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 grpid                   Give objects the same group ID as their creator.
 100 bsdgroups
 101
 102 nogrpid         (*)     New objects have the group ID of their creator.
 103 sysvgroups
 104
 105 resgid=n                The group ID which may use the reserved blocks.
 106
 107 resuid=n                The user ID which may use the reserved blocks.
 108
 109 sb=n                    Use alternate superblock at this location.
 110
 111 quota
 112 noquota
 113 grpquota
 114 usrquota
 115
 116 bh              (*)     ext3 associates buffer heads to data pages to
 117 nobh                    (a) cache disk block mapping information
 118                         (b) link pages into transaction to provide
 119                             ordering guarantees.
 120                         "bh" option forces use of buffer heads.
 121                         "nobh" option tries to avoid associating buffer
 122                         heads (supported only for "writeback" mode).
 123
 124
 125 Specification
 126 =============
 127 Ext3 shares all disk implementation with the ext2 filesystem, and adds
 128 transactions capabilities to ext2.  Journaling is done by the Journaling Block
 129 Device layer.
 130
 131 Journaling Block Device layer
 132 -----------------------------
 133 The Journaling Block Device layer (JBD) isn't ext3 specific.  It was designed
 134 to add journaling capabilities to a block device.  The ext3 filesystem code
 135 will inform the JBD of modifications it is performing (called a transaction).
 136 The journal supports the transactions start and stop, and in case of a crash,
 137 the journal can replay the transactions to quickly put the partition back into
 138 a consistent state.
 139
 140 Handles represent a single atomic update to a filesystem.  JBD can handle an
 141 external journal on a block device.
 142
 143 Data Mode
 144 ---------
 145 There are 3 different data modes:
 146
 147 * writeback mode
 148 In data=writeback mode, ext3 does not journal data at all.  This mode provides
 149 a similar level of journaling as that of XFS, JFS, and ReiserFS in its default
 150 mode - metadata journaling.  A crash+recovery can cause incorrect data to
 151 appear in files which were written shortly before the crash.  This mode will
 152 typically provide the best ext3 performance.
 153
 154 * ordered mode
 155 In data=ordered mode, ext3 only officially journals metadata, but it logically
 156 groups metadata and data blocks into a single unit called a transaction.  When
 157 it's time to write the new metadata out to disk, the associated data blocks
 158 are written first.  In general, this mode performs slightly slower than
 159 writeback but significantly faster than journal mode.
 160
 161 * journal mode
 162 data=journal mode provides full data and metadata journaling.  All new data is
 163 written to the journal first, and then to its final location.
 164 In the event of a crash, the journal can be replayed, bringing both data and
 165 metadata into a consistent state.  This mode is the slowest except when data
 166 needs to be read from and written to disk at the same time where it
 167 outperforms all other modes.
 168
 169 Compatibility
 170 -------------
 171
 172 Ext2 partitions can be easily convert to ext3, with `tune2fs -j <dev>`.
 173 Ext3 is fully compatible with Ext2.  Ext3 partitions can easily be mounted as
 174 Ext2.
 175
 176
 177 External Tools
 178 ==============
 179 See manual pages to learn more.
 180
 181 tune2fs:        create a ext3 journal on a ext2 partition with the -j flag.
 182 mke2fs:         create a ext3 partition with the -j flag.
 183 debugfs:        ext2 and ext3 file system debugger.
 184 ext2online:     online (mounted) ext2 and ext3 filesystem resizer
 185
 186
 187 References
 188 ==========
 189
 190 kernel source:  <file:fs/ext3/>
 191                 <file:fs/jbd/>
 192
 193 programs:       http://e2fsprogs.sourceforge.net/
 194                 http://ext2resize.sourceforge.net
 195
 196 useful links:   http://www.zip.com.au/~akpm/linux/ext3/ext3-usage.html
 197                 http://www-106.ibm.com/developerworks/linux/library/l-fs7/
 198                 http://www-106.ibm.com/developerworks/linux/library/l-fs8/