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Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 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	    https://www.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 for more information about
206				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 for more information about acl.
214	bsddf		(*)	Make 'df' act like BSD.
215	minixdf			Make 'df' act like Minix.
217	debug			Extra debugging information is sent to syslog.
219	abort			Simulate the effects of calling ext4_abort() for
220				debugging purposes.  This is normally used while
221				remounting a filesystem which is already mounted.
223	errors=remount-ro	Remount the filesystem read-only on an error.
224	errors=continue		Keep going on a filesystem error.
225	errors=panic		Panic and halt the machine if an error occurs.
226	                        (These mount options override the errors behavior
227	                        specified in the superblock, which can be configured
228	                        using tune2fs)
230	data_err=ignore(*)	Just print an error message if an error occurs
231				in a file data buffer in ordered mode.
232	data_err=abort		Abort the journal if an error occurs in a file
233				data buffer in ordered mode.
235	grpid			New objects have the group ID of their parent.
236	bsdgroups
238	nogrpid		(*)	New objects have the group ID of their creator.
239	sysvgroups
241	resgid=n		The group ID which may use the reserved blocks.
243	resuid=n		The user ID which may use the reserved blocks.
245	sb=n			Use alternate superblock at this location.
247	quota			These options are ignored by the filesystem. They
248	noquota			are used only by quota tools to recognize volumes
249	grpquota		where quota should be turned on. See documentation
250	usrquota		in the quota-tools package for more details
251				(http://sourceforge.net/projects/linuxquota).
253	jqfmt=<quota type>	These options tell filesystem details about quota
254	usrjquota=<file>	so that quota information can be properly updated
255	grpjquota=<file>	during journal replay. They replace the above
256				quota options. See documentation in the quota-tools
257				package for more details
258				(http://sourceforge.net/projects/linuxquota).
260	stripe=n		Number of filesystem blocks that mballoc will try
261				to use for allocation size and alignment. For RAID5/6
262				systems this should be the number of data
263				disks *  RAID chunk size in file system blocks.
265	delalloc	(*)	Defer block allocation until just before ext4
266				writes out the block(s) in question.  This
267				allows ext4 to better allocation decisions
268				more efficiently.
269	nodelalloc		Disable delayed allocation.  Blocks are allocated
270				when the data is copied from userspace to the
271				page cache, either via the write(2) system call
272				or when an mmap'ed page which was previously
273				unallocated is written for the first time.
275	max_batch_time=usec	Maximum amount of time ext4 should wait for
276				additional filesystem operations to be batch
277				together with a synchronous write operation.
278				Since a synchronous write operation is going to
279				force a commit and then a wait for the I/O
280				complete, it doesn't cost much, and can be a
281				huge throughput win, we wait for a small amount
282				of time to see if any other transactions can
283				piggyback on the synchronous write.   The
284				algorithm used is designed to automatically tune
285				for the speed of the disk, by measuring the
286				amount of time (on average) that it takes to
287				finish committing a transaction.  Call this time
288				the "commit time".  If the time that the
289				transaction has been running is less than the
290				commit time, ext4 will try sleeping for the
291				commit time to see if other operations will join
292				the transaction.   The commit time is capped by
293				the max_batch_time, which defaults to 15000us
294				(15ms).   This optimization can be turned off
295				entirely by setting max_batch_time to 0.
297	min_batch_time=usec	This parameter sets the commit time (as
298				described above) to be at least min_batch_time.
299				It defaults to zero microseconds.  Increasing
300				this parameter may improve the throughput of
301				multi-threaded, synchronous workloads on very
302				fast disks, at the cost of increasing latency.
304	journal_ioprio=prio	The I/O priority (from 0 to 7, where 0 is the
305				highest priority) which should be used for I/O
306				operations submitted by kjournald2 during a
307				commit operation.  This defaults to 3, which is
308				a slightly higher priority than the default I/O
309				priority.
311	auto_da_alloc(*)	Many broken applications don't use fsync() when 
312	noauto_da_alloc		replacing existing files via patterns such as
313				fd = open("foo.new")/write(fd,..)/close(fd)/
314				rename("foo.new", "foo"), or worse yet,
315				fd = open("foo", O_TRUNC)/write(fd,..)/close(fd).
316				If auto_da_alloc is enabled, ext4 will detect
317				the replace-via-rename and replace-via-truncate
318				patterns and force that any delayed allocation
319				blocks are allocated such that at the next
320				journal commit, in the default data=ordered
321				mode, the data blocks of the new file are forced
322				to disk before the rename() operation is
323				committed.  This provides roughly the same level
324				of guarantees as ext3, and avoids the
325				"zero-length" problem that can happen when a
326				system crashes before the delayed allocation
327				blocks are forced to disk.
329	noinit_itable		Do not initialize any uninitialized inode table
330				blocks in the background.  This feature may be
331				used by installation CD's so that the install
332				process can complete as quickly as possible; the
333				inode table initialization process would then be
334				deferred until the next time the  file system
335				is unmounted.
337	init_itable=n		The lazy itable init code will wait n times the
338				number of milliseconds it took to zero out the
339				previous block group's inode table.  This
340				minimizes the impact on the system performance
341				while file system's inode table is being initialized.
343	discard			Controls whether ext4 should issue discard/TRIM
344	nodiscard(*)		commands to the underlying block device when
345				blocks are freed.  This is useful for SSD devices
346				and sparse/thinly-provisioned LUNs, but it is off
347				by default until sufficient testing has been done.
349	nouid32			Disables 32-bit UIDs and GIDs.  This is for
350				interoperability  with  older kernels which only
351				store and expect 16-bit values.
353	block_validity(*)	These options enable or disable the in-kernel
354	noblock_validity	facility for tracking filesystem metadata blocks
355				within internal data structures.  This allows multi-
356				block allocator and other routines to notice
357				bugs or corrupted allocation bitmaps which cause
358				blocks to be allocated which overlap with
359				filesystem metadata blocks.
361	dioread_lock		Controls whether or not ext4 should use the DIO read
362	dioread_nolock		locking. If the dioread_nolock option is specified
363				ext4 will allocate uninitialized extent before buffer
364				write and convert the extent to initialized after IO
365				completes. This approach allows ext4 code to avoid
366				using inode mutex, which improves scalability on high
367				speed storages. However this does not work with
368				data journaling and dioread_nolock option will be
369				ignored with kernel warning. Note that dioread_nolock
370				code path is only used for extent-based files.
371				Because of the restrictions this options comprises
372				it is off by default (e.g. dioread_lock).
374	max_dir_size_kb=n	This limits the size of directories so that any
375				attempt to expand them beyond the specified
376				limit in kilobytes will cause an ENOSPC error.
377				This is useful in memory constrained
378				environments, where a very large directory can
379				cause severe performance problems or even
380				provoke the Out Of Memory killer.  (For example,
381				if there is only 512mb memory available, a 176mb
382				directory may seriously cramp the system's style.)
384	i_version		Enable 64-bit inode version support. This option is
385				off by default.
387	dax			Use direct access (no page cache).  See
388				Documentation/filesystems/dax.txt.  Note that
389				this option is incompatible with data=journal.
391	Data Mode
392	=========
393	There are 3 different data modes:
395	* writeback mode
396	In data=writeback mode, ext4 does not journal data at all.  This mode provides
397	a similar level of journaling as that of XFS, JFS, and ReiserFS in its default
398	mode - metadata journaling.  A crash+recovery can cause incorrect data to
399	appear in files which were written shortly before the crash.  This mode will
400	typically provide the best ext4 performance.
402	* ordered mode
403	In data=ordered mode, ext4 only officially journals metadata, but it logically
404	groups metadata information related to data changes with the data blocks into a
405	single unit called a transaction.  When it's time to write the new metadata
406	out to disk, the associated data blocks are written first.  In general,
407	this mode performs slightly slower than writeback but significantly faster than journal mode.
409	* journal mode
410	data=journal mode provides full data and metadata journaling.  All new data is
411	written to the journal first, and then to its final location.
412	In the event of a crash, the journal can be replayed, bringing both data and
413	metadata into a consistent state.  This mode is the slowest except when data
414	needs to be read from and written to disk at the same time where it
415	outperforms all others modes.  Enabling this mode will disable delayed
416	allocation and O_DIRECT support.
418	/proc entries
419	=============
421	Information about mounted ext4 file systems can be found in
422	/proc/fs/ext4.  Each mounted filesystem will have a directory in
423	/proc/fs/ext4 based on its device name (i.e., /proc/fs/ext4/hdc or
424	/proc/fs/ext4/dm-0).   The files in each per-device directory are shown
425	in table below.
427	Files in /proc/fs/ext4/<devname>
428	..............................................................................
429	 File            Content
430	 mb_groups       details of multiblock allocator buddy cache of free blocks
431	..............................................................................
433	/sys entries
434	============
436	Information about mounted ext4 file systems can be found in
437	/sys/fs/ext4.  Each mounted filesystem will have a directory in
438	/sys/fs/ext4 based on its device name (i.e., /sys/fs/ext4/hdc or
439	/sys/fs/ext4/dm-0).   The files in each per-device directory are shown
440	in table below.
442	Files in /sys/fs/ext4/<devname>
443	(see also Documentation/ABI/testing/sysfs-fs-ext4)
444	..............................................................................
445	 File                         Content
447	 delayed_allocation_blocks    This file is read-only and shows the number of
448	                              blocks that are dirty in the page cache, but
449	                              which do not have their location in the
450	                              filesystem allocated yet.
452	 inode_goal                   Tuning parameter which (if non-zero) controls
453	                              the goal inode used by the inode allocator in
454	                              preference to all other allocation heuristics.
455	                              This is intended for debugging use only, and
456	                              should be 0 on production systems.
458	 inode_readahead_blks         Tuning parameter which controls the maximum
459	                              number of inode table blocks that ext4's inode
460	                              table readahead algorithm will pre-read into
461	                              the buffer cache
463	 lifetime_write_kbytes        This file is read-only and shows the number of
464	                              kilobytes of data that have been written to this
465	                              filesystem since it was created.
467	 max_writeback_mb_bump        The maximum number of megabytes the writeback
468	                              code will try to write out before move on to
469	                              another inode.
471	 mb_group_prealloc            The multiblock allocator will round up allocation
472	                              requests to a multiple of this tuning parameter if
473	                              the stripe size is not set in the ext4 superblock
475	 mb_max_to_scan               The maximum number of extents the multiblock
476	                              allocator will search to find the best extent
478	 mb_min_to_scan               The minimum number of extents the multiblock
479	                              allocator will search to find the best extent
481	 mb_order2_req                Tuning parameter which controls the minimum size
482	                              for requests (as a power of 2) where the buddy
483	                              cache is used
485	 mb_stats                     Controls whether the multiblock allocator should
486	                              collect statistics, which are shown during the
487	                              unmount. 1 means to collect statistics, 0 means
488	                              not to collect statistics
490	 mb_stream_req                Files which have fewer blocks than this tunable
491	                              parameter will have their blocks allocated out
492	                              of a block group specific preallocation pool, so
493	                              that small files are packed closely together.
494	                              Each large file will have its blocks allocated
495	                              out of its own unique preallocation pool.
497	 session_write_kbytes         This file is read-only and shows the number of
498	                              kilobytes of data that have been written to this
499	                              filesystem since it was mounted.
501	 reserved_clusters            This is RW file and contains number of reserved
502	                              clusters in the file system which will be used
503	                              in the specific situations to avoid costly
504	                              zeroout, unexpected ENOSPC, or possible data
505	                              loss. The default is 2% or 4096 clusters,
506	                              whichever is smaller and this can be changed
507	                              however it can never exceed number of clusters
508	                              in the file system. If there is not enough space
509	                              for the reserved space when mounting the file
510	                              mount will _not_ fail.
511	..............................................................................
513	Ioctls
514	======
516	There is some Ext4 specific functionality which can be accessed by applications
517	through the system call interfaces. The list of all Ext4 specific ioctls are
518	shown in the table below.
520	Table of Ext4 specific ioctls
521	..............................................................................
522	 Ioctl			      Description
523	 EXT4_IOC_GETFLAGS	      Get additional attributes associated with inode.
524				      The ioctl argument is an integer bitfield, with
525				      bit values described in ext4.h. This ioctl is an
526				      alias for FS_IOC_GETFLAGS.
528	 EXT4_IOC_SETFLAGS	      Set additional attributes associated with inode.
529				      The ioctl argument is an integer bitfield, with
530				      bit values described in ext4.h. This ioctl is an
531				      alias for FS_IOC_SETFLAGS.
535				      Get the inode i_generation number stored for
536				      each inode. The i_generation number is normally
537				      changed only when new inode is created and it is
538				      particularly useful for network filesystems. The
539				      '_OLD' version of this ioctl is an alias for
540				      FS_IOC_GETVERSION.
544				      Set the inode i_generation number stored for
545				      each inode. The '_OLD' version of this ioctl
546				      is an alias for FS_IOC_SETVERSION.
548	 EXT4_IOC_GROUP_EXTEND	      This ioctl has the same purpose as the resize
549				      mount option. It allows to resize filesystem
550				      to the end of the last existing block group,
551				      further resize has to be done with resize2fs,
552				      either online, or offline. The argument points
553				      to the unsigned logn number representing the
554				      filesystem new block count.
556	 EXT4_IOC_MOVE_EXT	      Move the block extents from orig_fd (the one
557				      this ioctl is pointing to) to the donor_fd (the
558				      one specified in move_extent structure passed
559				      as an argument to this ioctl). Then, exchange
560				      inode metadata between orig_fd and donor_fd.
561				      This is especially useful for online
562				      defragmentation, because the allocator has the
563				      opportunity to allocate moved blocks better,
564				      ideally into one contiguous extent.
566	 EXT4_IOC_GROUP_ADD	      Add a new group descriptor to an existing or
567				      new group descriptor block. The new group
568				      descriptor is described by ext4_new_group_input
569				      structure, which is passed as an argument to
570				      this ioctl. This is especially useful in
571				      conjunction with EXT4_IOC_GROUP_EXTEND,
572				      which allows online resize of the filesystem
573				      to the end of the last existing block group.
574				      Those two ioctls combined is used in userspace
575				      online resize tool (e.g. resize2fs).
577	 EXT4_IOC_MIGRATE	      This ioctl operates on the filesystem itself.
578				      It converts (migrates) ext3 indirect block mapped
579				      inode to ext4 extent mapped inode by walking
580				      through indirect block mapping of the original
581				      inode and converting contiguous block ranges
582				      into ext4 extents of the temporary inode. Then,
583				      inodes are swapped. This ioctl might help, when
584				      migrating from ext3 to ext4 filesystem, however
585				      suggestion is to create fresh ext4 filesystem
586				      and copy data from the backup. Note, that
587				      filesystem has to support extents for this ioctl
588				      to work.
590	 EXT4_IOC_ALLOC_DA_BLKS	      Force all of the delay allocated blocks to be
591				      allocated to preserve application-expected ext3
592				      behaviour. Note that this will also start
593				      triggering a write of the data blocks, but this
594				      behaviour may change in the future as it is
595				      not necessary and has been done this way only
596				      for sake of simplicity.
598	 EXT4_IOC_RESIZE_FS	      Resize the filesystem to a new size.  The number
599				      of blocks of resized filesystem is passed in via
600				      64 bit integer argument.  The kernel allocates
601				      bitmaps and inode table, the userspace tool thus
602				      just passes the new number of blocks.
604	 EXT4_IOC_SWAP_BOOT	      Swap i_blocks and associated attributes
605				      (like i_blocks, i_size, i_flags, ...) from
606				      the specified inode with inode
607				      EXT4_BOOT_LOADER_INO (#5). This is typically
608				      used to store a boot loader in a secure part of
609				      the filesystem, where it can't be changed by a
610				      normal user by accident.
611				      The data blocks of the previous boot loader
612				      will be associated with the given inode.
614	..............................................................................
616	References
617	==========
619	kernel source:	<file:fs/ext4/>
620			<file:fs/jbd2/>
622	programs:	http://e2fsprogs.sourceforge.net/
624	useful links:	http://fedoraproject.org/wiki/ext3-devel
625			http://www.bullopensource.org/ext4/
626			http://ext4.wiki.kernel.org/index.php/Main_Page
627			http://fedoraproject.org/wiki/Features/Ext4
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