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Based on kernel version 3.9. Page generated on 2013-05-02 23:06 EST.

	
	Ext4 Filesystem
	===============
	
	Ext4 is an 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
	
	
	1. 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
	
	  - Compile and install the latest version of e2fsprogs (as of this
	    writing version 1.41.3) from:
	
	    http://sourceforge.net/project/showfiles.php?group_id=2406
		
		or
	
	    ftp://ftp.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs/
	
		or grab the latest git repository from:
	
	    git://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git
	
	  - Note that it is highly important to install the mke2fs.conf file
	    that comes with the e2fsprogs 1.41.x sources in /etc/mke2fs.conf. If
	    you have edited the /etc/mke2fs.conf file installed on your system,
	    you will need to merge your changes with the version from e2fsprogs
	    1.41.x.
	
	  - 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
	
	    (Note: we currently do not have tools to convert an ext4
	    filesystem back to ext3; so please do not do try this on production
	    filesystems.)
	
	  - 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.
	
	2. Features
	===========
	
	2.1 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.
	
	2.2 Candidate features for future inclusion
	
	* Online defrag (patches available but not well tested)
	* reduced mke2fs time via lazy itable initialization in conjunction with
	  the uninit_bg feature (capability to do this is available in e2fsprogs
	  but a kernel thread to do lazy zeroing of unused inode table blocks
	  after filesystem is first mounted is required for safety)
	
	There are several others under discussion, whether they all make it in is
	partly a function of how much time everyone has to work on them. Features like
	metadata checksumming have been discussed and planned for a bit but no patches
	exist yet so I'm not sure they're in the near-term roadmap.
	
	The big performance win will come with mballoc, delalloc and flex_bg
	grouping of bitmaps and inode tables.  Some test results available here:
	
	 - http://www.bullopensource.org/ext4/20080818-ffsb/ffsb-write-2.6.27-rc1.html
	 - http://www.bullopensource.org/ext4/20080818-ffsb/ffsb-readwrite-2.6.27-rc1.html
	
	3. Options
	==========
	
	When mounting an ext4 filesystem, the following option are accepted:
	(*) == default
	
	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_dev=devnum	When the external journal device's major/minor numbers
				have changed, this option allows the user to specify
				the new journal location.  The journal device is
				identified through its new major/minor numbers encoded
				in devnum.
	
	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 and http://acl.bestbits.at/
				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 and http://acl.bestbits.at/ 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			Give objects the same group ID as their creator.
	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		This options allows to enables/disables the in-kernel
	noblock_validity	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		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.
	
	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.
	..............................................................................
	
	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.
	
	..............................................................................
	
	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

• [ filesystems ]
• 00-INDEX
• 9p.txt
• adfs.txt
• affs.txt
• afs.txt
• autofs4-mount-control.txt
• automount-support.txt
• befs.txt
• bfs.txt
• btrfs.txt
• [ caching ]
• ceph.txt
• cifs.txt
• coda.txt
• [ configfs ]
• cramfs.txt
• debugfs.txt
• devpts.txt
• directory-locking
• dlmfs.txt
• dnotify.txt
• dnotify_test.c
• ecryptfs.txt
• efivarfs.txt
• exofs.txt
• ext2.txt
• ext3.txt
• ext4.txt
• f2fs.txt
• fiemap.txt
• files.txt
• fuse.txt
• gfs2-glocks.txt
• gfs2-uevents.txt
• gfs2.txt
• hfs.txt
• hfsplus.txt
• hpfs.txt
• inotify.txt
• isofs.txt
• jfs.txt
• Locking
• locks.txt
• logfs.txt
• Makefile
• mandatory-locking.txt
• ncpfs.txt
• [ nfs ]
• nilfs2.txt
• ntfs.txt
• ocfs2.txt
• omfs.txt
• path-lookup.txt
• [ pohmelfs ]
• porting
• proc.txt
• qnx6.txt
• quota.txt
• ramfs-rootfs-initramfs.txt
• relay.txt
• romfs.txt
• seq_file.txt
• sharedsubtree.txt
• spufs.txt
• squashfs.txt
• sysfs-pci.txt
• sysfs-tagging.txt
• sysfs.txt
• sysv-fs.txt
• tmpfs.txt
• ubifs.txt
• udf.txt
• ufs.txt
• vfat.txt
• vfs.txt
• xfs-delayed-logging-design.txt
• xfs.txt
• xip.txt
•