first, and then to its final location.
 169 In the event of a crash, the journal can be replayed, bringing both data and
 170 metadata into a consistent state.  This mode is the slowest except when data
 171 needs to be read from and written to disk at the same time where it
 172 outperforms all other modes.
 173
 174 Compatibility
 175 -------------
 176
 177 Ext2 partitions can be easily convert to ext3, with `tune2fs -j <dev>`.
 178 Ext3 is fully compatible with Ext2.  Ext3 partitions can easily be mounted as
 179 Ext2.
 180
 181
 182 External Tools
 183 ==============
 184 See manual pages to learn more.
 185
 186 tune2fs:        create a ext3 journal on a ext2 partition with the -j flag.
 187 mke2fs:         create a ext3 partition with the -j flag.
 188 debugfs:        ext2 and ext3 file system debugger.
 189 ext2online:     online (mounted) ext2 and ext3 filesystem resizer
 190
 191
 192 References
 193 ==========
 194
 195 kernel source:  <file:fs/ext3/>
 196                 <file:fs/jbd/>
 197
 198 programs:       http://e2fsprogs.sourceforge.net/
 199                 http://ext2resize.sourceforge.net
 200
 201 useful links:   http://www.ibm.com/developerworks/library/l-fs7.html
 202                 http://www.ibm.com/developerworks/library/l-fs8.html