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)
E2fsprogs version 1.42.13 May 2015 EXT4(5)