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EXT4(5) File Formats Manual EXT4(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 pre-
viously 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 imple-
mentations of the ext2, ext3, and ext4 file system drivers, depending on Linux kernel ver-
sion 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 fea-
ture 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, espe-
cially 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 nodelallocmountop-
tion).
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
Normally ext4 allows an inode to have no more than 65,000 hard links.
This applies to files as well as directories, which means that there
can be no more than 64,998 subdirectories in a directory (because each
of the '..' entries counts as a hard link). This feature lifts this
limit by causing ext4 to use a links count of 1 to indicate that the
number of hard links to a directory is not known.
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 sys-
tems. 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 com-
patibility 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 meta-
data (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 filesys-
tem 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 cre-
ated. 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 descrip-
tors. This scheme is also used to resize file sys-
tems 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 neces-
sary 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 stor-
age environments.
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 oper-
ation 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 behav-
ior 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 ker-
nel 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 con-
tinue, 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 sup-
port. 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 avail-
able 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. Other-
wise, it specifies the number of the inode which will represent
the ext3 filesystem's journal file; ext3 will create a new jour-
nal, 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 filesys-
tem, pass the mode to the kernel as boot parameter, e.g. root-
flags=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 quo-
tas 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 cor-
ruption 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 descrip-
tor 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 filesys-
tem 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 auto-
matically 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 oper-
ations will join the transaction. The commit time is capped by
the max_batch_time, which defaults to 15000 us (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 pur-
poses. 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 exist-
ing 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 mil-
liseconds 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 unini-
tialized 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 environ-
ments, 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 1.42.9 December 2013 EXT4(5)
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On Apache
Under GNU General Public License
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