Provided by: e2fsprogs_1.42.13-1ubuntu1.2_amd64 
NAME
ext2 - the second extended file system
ext2 - 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 handle mount requests for ext2 and ext3 file systems.
FILE SYSTEM FEATURES
A file system formated for ext2, ext3, or ext4 can be have some collection of the follow
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 -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
nodelallocmountoption).
This feature requires that the extent features 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.
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 current need to reserve this much
space. Without this feature, the kernel will reserve the amount of
space for features currently 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.
ext_attr
This feature enables the use of extended attributes. This feature is
supported by ext2, ext3, and ext4.
filetype
This feature enables the storage 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. 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.
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.
sparse_super2
This feature indicates that there will only at most two backup
superblock and block group descriptors. The block groups used to store
the backup superblock and blockgroup descriptors 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 is feature is essentially a more extreme version of sparse_super
and is designed to allow the a much larger percentage of the disk to
have contiguous blocks available for data files.
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 descriptor 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.
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.
Causes the quota files (i.e., user.quota and group.quota which existed
in the older quota design) to be hidden inodes.
resize_inode
This file system feature indicates that space has been reserved so the
block group descriptor table can be extended by the file system is
resized while the file system is mounted. 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
system. It indicates that backup copies of the superblock and block
group descriptors be present only on a few block groups, and not all of
them.
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 block 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).
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=update
Update the ext3 filesystem's journal to the current format.
journal=inum
When a journal already exists, this option is ignored. Otherwise, it specifies the
number of the inode which will represent the ext3 filesystem's journal file; ext3
will create a new journal, overwriting the old contents of the file whose inode
number is inum.
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.
acl Enable POSIX Access Control Lists. See the acl(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, 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 µs (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.
nouid32
Disables 32-bit UIDs and GIDs. This is for interoperability with older kernels
which only store and expect 16-bit values.
block_validity/noblock_validity
This options allows to 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.
SEE ALSO
mke2fs(8), mke2fs.conf(5), e2fsck(8), dumpe2fs(8), tune2fs(8), debugfs(8), mount(8),
chattr(1)