misc/ext4.5.in

   1 .\" -*- nroff -*-
   2 .\" Copyright 1993, 1994, 1995 by Theodore Ts'o.  All Rights Reserved.
   3 .\" This file may be copied under the terms of the GNU Public License.
   4 .\"
   5 .TH EXT4 5 "@E2FSPROGS_MONTH@ @E2FSPROGS_YEAR@" "E2fsprogs version @E2FSPROGS_VERSION@"
   6 .SH NAME
   7 ext2 \- the second extended file system
   8 .br
   9 ext2 \- the third extended file system
  10 .br
  11 ext4 \- the fourth extended file system
  12 .SH DESCRIPTION
  13 The second, third, and fourth extended file systems, or ext2, ext3, and
  14 ext4 as they are commonly known, are Linux file systems that have
  15 historically been the default file system for many Linux distributions.
  16 They are general purpose file systems that have been designed for
  17 extensibility and backwards compatibility.  In particular, file systems
  18 previously intended for use with the ext2 and ext3 file systems can be
  19 mounted using the ext4 file system driver, and indeed in many modern
  20 Linux distributions, the ext4 file system driver has been configured
  21 handle mount requests for ext2 and ext3 file systems.
  22 .SH FILE SYSTEM FEATURES
  23 A file system formated for ext2, ext3, or ext4 can be have some
  24 collection of the follow file system feature flags enabled.  Some of
  25 these features are not supported by all implementations of the ext2,
  26 ext3, and ext4 file system drivers, depending on Linux kernel version in
  27 use.  On other operating systems, such as the GNU/HURD or FreeBSD, only
  28 a very restrictive set of file system features may be supported in their
  29 implementations of ext2.
  30 .RS 1.2i
  31 .TP
  32 .B 64bit
  33 .br
  34 Enables the file system to be larger than 2^32 blocks.  This feature is set
  35 automatically, as needed, but it can be useful to specify this feature
  36 explicitly if the file system might need to be resized larger than 2^32
  37 blocks, even if it was smaller than that threshold when it was
  38 originally created.  Note that some older kernels and older versions
  39 of e2fsprogs will not support file systems with this ext4 feature enabled.
  40 .TP
  41 .B bigalloc
  42 .br
  43 This ext4 feature enables clustered block allocation, so that the unit of
  44 allocation is a power of two number of blocks.  That is, each bit in the
  45 what had traditionally been known as the block allocation bitmap now
  46 indicates whether a cluster is in use or not, where a cluster is by
  47 default composed of 16 blocks.  This feature can decrease the time
  48 spent on doing block allocation and brings smaller fragmentation, especially
  49 for large files.  The size can be specified using the
  50 .B \-C option.
  51 .IP
  52 .B Warning:
  53 The bigalloc feature is still under development, and may not be fully
  54 supported with your kernel or may have various bugs.  Please see the web
  55 page http://ext4.wiki.kernel.org/index.php/Bigalloc for details.
  56 May clash with delayed allocation (see
  57 .BR nodelalloc mount option).
  58 .IP
  59 This feature requires that the
  60 .B extent
  61 features be enabled.
  62 .TP
  63 .B dir_index
  64 .br
  65 Use hashed b-trees to speed up name lookups in large directories.  This
  66 feature is supported by ext3 and ext4 file systems, and is ignored by
  67 ext2 file systems.
  68 .TP
  69 .B dir_nlink
  70 .br
  71 This ext4 feature allows more than 65000 subdirectories per directory.
  72 .TP
  73 .B extent
  74 .br
  75 This ext4 feature allows the mapping of logical block numbers for a
  76 particular inode to physical blocks on the storage device to be stored
  77 using an extent tree, which is a more efficient data structure than the
  78 traditional indirect block scheme used by the ext2 and ext3 file
  79 systems.  The use of the extent tree decreases metadata block overhead,
  80 improves file system performance, and decreases the needed to run
  81 .BR e2fsck (8)
  82 on the file system.
  83 (Note: both
  84 .B extent
  85 and
  86 .B extents
  87 are accepted as valid names for this feature for
  88 historical/backwards compatibility reasons.)
  89 .TP
  90 .B extra_isize
  91 .br
  92 This ext4 feature reserves a specific amount of space in each inode for
  93 extended metadata such as nanosecond timestamps and file creation time,
  94 even if the current kernel does not current need to reserve this much
  95 space.  Without this feature, the kernel will reserve the amount of
  96 space for features currently it currently needs, and the rest may be
  97 consumed by extended attributes.
  98
  99 For this feature to be useful the inode size must be 256 bytes in size
 100 or larger.
 101 .TP
 102 .B ext_attr
 103 .br
 104 This feature enables the use of extended attributes.  This feature is
 105 supported by ext2, ext3, and ext4.
 106 .TP
 107 .B filetype
 108 .br
 109 This feature enables the storage file type information in directory
 110 entries.  This feature is supported by ext2, ext3, and ext4.
 111 .TP
 112 .TP
 113 .B flex_bg
 114 .br
 115 This ext4 feature allows the per-block group metadata (allocation
 116 bitmaps
 117 and inode tables)
 118 to be placed anywhere on the storage media.  In addition,
 119 .B mke2fs
 120 will place the per-block group metadata together starting at the first
 121 block group of each "flex_bg group".   The size of the flex_bg group
 122 can be specified using the
 123 .B \-G
 124 option.
 125 .TP
 126 .B has_journal
 127 .br
 128 Create a journal to ensure filesystem consistency even across unclean
 129 shutdowns.  Setting the filesystem feature is equivalent to using the
 130 .B \-j
 131 option.  This feature is supported by ext3 and ext4, and ignored by the
 132 ext2 file system driver.
 133 .TP
 134 .B huge_file
 135 .br
 136 This ext4 feature allows files to be larger than 2 terabytes in size.
 137 .TP
 138 .B journal_dev
 139 .br
 140 This feature is enabled on the superblock found on an external journal
 141 device.  The block size for the external journal must be the same as the
 142 file system which uses it.
 143 .IP
 144 The external journal device can be used by a file system by specifying
 145 the
 146 .B \-J
 147 .BR device= <external-device>
 148 option to
 149 .BR mke2fs (8)
 150 or
 151 .BR tune2fs(8).
 152 .TP
 153 .B large_file
 154 .br
 155 This feature flag is set automatically by modern kernels when a file
 156 larger than 2 gigabytes is created.  Very old kernels could not
 157 handle large files, so this feature flag was used to prohibit those
 158 kernels from mounting file systems that they could not understand.
 159 .\" .TP
 160 .\" .B metadata_csum
 161 .\" .br
 162 .\" This ext4 feature enables metadata checksumming.  This feature stores
 163 .\" checksums for all of the filesystem metadata (superblock, group
 164 .\" descriptor blocks, inode and block bitmaps, directories, and
 165 .\" extent tree blocks).  The checksum algorithm used for the metadata
 166 .\" blocks is different than the one used for group descriptors with the
 167 .\" .B uninit_bg
 168 .\" feature, these two features are incompatible and
 169 .\" .B metadata_csum
 170 .\" will be used preferentially instead of
 171 .\" .BR uninit_bg .
 172 .\" .br
 173 .\" .B Future feature, available in e2fsprogs 1.43-WIP
 174 .TP
 175 .B meta_bg
 176 .br
 177 This ext4 feature allows file systems to be resized on-line without explicitly
 178 needing to reserve space for growth in the size of the block group
 179 descriptors.  This scheme is also used to resize file systems which are
 180 larger than 2^32 blocks.  It is not recommended that this feature be set
 181 when a file system is created, since this alternate method of storing
 182 the block group descriptor will slow down the time needed to mount the
 183 file system, and newer kernels can automatically set this feature as
 184 necessary when doing an online resize and no more reserved space is
 185 available in the resize inode.
 186 .TP
 187 .B mmp
 188 .br
 189 This ext4 feature provides multiple mount protection (MMP).  MMP helps to
 190 protect the filesystem from being multiply mounted and is useful in
 191 shared storage environments.
 192 @QUOTA_MAN_COMMENT@.TP
 193 @QUOTA_MAN_COMMENT@.B quota
 194 @QUOTA_MAN_COMMENT@.br
 195 @QUOTA_MAN_COMMENT@Create quota inodes (inode #3 for userquota and inode
 196 @QUOTA_MAN_COMMENT@#4 for group quota) and set them in the superblock.
 197 @QUOTA_MAN_COMMENT@With this feature, the quotas will be enabled
 198 @QUOTA_MAN_COMMENT@automatically when the filesystem is mounted.
 199 @QUOTA_MAN_COMMENT@.IP
 200 @QUOTA_MAN_COMMENT@Causes the quota files (i.e., user.quota and
 201 @QUOTA_MAN_COMMENT@group.quota which existed
 202 @QUOTA_MAN_COMMENT@in the older quota design) to be hidden inodes.
 203 @QUOTA_MAN_COMMENT@.IP
 204 @QUOTA_MAN_COMMENT@.B Warning:
 205 @QUOTA_MAN_COMMENT@The quota feature is still under development,
 206 @QUOTA_MAN_COMMENT@and may not be fully supported with your kernel
 207 @QUOTA_MAN_COMMENT@or may have various bugs.  Please
 208 @QUOTA_MAN_COMMENT@see https://ext4.wiki.kernel.org/index.php/Quota
 209 @QUOTA_MAN_COMMENT@for more details.
 210 .TP
 211 .B resize_inode
 212 .br
 213 This file system feature indicates that space has been reserved so
 214 the block group descriptor table can be extended by the file system is
 215 resized while the file system is mounted.  The online resize operation
 216 is carried out by the kernel, triggered, by
 217 .BR resize2fs (8).
 218 By default
 219 .B mke2fs
 220 will attempt to reserve enough space so that the
 221 filesystem may grow to 1024 times its initial size.  This can be changed
 222 using the
 223 .B resize
 224 extended option.
 225 .IP
 226 This feature requires that the
 227 .B sparse_super
 228 feature be enabled.
 229 .TP
 230 .B sparse_super
 231 .br
 232 This file system feature is set on all modern ext2, ext3, and ext4 file
 233 system.  It indicates that backup copies of the superblock and block
 234 group descriptors be present only on a few block groups, and not all of
 235 them.
 236 .TP
 237 .B uninit_bg
 238 .br
 239 This ext4 file system feature indicates that the block group descriptors
 240 will be protected using checksums, making it safe for
 241 .BR mke2fs (8)
 242 to create a file system without initializing all of the block groups.
 243 The kernel will keep a high watermark of unused inodes, and initialize
 244 inode tables and block lazily.  This feature speeds up the time to check
 245 the file system using
 246 .BR e2fsck (8),
 247 and it also speeds up the time required for
 248 .BR mke2fs (8)
 249 to create the file system.
 250 .RE
 251 .SH SEE ALSO
 252 .BR mke2fs (8),
 253 .BR mke2fs.conf (5),
 254 .BR e2fsck (8),
 255 .BR dumpe2fs (8),
 256 .BR tune2fs (8),
 257 .BR debugfs (8)