ext2(5)
NAME
ext2 - the second extended file system ext3 - the third extended file system ext4 - the fourth extended file system
DESCRIPTION
The second, third, and fourth extended file systems, or ext2, ext3, and ext4 as they are commonly known, are Linux file systems that have historically been the default file system for many Linux distributions. They are general purpose file systems that have been designed for extensibility and backwards compatibility. In particular, file systems previously intended for use with the ext2 and ext3 file systems can be mounted using the ext4 file system driver, and indeed in many modern Linux distributions, the ext4 file system driver has been configured to handle mount requests for ext2 and ext3 file systems.
FILE SYSTEM FEATURES
A file system formatted for ext2, ext3, or ext4 can have some
collection of the following file system feature flags enabled. Some of
these features are not supported by all implementations of the ext2,
ext3, and ext4 file system drivers, depending on Linux kernel version
in use. On other operating systems, such as the GNU/HURD or FreeBSD,
only a very restrictive set of file system features may be supported in
their implementations of ext2.
64bit
Enables the file system to be larger than 2^32 blocks. This
feature is set automatically, as needed, but it can be useful to
specify this feature explicitly if the file system might need to
be resized larger than 2^32 blocks, even if it was smaller than
that threshold when it was originally created. Note that some
older kernels and older versions of e2fsprogs will not support
file systems with this ext4 feature enabled.
bigalloc
This ext4 feature enables clustered block allocation, so that
the unit of allocation is a power of two number of blocks. That
is, each bit in the what had traditionally been known as the
block allocation bitmap now indicates whether a cluster is in
use or not, where a cluster is by default composed of 16 blocks.
This feature can decrease the time spent on doing block
allocation and brings smaller fragmentation, especially for
large files. The size can be specified using the mke2fs -C
option.
Warning: The bigalloc feature is still under development, and
may not be fully supported with your kernel or may have various
bugs. Please see the web page
http://ext4.wiki.kernel.org/index.php/Bigalloc for details. May
clash with delayed allocation (see nodelalloc mount option).
This feature requires that the extent feature be enabled.
dir_index
Use hashed b-trees to speed up name lookups in large
directories. This feature is supported by ext3 and ext4 file
systems, and is ignored by ext2 file systems.
dir_nlink
This ext4 feature allows more than 65000 subdirectories per
directory.
encrypt
This ext4 feature provides file-system level encryption of data
blocks and file names. The inode metadata (timestamps, file
size, user/group ownership, etc.) is not encrypted.
This feature is most useful on file systems with multiple users,
or where not all files should be encrypted. In many use cases,
especially on single-user systems, encryption at the block
device layer using dm-crypt may provide much better security.
ext_attr
This feature enables the use of extended attributes. This
feature is supported by ext2, ext3, and ext4.
extent
This ext4 feature allows the mapping of logical block numbers
for a particular inode to physical blocks on the storage device
to be stored using an extent tree, which is a more efficient
data structure than the traditional indirect block scheme used
by the ext2 and ext3 file systems. The use of the extent tree
decreases metadata block overhead, improves file system
performance, and decreases the needed to run e2fsck(8) on the
file system. (Note: both extent and extents are accepted as
valid names for this feature for historical/backwards
compatibility reasons.)
extra_isize
This ext4 feature reserves a specific amount of space in each
inode for extended metadata such as nanosecond timestamps and
file creation time, even if the current kernel does not
currently need to reserve this much space. Without this
feature, the kernel will reserve the amount of space for
features it currently needs, and the rest may be consumed by
extended attributes.
For this feature to be useful the inode size must be 256 bytes
in size or larger.
filetype
This feature enables the storage of file type information in
directory entries. This feature is supported by ext2, ext3, and
ext4.
flex_bg
This ext4 feature allows the per-block group metadata
(allocation bitmaps and inode tables) to be placed anywhere on
the storage media. In addition, mke2fs will place the per-block
group metadata together starting at the first block group of
each "flex_bg group". The size of the flex_bg group can be
specified using the -G option.
has_journal
Create a journal to ensure filesystem consistency even across
unclean shutdowns. Setting the filesystem feature is equivalent
to using the -j option with mke2fs or tune2fs. This feature is
supported by ext3 and ext4, and ignored by the ext2 file system
driver.
huge_file
This ext4 feature allows files to be larger than 2 terabytes in
size.
inline_data
Allow data to be stored in the inode and extended attribute
area.
journal_dev
This feature is enabled on the superblock found on an external
journal device. The block size for the external journal must be
the same as the file system which uses it.
The external journal device can be used by a file system by
specifying the -J device=<external-device> option to mke2fs(8)
or tune2fs(8).
large_file
This feature flag is set automatically by modern kernels when a
file larger than 2 gigabytes is created. Very old kernels could
not handle large files, so this feature flag was used to
prohibit those kernels from mounting file systems that they
could not understand.
meta_bg
This ext4 feature allows file systems to be resized on-line
without explicitly needing to reserve space for growth in the
size of the block group descriptors. This scheme is also used
to resize file systems which are larger than 2^32 blocks. It is
not recommended that this feature be set when a file system is
created, since this alternate method of storing the block group
descriptors will slow down the time needed to mount the file
system, and newer kernels can automatically set this feature as
necessary when doing an online resize and no more reserved space
is available in the resize inode.
mmp
This ext4 feature provides multiple mount protection (MMP). MMP
helps to protect the filesystem from being multiply mounted and
is useful in shared storage environments.
Causes the quota files (i.e., user.quota and group.quota which
existed in the older quota design) to be hidden inodes.
project
This ext4 feature provides project quota support. With this
feature, the project ID of inode will be managed when the
filesystem is mounted.
quota
Create quota inodes (inode #3 for userquota and inode #4 for
group quota) and set them in the superblock. With this feature,
the quotas will be enabled automatically when the filesystem is
mounted.
resize_inode
This file system feature indicates that space has been reserved
so that the block group descriptor table can be extended while
resizing a mounted file system. The online resize operation is
carried out by the kernel, triggered by resize2fs(8). By
default mke2fs will attempt to reserve enough space so that the
filesystem may grow to 1024 times its initial size. This can be
changed using the resize extended option.
This feature requires that the sparse_super feature be enabled.
sparse_super
This file system feature is set on all modern ext2, ext3, and
ext4 file systems. It indicates that backup copies of the
superblock and block group descriptors are present only in a few
block groups, not all of them.
sparse_super2
This feature indicates that there will only be at most two
backup superblocks and block group descriptors. The block
groups used to store the backup superblock(s) and blockgroup
descriptor(s) are stored in the superblock, but typically, one
will be located at the beginning of block group #1, and one in
the last block group in the file system. This feature is
essentially a more extreme version of sparse_super and is
designed to allow a much larger percentage of the disk to have
contiguous blocks available for data files.
uninit_bg
This ext4 file system feature indicates that the block group
descriptors will be protected using checksums, making it safe
for mke2fs(8) to create a file system without initializing all
of the block groups. The kernel will keep a high watermark of
unused inodes, and initialize inode tables and blocks lazily.
This feature speeds up the time to check the file system using
e2fsck(8), and it also speeds up the time required for mke2fs(8)
to create the file system.
MOUNT OPTIONS
This section describes mount options which are specific to ext2, ext3, and ext4. Other generic mount options may be used as well; see mount(8) for details.
Mount options for ext2
The `ext2' filesystem is the standard Linux filesystem. Since Linux 2.5.46, for most mount options the default is determined by the filesystem superblock. Set them with tune2fs(8). acl|noacl Support POSIX Access Control Lists (or not). See the acl(5) manual page. bsddf|minixdf Set the behavior for the statfs system call. The minixdf behavior is to return in the f_blocks field the total number of blocks of the filesystem, while the bsddf behavior (which is the default) is to subtract the overhead blocks used by the ext2 filesystem and not available for file storage. Thus % mount /k -o minixdf; df /k; umount /k Filesystem 1024-blocks Used Available Capacity Mounted on /dev/sda6 2630655 86954 2412169 3% /k % mount /k -o bsddf; df /k; umount /k Filesystem 1024-blocks Used Available Capacity Mounted on /dev/sda6 2543714 13 2412169 0% /k (Note that this example shows that one can add command line options to the options given in /etc/fstab.) check=none or nocheck No checking is done at mount time. This is the default. This is fast. It is wise to invoke e2fsck(8) every now and then, e.g. at boot time. The non-default behavior is unsupported (check=normal and check=strict options have been removed). Note that these mount options don't have to be supported if ext4 kernel driver is used for ext2 and ext3 filesystems. debug Print debugging info upon each (re)mount. errors={continue|remount-ro|panic} Define the behavior when an error is encountered. (Either ignore errors and just mark the filesystem erroneous and continue, or remount the filesystem read-only, or panic and halt the system.) The default is set in the filesystem superblock, and can be changed using tune2fs(8). grpid|bsdgroups and nogrpid|sysvgroups These options define what group id a newly created file gets. When grpid is set, it takes the group id of the directory in which it is created; otherwise (the default) it takes the fsgid of the current process, unless the directory has the setgid bit set, in which case it takes the gid from the parent directory, and also gets the setgid bit set if it is a directory itself. grpquota|noquota|quota|usrquota The usrquota (same as quota) mount option enables user quota support on the filesystem. grpquota enables group quotas support. You need the quota utilities to actually enable and manage the quota system. nouid32 Disables 32-bit UIDs and GIDs. This is for interoperability with older kernels which only store and expect 16-bit values. oldalloc or orlov Use old allocator or Orlov allocator for new inodes. Orlov is default. resgid=n and resuid=n The ext2 filesystem reserves a certain percentage of the available space (by default 5%, see mke2fs(8) and tune2fs(8)). These options determine who can use the reserved blocks. (Roughly: whoever has the specified uid, or belongs to the specified group.) sb=n Instead of block 1, use block n as superblock. This could be useful when the filesystem has been damaged. (Earlier, copies of the superblock would be made every 8192 blocks: in block 1, 8193, 16385, ... (and one got thousands of copies on a big filesystem). Since version 1.08, mke2fs has a -s (sparse superblock) option to reduce the number of backup superblocks, and since version 1.15 this is the default. Note that this may mean that ext2 filesystems created by a recent mke2fs cannot be mounted r/w under Linux 2.0.*.) The block number here uses 1 k units. Thus, if you want to use logical block 32768 on a filesystem with 4 k blocks, use "sb=131072". user_xattr|nouser_xattr Support "user." extended attributes (or not).
Mount options for ext3
The ext3 filesystem is a version of the ext2 filesystem which has been
enhanced with journaling. It supports the same options as ext2 as well
as the following additions:
journal_dev=devnum/journal_path=path
When the external journal device's major/minor numbers have
changed, these options allow the user to specify the new journal
location. The journal device is identified either through its
new major/minor numbers encoded in devnum, or via a path to the
device.
norecovery/noload
Don't load the journal on mounting. Note that if the filesystem
was not unmounted cleanly, skipping the journal replay will lead
to the filesystem containing inconsistencies that can lead to
any number of problems.
data={journal|ordered|writeback}
Specifies the journaling mode for file data. Metadata is always
journaled. To use modes other than ordered on the root
filesystem, pass the mode to the kernel as boot parameter, e.g.
rootflags=data=journal.
journal
All data is committed into the journal prior to being
written into the main filesystem.
ordered
This is the default mode. All data is forced directly
out to the main file system prior to its metadata being
committed to the journal.
writeback
Data ordering is not preserved -- data may be written into
the main filesystem after its metadata has been committed
to the journal. This is rumoured to be the highest-
throughput option. It guarantees internal filesystem
integrity, however it can allow old data to appear in
files after a crash and journal recovery.
data_err=ignore
Just print an error message if an error occurs in a file data
buffer in ordered mode.
data_err=abort
Abort the journal if an error occurs in a file data buffer in
ordered mode.
barrier=0 / barrier=1
This disables / enables the use of write barriers in the jbd
code. barrier=0 disables, barrier=1 enables (default). This
also requires an IO stack which can support barriers, and if jbd
gets an error on a barrier write, it will disable barriers again
with a warning. Write barriers enforce proper on-disk ordering
of journal commits, making volatile disk write caches safe to
use, at some performance penalty. If your disks are battery-
backed in one way or another, disabling barriers may safely
improve performance.
commit=nrsec
Sync all data and metadata every nrsec seconds. The default
value is 5 seconds. Zero means default.
user_xattr
Enable Extended User Attributes. See the attr(5) manual page.
usrjquota=aquota.user|grpjquota=aquota.group|jqfmt=vfsv0
Apart from the old quota system (as in ext2, jqfmt=vfsold aka
version 1 quota) ext3 also supports journaled quotas (version 2
quota). jqfmt=vfsv0 enables journaled quotas. For journaled
quotas the mount options usrjquota=aquota.user and
grpjquota=aquota.group are required to tell the quota system
which quota database files to use. Journaled quotas have the
advantage that even after a crash no quota check is required.
Mount options for ext4
The ext4 filesystem is an advanced level of the ext3 filesystem which
incorporates scalability and reliability enhancements for supporting
large filesystem.
The options journal_dev, norecovery, noload, data, commit, orlov,
oldalloc, [no]user_xattr [no]acl, bsddf, minixdf, debug, errors,
data_err, grpid, bsdgroups, nogrpid sysvgroups, resgid, resuid, sb,
quota, noquota, nouid32, grpquota, usrquota usrjquota, grpjquota and
jqfmt are backwardly compatible with ext3 or ext2.
journal_checksum
Enable checksumming of the journal transactions. This will
allow the recovery code in e2fsck and the kernel to detect
corruption in the kernel. It is a compatible change and will be
ignored by older kernels.
journal_async_commit
Commit block can be written to disk without waiting for
descriptor blocks. If enabled older kernels cannot mount the
device. This will enable 'journal_checksum' internally.
barrier=0 / barrier=1 / barrier / nobarrier
These mount options have the same effect as in ext3. The mount
options "barrier" and "nobarrier" are added for consistency with
other ext4 mount options.
The ext4 filesystem enables write barriers by default.
inode_readahead_blks=n
This tuning parameter controls the maximum number of inode table
blocks that ext4's inode table readahead algorithm will pre-read
into the buffer cache. The value must be a power of 2. The
default value is 32 blocks.
stripe=n
Number of filesystem blocks that mballoc will try to use for
allocation size and alignment. For RAID5/6 systems this should
be the number of data disks * RAID chunk size in filesystem
blocks.
delalloc
Deferring block allocation until write-out time.
nodelalloc
Disable delayed allocation. Blocks are allocated when data is
copied from user to page cache.
max_batch_time=usec
Maximum amount of time ext4 should wait for additional
filesystem operations to be batch together with a synchronous
write operation. Since a synchronous write operation is going to
force a commit and then a wait for the I/O complete, it doesn't
cost much, and can be a huge throughput win, we wait for a small
amount of time to see if any other transactions can piggyback on
the synchronous write. The algorithm used is designed to
automatically tune for the speed of the disk, by measuring the
amount of time (on average) that it takes to finish committing a
transaction. Call this time the "commit time". If the time that
the transaction has been running is less than the commit time,
ext4 will try sleeping for the commit time to see if other
operations will join the transaction. The commit time is capped
by the max_batch_time, which defaults to 15000 s (15 ms). This
optimization can be turned off entirely by setting
max_batch_time to 0.
min_batch_time=usec
This parameter sets the commit time (as described above) to be
at least min_batch_time. It defaults to zero microseconds.
Increasing this parameter may improve the throughput of multi-
threaded, synchronous workloads on very fast disks, at the cost
of increasing latency.
journal_ioprio=prio
The I/O priority (from 0 to 7, where 0 is the highest priority)
which should be used for I/O operations submitted by kjournald2
during a commit operation. This defaults to 3, which is a
slightly higher priority than the default I/O priority.
abort Simulate the effects of calling ext4_abort() for debugging
purposes. This is normally used while remounting a filesystem
which is already mounted.
auto_da_alloc|noauto_da_alloc
Many broken applications don't use fsync() when replacing
existing files via patterns such as
fd = open("foo.new")/write(fd,...)/close(fd)/ rename("foo.new",
"foo")
or worse yet
fd = open("foo", O_TRUNC)/write(fd,...)/close(fd).
If auto_da_alloc is enabled, ext4 will detect the replace-via-
rename and replace-via-truncate patterns and force that any
delayed allocation blocks are allocated such that at the next
journal commit, in the default data=ordered mode, the data
blocks of the new file are forced to disk before the rename()
operation is committed. This provides roughly the same level of
guarantees as ext3, and avoids the "zero-length" problem that
can happen when a system crashes before the delayed allocation
blocks are forced to disk.
noinit_itable
Do not initialize any uninitialized inode table blocks in the
background. This feature may be used by installation CD's so
that the install process can complete as quickly as possible;
the inode table initialization process would then be deferred
until the next time the filesystem is mounted.
init_itable=n
The lazy itable init code will wait n times the number of
milliseconds it took to zero out the previous block group's
inode table. This minimizes the impact on system performance
while the filesystem's inode table is being initialized.
discard/nodiscard
Controls whether ext4 should issue discard/TRIM commands to the
underlying block device when blocks are freed. This is useful
for SSD devices and sparse/thinly-provisioned LUNs, but it is
off by default until sufficient testing has been done.
block_validity/noblock_validity
This options enables/disables the in-kernel facility for
tracking filesystem metadata blocks within internal data
structures. This allows multi-block allocator and other routines
to quickly locate extents which might overlap with filesystem
metadata blocks. This option is intended for debugging purposes
and since it negatively affects the performance, it is off by
default.
dioread_lock/dioread_nolock
Controls whether or not ext4 should use the DIO read locking. If
the dioread_nolock option is specified ext4 will allocate
uninitialized extent before buffer write and convert the extent
to initialized after IO completes. This approach allows ext4
code to avoid using inode mutex, which improves scalability on
high speed storages. However this does not work with data
journaling and dioread_nolock option will be ignored with kernel
warning. Note that dioread_nolock code path is only used for
extent-based files. Because of the restrictions this options
comprises it is off by default (e.g. dioread_lock).
max_dir_size_kb=n
This limits the size of the directories so that any attempt to
expand them beyond the specified limit in kilobytes will cause
an ENOSPC error. This is useful in memory-constrained
environments, where a very large directory can cause severe
performance problems or even provoke the Out Of Memory killer.
(For example, if there is only 512 MB memory available, a 176 MB
directory may seriously cramp the system's style.)
i_version
Enable 64-bit inode version support. This option is off by
default.
FILE ATTRIBUTES
The ext2, ext3, and ext4 filesystems support setting the following file attributes on Linux systems using the chattr(1) utility: a - append only A - no atime updates d - no dump D - synchronous directory updates i - immutable S - synchronous updates u - undeletable In addition, the ext3 and ext4 filesystems support the following flag: j - data journaling Finally, the ext4 filesystem also supports the following flag: e - extents format For descriptions of these attribute flags, please refer to the chattr(1) man page.
KERNEL SUPPORT
This section lists the file system driver (e.g., ext2, ext3, ext4) and upstream kernel version where a particular file system feature was supported. Note that in some cases the feature was present in earlier kernel versions, but there were known, serious bugs. In other cases the feature may still be considered in an experimental state. Finally, note that some distributions may have backported features into older kernels; in particular the kernel versions in certain "enterprise distributions" can be extremely misleading. filetype ext2, 2.2.0 sparse_super ext2, 2.2.0 large_file ext2, 2.2.0 has_journal ext3, 2.4.15 ext_attr ext2/ext3, 2.6.0 dir_index ext3, 2.6.0 resize_inode ext3, 2.6.10 (online resizing) 64bit ext4, 2.6.28 dir_nlink ext4, 2.6.28 extent ext4, 2.6.28 extra_isize ext4, 2.6.28 flex_bg ext4, 2.6.28 huge_file ext4, 2.6.28 meta_bg ext4, 2.6.28 uninit_bg ext4, 2.6.28 mmp ext4, 3.0 bigalloc ext4, 3.2 quota ext4, 3.6 inline_data ext4, 3.8 sparse_super2 ext4, 3.16 metdata_csum ext4, 3.18 encrypt ext4, 4.1 project ext4, 4.5
SEE ALSO
mke2fs(8), mke2fs.conf(5), e2fsck(8), dumpe2fs(8), tune2fs(8), debugfs(8), mount(8), chattr(1)
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