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Based on kernel version 4.2. Page generated on 2015-09-09 12:13 EST.

2	Ext4 Filesystem
3	===============
5	Ext4 is an advanced level of the ext3 filesystem which incorporates
6	scalability and reliability enhancements for supporting large filesystems
7	(64 bit) in keeping with increasing disk capacities and state-of-the-art
8	feature requirements.
10	Mailing list:	linux-ext4@vger.kernel.org
11	Web site:	http://ext4.wiki.kernel.org
14	1. Quick usage instructions:
15	===========================
17	Note: More extensive information for getting started with ext4 can be
18	      found at the ext4 wiki site at the URL:
19	      http://ext4.wiki.kernel.org/index.php/Ext4_Howto
21	  - Compile and install the latest version of e2fsprogs (as of this
22	    writing version 1.41.3) from:
24	    http://sourceforge.net/project/showfiles.php?group_id=2406
26		or
28	    ftp://ftp.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs/
30		or grab the latest git repository from:
32	    git://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git
34	  - Note that it is highly important to install the mke2fs.conf file
35	    that comes with the e2fsprogs 1.41.x sources in /etc/mke2fs.conf. If
36	    you have edited the /etc/mke2fs.conf file installed on your system,
37	    you will need to merge your changes with the version from e2fsprogs
38	    1.41.x.
40	  - Create a new filesystem using the ext4 filesystem type:
42	    	# mke2fs -t ext4 /dev/hda1
44	    Or to configure an existing ext3 filesystem to support extents: 
46		# tune2fs -O extents /dev/hda1
48	    If the filesystem was created with 128 byte inodes, it can be
49	    converted to use 256 byte for greater efficiency via:
51	        # tune2fs -I 256 /dev/hda1
53	    (Note: we currently do not have tools to convert an ext4
54	    filesystem back to ext3; so please do not do try this on production
55	    filesystems.)
57	  - Mounting:
59		# mount -t ext4 /dev/hda1 /wherever
61	  - When comparing performance with other filesystems, it's always
62	    important to try multiple workloads; very often a subtle change in a
63	    workload parameter can completely change the ranking of which
64	    filesystems do well compared to others.  When comparing versus ext3,
65	    note that ext4 enables write barriers by default, while ext3 does
66	    not enable write barriers by default.  So it is useful to use
67	    explicitly specify whether barriers are enabled or not when via the
68	    '-o barriers=[0|1]' mount option for both ext3 and ext4 filesystems
69	    for a fair comparison.  When tuning ext3 for best benchmark numbers,
70	    it is often worthwhile to try changing the data journaling mode; '-o
71	    data=writeback' can be faster for some workloads.  (Note however that
72	    running mounted with data=writeback can potentially leave stale data
73	    exposed in recently written files in case of an unclean shutdown,
74	    which could be a security exposure in some situations.)  Configuring
75	    the filesystem with a large journal can also be helpful for
76	    metadata-intensive workloads.
78	2. Features
79	===========
81	2.1 Currently available
83	* ability to use filesystems > 16TB (e2fsprogs support not available yet)
84	* extent format reduces metadata overhead (RAM, IO for access, transactions)
85	* extent format more robust in face of on-disk corruption due to magics,
86	* internal redundancy in tree
87	* improved file allocation (multi-block alloc)
88	* lift 32000 subdirectory limit imposed by i_links_count[1]
89	* nsec timestamps for mtime, atime, ctime, create time
90	* inode version field on disk (NFSv4, Lustre)
91	* reduced e2fsck time via uninit_bg feature
92	* journal checksumming for robustness, performance
93	* persistent file preallocation (e.g for streaming media, databases)
94	* ability to pack bitmaps and inode tables into larger virtual groups via the
95	  flex_bg feature
96	* large file support
97	* Inode allocation using large virtual block groups via flex_bg
98	* delayed allocation
99	* large block (up to pagesize) support
100	* efficient new ordered mode in JBD2 and ext4(avoid using buffer head to force
101	  the ordering)
103	[1] Filesystems with a block size of 1k may see a limit imposed by the
104	directory hash tree having a maximum depth of two.
106	2.2 Candidate features for future inclusion
108	* Online defrag (patches available but not well tested)
109	* reduced mke2fs time via lazy itable initialization in conjunction with
110	  the uninit_bg feature (capability to do this is available in e2fsprogs
111	  but a kernel thread to do lazy zeroing of unused inode table blocks
112	  after filesystem is first mounted is required for safety)
114	There are several others under discussion, whether they all make it in is
115	partly a function of how much time everyone has to work on them. Features like
116	metadata checksumming have been discussed and planned for a bit but no patches
117	exist yet so I'm not sure they're in the near-term roadmap.
119	The big performance win will come with mballoc, delalloc and flex_bg
120	grouping of bitmaps and inode tables.  Some test results available here:
122	 - http://www.bullopensource.org/ext4/20080818-ffsb/ffsb-write-2.6.27-rc1.html
123	 - http://www.bullopensource.org/ext4/20080818-ffsb/ffsb-readwrite-2.6.27-rc1.html
125	3. Options
126	==========
128	When mounting an ext4 filesystem, the following option are accepted:
129	(*) == default
131	ro                   	Mount filesystem read only. Note that ext4 will
132	                     	replay the journal (and thus write to the
133	                     	partition) even when mounted "read only". The
134	                     	mount options "ro,noload" can be used to prevent
135			     	writes to the filesystem.
137	journal_checksum	Enable checksumming of the journal transactions.
138				This will allow the recovery code in e2fsck and the
139				kernel to detect corruption in the kernel.  It is a
140				compatible change and will be ignored by older kernels.
142	journal_async_commit	Commit block can be written to disk without waiting
143				for descriptor blocks. If enabled older kernels cannot
144				mount the device. This will enable 'journal_checksum'
145				internally.
147	journal_path=path
148	journal_dev=devnum	When the external journal device's major/minor numbers
149				have changed, these options allow the user to specify
150				the new journal location.  The journal device is
151				identified through either its new major/minor numbers
152				encoded in devnum, or via a path to the device.
154	norecovery		Don't load the journal on mounting.  Note that
155	noload			if the filesystem was not unmounted cleanly,
156	                     	skipping the journal replay will lead to the
157	                     	filesystem containing inconsistencies that can
158	                     	lead to any number of problems.
160	data=journal		All data are committed into the journal prior to being
161				written into the main file system.  Enabling
162				this mode will disable delayed allocation and
163				O_DIRECT support.
165	data=ordered	(*)	All data are forced directly out to the main file
166				system prior to its metadata being committed to the
167				journal.
169	data=writeback		Data ordering is not preserved, data may be written
170				into the main file system after its metadata has been
171				committed to the journal.
173	commit=nrsec	(*)	Ext4 can be told to sync all its data and metadata
174				every 'nrsec' seconds. The default value is 5 seconds.
175				This means that if you lose your power, you will lose
176				as much as the latest 5 seconds of work (your
177				filesystem will not be damaged though, thanks to the
178				journaling).  This default value (or any low value)
179				will hurt performance, but it's good for data-safety.
180				Setting it to 0 will have the same effect as leaving
181				it at the default (5 seconds).
182				Setting it to very large values will improve
183				performance.
185	barrier=<0|1(*)>	This enables/disables the use of write barriers in
186	barrier(*)		the jbd code.  barrier=0 disables, barrier=1 enables.
187	nobarrier		This also requires an IO stack which can support
188				barriers, and if jbd gets an error on a barrier
189				write, it will disable again with a warning.
190				Write barriers enforce proper on-disk ordering
191				of journal commits, making volatile disk write caches
192				safe to use, at some performance penalty.  If
193				your disks are battery-backed in one way or another,
194				disabling barriers may safely improve performance.
195				The mount options "barrier" and "nobarrier" can
196				also be used to enable or disable barriers, for
197				consistency with other ext4 mount options.
199	inode_readahead_blks=n	This tuning parameter controls the maximum
200				number of inode table blocks that ext4's inode
201				table readahead algorithm will pre-read into
202				the buffer cache.  The default value is 32 blocks.
204	nouser_xattr		Disables Extended User Attributes.  See the
205				attr(5) manual page and http://acl.bestbits.at/
206				for more information about extended attributes.
208	noacl			This option disables POSIX Access Control List
209				support. If ACL support is enabled in the kernel
210				configuration (CONFIG_EXT4_FS_POSIX_ACL), ACL is
211				enabled by default on mount. See the acl(5) manual
212				page and http://acl.bestbits.at/ for more information
213				about acl.
215	bsddf		(*)	Make 'df' act like BSD.
216	minixdf			Make 'df' act like Minix.
218	debug			Extra debugging information is sent to syslog.
220	abort			Simulate the effects of calling ext4_abort() for
221				debugging purposes.  This is normally used while
222				remounting a filesystem which is already mounted.
224	errors=remount-ro	Remount the filesystem read-only on an error.
225	errors=continue		Keep going on a filesystem error.
226	errors=panic		Panic and halt the machine if an error occurs.
227	                        (These mount options override the errors behavior
228	                        specified in the superblock, which can be configured
229	                        using tune2fs)
231	data_err=ignore(*)	Just print an error message if an error occurs
232				in a file data buffer in ordered mode.
233	data_err=abort		Abort the journal if an error occurs in a file
234				data buffer in ordered mode.
236	grpid			Give objects the same group ID as their creator.
237	bsdgroups
239	nogrpid		(*)	New objects have the group ID of their creator.
240	sysvgroups
242	resgid=n		The group ID which may use the reserved blocks.
244	resuid=n		The user ID which may use the reserved blocks.
246	sb=n			Use alternate superblock at this location.
248	quota			These options are ignored by the filesystem. They
249	noquota			are used only by quota tools to recognize volumes
250	grpquota		where quota should be turned on. See documentation
251	usrquota		in the quota-tools package for more details
252				(http://sourceforge.net/projects/linuxquota).
254	jqfmt=<quota type>	These options tell filesystem details about quota
255	usrjquota=<file>	so that quota information can be properly updated
256	grpjquota=<file>	during journal replay. They replace the above
257				quota options. See documentation in the quota-tools
258				package for more details
259				(http://sourceforge.net/projects/linuxquota).
261	stripe=n		Number of filesystem blocks that mballoc will try
262				to use for allocation size and alignment. For RAID5/6
263				systems this should be the number of data
264				disks *  RAID chunk size in file system blocks.
266	delalloc	(*)	Defer block allocation until just before ext4
267				writes out the block(s) in question.  This
268				allows ext4 to better allocation decisions
269				more efficiently.
270	nodelalloc		Disable delayed allocation.  Blocks are allocated
271				when the data is copied from userspace to the
272				page cache, either via the write(2) system call
273				or when an mmap'ed page which was previously
274				unallocated is written for the first time.
276	max_batch_time=usec	Maximum amount of time ext4 should wait for
277				additional filesystem operations to be batch
278				together with a synchronous write operation.
279				Since a synchronous write operation is going to
280				force a commit and then a wait for the I/O
281				complete, it doesn't cost much, and can be a
282				huge throughput win, we wait for a small amount
283				of time to see if any other transactions can
284				piggyback on the synchronous write.   The
285				algorithm used is designed to automatically tune
286				for the speed of the disk, by measuring the
287				amount of time (on average) that it takes to
288				finish committing a transaction.  Call this time
289				the "commit time".  If the time that the
290				transaction has been running is less than the
291				commit time, ext4 will try sleeping for the
292				commit time to see if other operations will join
293				the transaction.   The commit time is capped by
294				the max_batch_time, which defaults to 15000us
295				(15ms).   This optimization can be turned off
296				entirely by setting max_batch_time to 0.
298	min_batch_time=usec	This parameter sets the commit time (as
299				described above) to be at least min_batch_time.
300				It defaults to zero microseconds.  Increasing
301				this parameter may improve the throughput of
302				multi-threaded, synchronous workloads on very
303				fast disks, at the cost of increasing latency.
305	journal_ioprio=prio	The I/O priority (from 0 to 7, where 0 is the
306				highest priority) which should be used for I/O
307				operations submitted by kjournald2 during a
308				commit operation.  This defaults to 3, which is
309				a slightly higher priority than the default I/O
310				priority.
312	auto_da_alloc(*)	Many broken applications don't use fsync() when 
313	noauto_da_alloc		replacing existing files via patterns such as
314				fd = open("foo.new")/write(fd,..)/close(fd)/
315				rename("foo.new", "foo"), or worse yet,
316				fd = open("foo", O_TRUNC)/write(fd,..)/close(fd).
317				If auto_da_alloc is enabled, ext4 will detect
318				the replace-via-rename and replace-via-truncate
319				patterns and force that any delayed allocation
320				blocks are allocated such that at the next
321				journal commit, in the default data=ordered
322				mode, the data blocks of the new file are forced
323				to disk before the rename() operation is
324				committed.  This provides roughly the same level
325				of guarantees as ext3, and avoids the
326				"zero-length" problem that can happen when a
327				system crashes before the delayed allocation
328				blocks are forced to disk.
330	noinit_itable		Do not initialize any uninitialized inode table
331				blocks in the background.  This feature may be
332				used by installation CD's so that the install
333				process can complete as quickly as possible; the
334				inode table initialization process would then be
335				deferred until the next time the  file system
336				is unmounted.
338	init_itable=n		The lazy itable init code will wait n times the
339				number of milliseconds it took to zero out the
340				previous block group's inode table.  This
341				minimizes the impact on the system performance
342				while file system's inode table is being initialized.
344	discard			Controls whether ext4 should issue discard/TRIM
345	nodiscard(*)		commands to the underlying block device when
346				blocks are freed.  This is useful for SSD devices
347				and sparse/thinly-provisioned LUNs, but it is off
348				by default until sufficient testing has been done.
350	nouid32			Disables 32-bit UIDs and GIDs.  This is for
351				interoperability  with  older kernels which only
352				store and expect 16-bit values.
354	block_validity		This options allows to enables/disables the in-kernel
355	noblock_validity	facility for tracking filesystem metadata blocks
356				within internal data structures. This allows multi-
357				block allocator and other routines to quickly locate
358				extents which might overlap with filesystem metadata
359				blocks. This option is intended for debugging
360				purposes and since it negatively affects the
361				performance, it is off by default.
363	dioread_lock		Controls whether or not ext4 should use the DIO read
364	dioread_nolock		locking. If the dioread_nolock option is specified
365				ext4 will allocate uninitialized extent before buffer
366				write and convert the extent to initialized after IO
367				completes. This approach allows ext4 code to avoid
368				using inode mutex, which improves scalability on high
369				speed storages. However this does not work with
370				data journaling and dioread_nolock option will be
371				ignored with kernel warning. Note that dioread_nolock
372				code path is only used for extent-based files.
373				Because of the restrictions this options comprises
374				it is off by default (e.g. dioread_lock).
376	max_dir_size_kb=n	This limits the size of directories so that any
377				attempt to expand them beyond the specified
378				limit in kilobytes will cause an ENOSPC error.
379				This is useful in memory constrained
380				environments, where a very large directory can
381				cause severe performance problems or even
382				provoke the Out Of Memory killer.  (For example,
383				if there is only 512mb memory available, a 176mb
384				directory may seriously cramp the system's style.)
386	i_version		Enable 64-bit inode version support. This option is
387				off by default.
389	dax			Use direct access (no page cache).  See
390				Documentation/filesystems/dax.txt.  Note that
391				this option is incompatible with data=journal.
393	Data Mode
394	=========
395	There are 3 different data modes:
397	* writeback mode
398	In data=writeback mode, ext4 does not journal data at all.  This mode provides
399	a similar level of journaling as that of XFS, JFS, and ReiserFS in its default
400	mode - metadata journaling.  A crash+recovery can cause incorrect data to
401	appear in files which were written shortly before the crash.  This mode will
402	typically provide the best ext4 performance.
404	* ordered mode
405	In data=ordered mode, ext4 only officially journals metadata, but it logically
406	groups metadata information related to data changes with the data blocks into a
407	single unit called a transaction.  When it's time to write the new metadata
408	out to disk, the associated data blocks are written first.  In general,
409	this mode performs slightly slower than writeback but significantly faster than journal mode.
411	* journal mode
412	data=journal mode provides full data and metadata journaling.  All new data is
413	written to the journal first, and then to its final location.
414	In the event of a crash, the journal can be replayed, bringing both data and
415	metadata into a consistent state.  This mode is the slowest except when data
416	needs to be read from and written to disk at the same time where it
417	outperforms all others modes.  Enabling this mode will disable delayed
418	allocation and O_DIRECT support.
420	/proc entries
421	=============
423	Information about mounted ext4 file systems can be found in
424	/proc/fs/ext4.  Each mounted filesystem will have a directory in
425	/proc/fs/ext4 based on its device name (i.e., /proc/fs/ext4/hdc or
426	/proc/fs/ext4/dm-0).   The files in each per-device directory are shown
427	in table below.
429	Files in /proc/fs/ext4/<devname>
430	..............................................................................
431	 File            Content
432	 mb_groups       details of multiblock allocator buddy cache of free blocks
433	..............................................................................
435	/sys entries
436	============
438	Information about mounted ext4 file systems can be found in
439	/sys/fs/ext4.  Each mounted filesystem will have a directory in
440	/sys/fs/ext4 based on its device name (i.e., /sys/fs/ext4/hdc or
441	/sys/fs/ext4/dm-0).   The files in each per-device directory are shown
442	in table below.
444	Files in /sys/fs/ext4/<devname>
445	(see also Documentation/ABI/testing/sysfs-fs-ext4)
446	..............................................................................
447	 File                         Content
449	 delayed_allocation_blocks    This file is read-only and shows the number of
450	                              blocks that are dirty in the page cache, but
451	                              which do not have their location in the
452	                              filesystem allocated yet.
454	 inode_goal                   Tuning parameter which (if non-zero) controls
455	                              the goal inode used by the inode allocator in
456	                              preference to all other allocation heuristics.
457	                              This is intended for debugging use only, and
458	                              should be 0 on production systems.
460	 inode_readahead_blks         Tuning parameter which controls the maximum
461	                              number of inode table blocks that ext4's inode
462	                              table readahead algorithm will pre-read into
463	                              the buffer cache
465	 lifetime_write_kbytes        This file is read-only and shows the number of
466	                              kilobytes of data that have been written to this
467	                              filesystem since it was created.
469	 max_writeback_mb_bump        The maximum number of megabytes the writeback
470	                              code will try to write out before move on to
471	                              another inode.
473	 mb_group_prealloc            The multiblock allocator will round up allocation
474	                              requests to a multiple of this tuning parameter if
475	                              the stripe size is not set in the ext4 superblock
477	 mb_max_to_scan               The maximum number of extents the multiblock
478	                              allocator will search to find the best extent
480	 mb_min_to_scan               The minimum number of extents the multiblock
481	                              allocator will search to find the best extent
483	 mb_order2_req                Tuning parameter which controls the minimum size
484	                              for requests (as a power of 2) where the buddy
485	                              cache is used
487	 mb_stats                     Controls whether the multiblock allocator should
488	                              collect statistics, which are shown during the
489	                              unmount. 1 means to collect statistics, 0 means
490	                              not to collect statistics
492	 mb_stream_req                Files which have fewer blocks than this tunable
493	                              parameter will have their blocks allocated out
494	                              of a block group specific preallocation pool, so
495	                              that small files are packed closely together.
496	                              Each large file will have its blocks allocated
497	                              out of its own unique preallocation pool.
499	 session_write_kbytes         This file is read-only and shows the number of
500	                              kilobytes of data that have been written to this
501	                              filesystem since it was mounted.
503	 reserved_clusters            This is RW file and contains number of reserved
504	                              clusters in the file system which will be used
505	                              in the specific situations to avoid costly
506	                              zeroout, unexpected ENOSPC, or possible data
507	                              loss. The default is 2% or 4096 clusters,
508	                              whichever is smaller and this can be changed
509	                              however it can never exceed number of clusters
510	                              in the file system. If there is not enough space
511	                              for the reserved space when mounting the file
512	                              mount will _not_ fail.
513	..............................................................................
515	Ioctls
516	======
518	There is some Ext4 specific functionality which can be accessed by applications
519	through the system call interfaces. The list of all Ext4 specific ioctls are
520	shown in the table below.
522	Table of Ext4 specific ioctls
523	..............................................................................
524	 Ioctl			      Description
525	 EXT4_IOC_GETFLAGS	      Get additional attributes associated with inode.
526				      The ioctl argument is an integer bitfield, with
527				      bit values described in ext4.h. This ioctl is an
528				      alias for FS_IOC_GETFLAGS.
530	 EXT4_IOC_SETFLAGS	      Set additional attributes associated with inode.
531				      The ioctl argument is an integer bitfield, with
532				      bit values described in ext4.h. This ioctl is an
533				      alias for FS_IOC_SETFLAGS.
537				      Get the inode i_generation number stored for
538				      each inode. The i_generation number is normally
539				      changed only when new inode is created and it is
540				      particularly useful for network filesystems. The
541				      '_OLD' version of this ioctl is an alias for
542				      FS_IOC_GETVERSION.
546				      Set the inode i_generation number stored for
547				      each inode. The '_OLD' version of this ioctl
548				      is an alias for FS_IOC_SETVERSION.
550	 EXT4_IOC_GROUP_EXTEND	      This ioctl has the same purpose as the resize
551				      mount option. It allows to resize filesystem
552				      to the end of the last existing block group,
553				      further resize has to be done with resize2fs,
554				      either online, or offline. The argument points
555				      to the unsigned logn number representing the
556				      filesystem new block count.
558	 EXT4_IOC_MOVE_EXT	      Move the block extents from orig_fd (the one
559				      this ioctl is pointing to) to the donor_fd (the
560				      one specified in move_extent structure passed
561				      as an argument to this ioctl). Then, exchange
562				      inode metadata between orig_fd and donor_fd.
563				      This is especially useful for online
564				      defragmentation, because the allocator has the
565				      opportunity to allocate moved blocks better,
566				      ideally into one contiguous extent.
568	 EXT4_IOC_GROUP_ADD	      Add a new group descriptor to an existing or
569				      new group descriptor block. The new group
570				      descriptor is described by ext4_new_group_input
571				      structure, which is passed as an argument to
572				      this ioctl. This is especially useful in
573				      conjunction with EXT4_IOC_GROUP_EXTEND,
574				      which allows online resize of the filesystem
575				      to the end of the last existing block group.
576				      Those two ioctls combined is used in userspace
577				      online resize tool (e.g. resize2fs).
579	 EXT4_IOC_MIGRATE	      This ioctl operates on the filesystem itself.
580				      It converts (migrates) ext3 indirect block mapped
581				      inode to ext4 extent mapped inode by walking
582				      through indirect block mapping of the original
583				      inode and converting contiguous block ranges
584				      into ext4 extents of the temporary inode. Then,
585				      inodes are swapped. This ioctl might help, when
586				      migrating from ext3 to ext4 filesystem, however
587				      suggestion is to create fresh ext4 filesystem
588				      and copy data from the backup. Note, that
589				      filesystem has to support extents for this ioctl
590				      to work.
592	 EXT4_IOC_ALLOC_DA_BLKS	      Force all of the delay allocated blocks to be
593				      allocated to preserve application-expected ext3
594				      behaviour. Note that this will also start
595				      triggering a write of the data blocks, but this
596				      behaviour may change in the future as it is
597				      not necessary and has been done this way only
598				      for sake of simplicity.
600	 EXT4_IOC_RESIZE_FS	      Resize the filesystem to a new size.  The number
601				      of blocks of resized filesystem is passed in via
602				      64 bit integer argument.  The kernel allocates
603				      bitmaps and inode table, the userspace tool thus
604				      just passes the new number of blocks.
606	EXT4_IOC_SWAP_BOOT	      Swap i_blocks and associated attributes
607				      (like i_blocks, i_size, i_flags, ...) from
608				      the specified inode with inode
609				      EXT4_BOOT_LOADER_INO (#5). This is typically
610				      used to store a boot loader in a secure part of
611				      the filesystem, where it can't be changed by a
612				      normal user by accident.
613				      The data blocks of the previous boot loader
614				      will be associated with the given inode.
616	..............................................................................
618	References
619	==========
621	kernel source:	<file:fs/ext4/>
622			<file:fs/jbd2/>
624	programs:	http://e2fsprogs.sourceforge.net/
626	useful links:	http://fedoraproject.org/wiki/ext3-devel
627			http://www.bullopensource.org/ext4/
628			http://ext4.wiki.kernel.org/index.php/Main_Page
629			http://fedoraproject.org/wiki/Features/Ext4
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