linux_kernel/Documentation/filesystems/ext4/ext4.rst

.. SPDX-License-Identifier: GPL-2.0

========================
General Information
========================

Ext4 is an advanced level of the ext3 filesystem which incorporates
scalability and reliability enhancements for supporting large filesystems
(64 bit) in keeping with increasing disk capacities and state-of-the-art
feature requirements.

Mailing list:	linux-ext4@vger.kernel.org
Web site:	http://ext4.wiki.kernel.org


Quick usage instructions
========================

Note: More extensive information for getting started with ext4 can be
found at the ext4 wiki site at the URL:
http://ext4.wiki.kernel.org/index.php/Ext4_Howto

  - The latest version of e2fsprogs can be found at:

    https://www.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs/

	or

    http://sourceforge.net/project/showfiles.php?group_id=2406

	or grab the latest git repository from:

   https://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git

  - Create a new filesystem using the ext4 filesystem type:

        # mke2fs -t ext4 /dev/hda1

    Or to configure an existing ext3 filesystem to support extents:

	# tune2fs -O extents /dev/hda1

    If the filesystem was created with 128 byte inodes, it can be
    converted to use 256 byte for greater efficiency via:

        # tune2fs -I 256 /dev/hda1

  - Mounting:

	# mount -t ext4 /dev/hda1 /wherever

  - When comparing performance with other filesystems, it's always
    important to try multiple workloads; very often a subtle change in a
    workload parameter can completely change the ranking of which
    filesystems do well compared to others.  When comparing versus ext3,
    note that ext4 enables write barriers by default, while ext3 does
    not enable write barriers by default.  So it is useful to use
    explicitly specify whether barriers are enabled or not when via the
    '-o barriers=[0|1]' mount option for both ext3 and ext4 filesystems
    for a fair comparison.  When tuning ext3 for best benchmark numbers,
    it is often worthwhile to try changing the data journaling mode; '-o
    data=writeback' can be faster for some workloads.  (Note however that
    running mounted with data=writeback can potentially leave stale data
    exposed in recently written files in case of an unclean shutdown,
    which could be a security exposure in some situations.)  Configuring
    the filesystem with a large journal can also be helpful for
    metadata-intensive workloads.

Features
========

Currently Available
-------------------

* ability to use filesystems > 16TB (e2fsprogs support not available yet)
* extent format reduces metadata overhead (RAM, IO for access, transactions)
* extent format more robust in face of on-disk corruption due to magics,
* internal redundancy in tree
* improved file allocation (multi-block alloc)
* lift 32000 subdirectory limit imposed by i_links_count[1]
* nsec timestamps for mtime, atime, ctime, create time
* inode version field on disk (NFSv4, Lustre)
* reduced e2fsck time via uninit_bg feature
* journal checksumming for robustness, performance
* persistent file preallocation (e.g for streaming media, databases)
* ability to pack bitmaps and inode tables into larger virtual groups via the
  flex_bg feature
* large file support
* inode allocation using large virtual block groups via flex_bg
* delayed allocation
* large block (up to pagesize) support
* efficient new ordered mode in JBD2 and ext4 (avoid using buffer head to force
  the ordering)

[1] Filesystems with a block size of 1k may see a limit imposed by the
directory hash tree having a maximum depth of two.

Options
=======

When mounting an ext4 filesystem, the following option are accepted:
(*) == default

======================= =======================================================
Mount Option            Description
======================= =======================================================
ro                   	Mount filesystem read only. Note that ext4 will
                     	replay the journal (and thus write to the
                     	partition) even when mounted "read only". The
                     	mount options "ro,noload" can be used to prevent
		     	writes to the filesystem.

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.

journal_path=path
journal_dev=devnum	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 through either its new major/minor numbers
			encoded in devnum, or via a path to the device.

norecovery		Don't load the journal on mounting.  Note that
noload			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		All data are committed into the journal prior to being
			written into the main file system.  Enabling
			this mode will disable delayed allocation and
			O_DIRECT support.

data=ordered	(*)	All data are forced directly out to the main file
			system prior to its metadata being committed to the
			journal.

data=writeback		Data ordering is not preserved, data may be written
			into the main file system after its metadata has been
			committed to the journal.

commit=nrsec	(*)	Ext4 can be told to sync all its data and metadata
			every 'nrsec' seconds. The default value is 5 seconds.
			This means that if you lose your power, you will lose
			as much as the latest 5 seconds of work (your
			filesystem will not be damaged though, thanks to the
			journaling).  This default value (or any low value)
			will hurt performance, but it's good for data-safety.
			Setting it to 0 will have the same effect as leaving
			it at the default (5 seconds).
			Setting it to very large values will improve
			performance.

barrier=<0|1(*)>	This enables/disables the use of write barriers in
barrier(*)		the jbd code.  barrier=0 disables, barrier=1 enables.
nobarrier		This also requires an IO stack which can support
			barriers, and if jbd gets an error on a barrier
			write, it will disable 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.
			The mount options "barrier" and "nobarrier" can
			also be used to enable or disable barriers, for
			consistency with other ext4 mount options.

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 default value is 32 blocks.

nouser_xattr		Disables Extended User Attributes.  See the
			attr(5) manual page for more information about
			extended attributes.

noacl			This option disables POSIX Access Control List
			support. If ACL support is enabled in the kernel
			configuration (CONFIG_EXT4_FS_POSIX_ACL), ACL is
			enabled by default on mount. See the acl(5) manual
			page for more information about acl.

bsddf		(*)	Make 'df' act like BSD.
minixdf			Make 'df' act like Minix.

debug			Extra debugging information is sent to syslog.

abort			Simulate the effects of calling ext4_abort() for
			debugging purposes.  This is normally used while
			remounting a filesystem which is already mounted.

errors=remount-ro	Remount the filesystem read-only on an error.
errors=continue		Keep going on a filesystem error.
errors=panic		Panic and halt the machine if an error occurs.
                        (These mount options override the errors behavior
                        specified in the superblock, which can be configured
                        using tune2fs)

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.

grpid			New objects have the group ID of their parent.
bsdgroups

nogrpid		(*)	New objects have the group ID of their creator.
sysvgroups

resgid=n		The group ID which may use the reserved blocks.

resuid=n		The user ID which may use the reserved blocks.

sb=n			Use alternate superblock at this location.

quota			These options are ignored by the filesystem. They
noquota			are used only by quota tools to recognize volumes
grpquota		where quota should be turned on. See documentation
usrquota		in the quota-tools package for more details
			(http://sourceforge.net/projects/linuxquota).

jqfmt=<quota type>	These options tell filesystem details about quota
usrjquota=<file>	so that quota information can be properly updated
grpjquota=<file>	during journal replay. They replace the above
			quota options. See documentation in the quota-tools
			package for more details
			(http://sourceforge.net/projects/linuxquota).

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 file system blocks.

delalloc	(*)	Defer block allocation until just before ext4
			writes out the block(s) in question.  This
			allows ext4 to better allocation decisions
			more efficiently.
nodelalloc		Disable delayed allocation.  Blocks are allocated
			when the data is copied from userspace to the
			page cache, either via the write(2) system call
			or when an mmap'ed page which was previously
			unallocated is written for the first time.

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 15000us
			(15ms).   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.

auto_da_alloc(*)	Many broken applications don't use fsync() when 
noauto_da_alloc		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  file system
			is unmounted.

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 the system performance
			while file system's inode table is being initialized.

discard			Controls whether ext4 should issue discard/TRIM
nodiscard(*)		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(*)	These options enable or disable the in-kernel
noblock_validity	facility for tracking filesystem metadata blocks
			within internal data structures.  This allows multi-
			block allocator and other routines to notice
			bugs or corrupted allocation bitmaps which cause
			blocks to be allocated which overlap with
			filesystem metadata blocks.

dioread_lock		Controls whether or not ext4 should use the DIO read
dioread_nolock		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 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 512mb memory available, a 176mb
			directory may seriously cramp the system's style.)

i_version		Enable 64-bit inode version support. This option is
			off by default.

dax			Use direct access (no page cache).  See
			Documentation/filesystems/dax.txt.  Note that
			this option is incompatible with data=journal.
======================= =======================================================

Data Mode
=========
There are 3 different data modes:

* writeback mode

  In data=writeback mode, ext4 does not journal data at all.  This mode provides
  a similar level of journaling as that of XFS, JFS, and ReiserFS in its default
  mode - metadata journaling.  A crash+recovery can cause incorrect data to
  appear in files which were written shortly before the crash.  This mode will
  typically provide the best ext4 performance.

* ordered mode

  In data=ordered mode, ext4 only officially journals metadata, but it logically
  groups metadata information related to data changes with the data blocks into
  a single unit called a transaction.  When it's time to write the new metadata
  out to disk, the associated data blocks are written first.  In general, this
  mode performs slightly slower than writeback but significantly faster than
  journal mode.

* journal mode

  data=journal mode provides full data and metadata journaling.  All new data is
  written to the journal first, and then to its final location.  In the event of
  a crash, the journal can be replayed, bringing both data and metadata into a
  consistent state.  This mode is the slowest except when data needs to be read
  from and written to disk at the same time where it outperforms all others
  modes.  Enabling this mode will disable delayed allocation and O_DIRECT
  support.

/proc entries
=============

Information about mounted ext4 file systems can be found in
/proc/fs/ext4.  Each mounted filesystem will have a directory in
/proc/fs/ext4 based on its device name (i.e., /proc/fs/ext4/hdc or
/proc/fs/ext4/dm-0).   The files in each per-device directory are shown
in table below.

Files in /proc/fs/ext4/<devname>

================ =======
 File            Content
================ =======
 mb_groups       details of multiblock allocator buddy cache of free blocks
================ =======

/sys entries
============

Information about mounted ext4 file systems can be found in
/sys/fs/ext4.  Each mounted filesystem will have a directory in
/sys/fs/ext4 based on its device name (i.e., /sys/fs/ext4/hdc or
/sys/fs/ext4/dm-0).   The files in each per-device directory are shown
in table below.

Files in /sys/fs/ext4/<devname>:

(see also Documentation/ABI/testing/sysfs-fs-ext4)

============================= =================================================
File                          Content
============================= =================================================
 delayed_allocation_blocks    This file is read-only and shows the number of
                              blocks that are dirty in the page cache, but
                              which do not have their location in the
                              filesystem allocated yet.

inode_goal                    Tuning parameter which (if non-zero) controls
                              the goal inode used by the inode allocator in
                              preference to all other allocation heuristics.
                              This is intended for debugging use only, and
                              should be 0 on production systems.

inode_readahead_blks          Tuning parameter which controls the maximum
                              number of inode table blocks that ext4's inode
                              table readahead algorithm will pre-read into
                              the buffer cache

lifetime_write_kbytes         This file is read-only and shows the number of
                              kilobytes of data that have been written to this
                              filesystem since it was created.

 max_writeback_mb_bump        The maximum number of megabytes the writeback
                              code will try to write out before move on to
                              another inode.

 mb_group_prealloc            The multiblock allocator will round up allocation
                              requests to a multiple of this tuning parameter if
                              the stripe size is not set in the ext4 superblock

 mb_max_to_scan               The maximum number of extents the multiblock
                              allocator will search to find the best extent

 mb_min_to_scan               The minimum number of extents the multiblock
                              allocator will search to find the best extent

 mb_order2_req                Tuning parameter which controls the minimum size
                              for requests (as a power of 2) where the buddy
                              cache is used

 mb_stats                     Controls whether the multiblock allocator should
                              collect statistics, which are shown during the
                              unmount. 1 means to collect statistics, 0 means
                              not to collect statistics

 mb_stream_req                Files which have fewer blocks than this tunable
                              parameter will have their blocks allocated out
                              of a block group specific preallocation pool, so
                              that small files are packed closely together.
                              Each large file will have its blocks allocated
                              out of its own unique preallocation pool.

 session_write_kbytes         This file is read-only and shows the number of
                              kilobytes of data that have been written to this
                              filesystem since it was mounted.

 reserved_clusters            This is RW file and contains number of reserved
                              clusters in the file system which will be used
                              in the specific situations to avoid costly
                              zeroout, unexpected ENOSPC, or possible data
                              loss. The default is 2% or 4096 clusters,
                              whichever is smaller and this can be changed
                              however it can never exceed number of clusters
                              in the file system. If there is not enough space
                              for the reserved space when mounting the file
                              mount will _not_ fail.
============================= =================================================

Ioctls
======

There is some Ext4 specific functionality which can be accessed by applications
through the system call interfaces. The list of all Ext4 specific ioctls are
shown in the table below.

Table of Ext4 specific ioctls

============================= =================================================
Ioctl			      Description
============================= =================================================
 EXT4_IOC_GETFLAGS	      Get additional attributes associated with inode.
			      The ioctl argument is an integer bitfield, with
			      bit values described in ext4.h. This ioctl is an
			      alias for FS_IOC_GETFLAGS.

 EXT4_IOC_SETFLAGS	      Set additional attributes associated with inode.
			      The ioctl argument is an integer bitfield, with
			      bit values described in ext4.h. This ioctl is an
			      alias for FS_IOC_SETFLAGS.

 EXT4_IOC_GETVERSION
 EXT4_IOC_GETVERSION_OLD
			      Get the inode i_generation number stored for
			      each inode. The i_generation number is normally
			      changed only when new inode is created and it is
			      particularly useful for network filesystems. The
			      '_OLD' version of this ioctl is an alias for
			      FS_IOC_GETVERSION.

 EXT4_IOC_SETVERSION
 EXT4_IOC_SETVERSION_OLD
			      Set the inode i_generation number stored for
			      each inode. The '_OLD' version of this ioctl
			      is an alias for FS_IOC_SETVERSION.

 EXT4_IOC_GROUP_EXTEND	      This ioctl has the same purpose as the resize
			      mount option. It allows to resize filesystem
			      to the end of the last existing block group,
			      further resize has to be done with resize2fs,
			      either online, or offline. The argument points
			      to the unsigned logn number representing the
			      filesystem new block count.

 EXT4_IOC_MOVE_EXT	      Move the block extents from orig_fd (the one
			      this ioctl is pointing to) to the donor_fd (the
			      one specified in move_extent structure passed
			      as an argument to this ioctl). Then, exchange
			      inode metadata between orig_fd and donor_fd.
			      This is especially useful for online
			      defragmentation, because the allocator has the
			      opportunity to allocate moved blocks better,
			      ideally into one contiguous extent.

 EXT4_IOC_GROUP_ADD	      Add a new group descriptor to an existing or
			      new group descriptor block. The new group
			      descriptor is described by ext4_new_group_input
			      structure, which is passed as an argument to
			      this ioctl. This is especially useful in
			      conjunction with EXT4_IOC_GROUP_EXTEND,
			      which allows online resize of the filesystem
			      to the end of the last existing block group.
			      Those two ioctls combined is used in userspace
			      online resize tool (e.g. resize2fs).

 EXT4_IOC_MIGRATE	      This ioctl operates on the filesystem itself.
			      It converts (migrates) ext3 indirect block mapped
			      inode to ext4 extent mapped inode by walking
			      through indirect block mapping of the original
			      inode and converting contiguous block ranges
			      into ext4 extents of the temporary inode. Then,
			      inodes are swapped. This ioctl might help, when
			      migrating from ext3 to ext4 filesystem, however
			      suggestion is to create fresh ext4 filesystem
			      and copy data from the backup. Note, that
			      filesystem has to support extents for this ioctl
			      to work.

 EXT4_IOC_ALLOC_DA_BLKS	      Force all of the delay allocated blocks to be
			      allocated to preserve application-expected ext3
			      behaviour. Note that this will also start
			      triggering a write of the data blocks, but this
			      behaviour may change in the future as it is
			      not necessary and has been done this way only
			      for sake of simplicity.

 EXT4_IOC_RESIZE_FS	      Resize the filesystem to a new size.  The number
			      of blocks of resized filesystem is passed in via
			      64 bit integer argument.  The kernel allocates
			      bitmaps and inode table, the userspace tool thus
			      just passes the new number of blocks.

 EXT4_IOC_SWAP_BOOT	      Swap i_blocks and associated attributes
			      (like i_blocks, i_size, i_flags, ...) from
			      the specified inode with inode
			      EXT4_BOOT_LOADER_INO (#5). This is typically
			      used to store a boot loader in a secure part of
			      the filesystem, where it can't be changed by a
			      normal user by accident.
			      The data blocks of the previous boot loader
			      will be associated with the given inode.
============================= =================================================

References
==========

kernel source:	<file:fs/ext4/>
		<file:fs/jbd2/>

programs:	http://e2fsprogs.sourceforge.net/

useful links:	http://fedoraproject.org/wiki/ext3-devel
		http://www.bullopensource.org/ext4/
		http://ext4.wiki.kernel.org/index.php/Main_Page
		http://fedoraproject.org/wiki/Features/Ext4