Documentation / filesystems / ext4.txt

Custom Search

Based on kernel version 2.6.33. Page generated on 2010-02-24 15:36 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[AT]vger.kernel[DOT]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,nobh' 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
	* efficent 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 conjuction 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=update		Update the ext4 file system's journal to the current
				format.
	
	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.
	
	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.
	
	orlov		(*)	This enables the new Orlov block allocator. It is
				enabled by default.
	
	oldalloc		This disables the Orlov block allocator and enables
				the old block allocator.  Orlov should have better
				performance - we'd like to get some feedback if it's
				the contrary for you.
	
	user_xattr		Enables Extended User Attributes.  Additionally, you
				need to have extended attribute support enabled in the
				kernel configuration (CONFIG_EXT4_FS_XATTR).  See the
				attr(5) manual page and http://acl.bestbits.at/ to
				learn more about extended attributes.
	
	nouser_xattr		Disables Extended User Attributes.
	
	acl			Enables POSIX Access Control Lists support.
				Additionally, you need to have ACL support enabled in
				the kernel configuration (CONFIG_EXT4_FS_POSIX_ACL).
				See the acl(5) manual page and http://acl.bestbits.at/
				for more information.
	
	noacl			This option disables POSIX Access Control List
				support.
	
	reservation
	
	noreservation
	
	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).
	
	bh		(*)	ext4 associates buffer heads to data pages to
	nobh			(a) cache disk block mapping information
				(b) link pages into transaction to provide
				    ordering guarantees.
				"bh" option forces use of buffer heads.
				"nobh" option tries to avoid associating buffer
				heads (supported only for "writeback" mode).
	
	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 priorty) 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.
	
	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.
	
	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.  Currently ext4 does not have delayed
	allocation support if this data journalling mode is selected.
	
	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 ]
• cifs.txt
• coda.txt
• [ configfs ]
• cramfs.txt
• debugfs.txt
• dentry-locking.txt
• devpts.txt
• directory-locking
• dlmfs.txt
• dnotify.txt
• ecryptfs.txt
• exofs.txt
• ext2.txt
• ext3.txt
• ext4.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
• mandatory-locking.txt
• ncpfs.txt
• [ nfs ]
• nilfs2.txt
• ntfs.txt
• ocfs2.txt
• omfs.txt
• [ pohmelfs ]
• porting
• proc.txt
• quota.txt
• ramfs-rootfs-initramfs.txt
• relay.txt
• romfs.txt
• seq_file.txt
• sharedsubtree.txt
• smbfs.txt
• spufs.txt
• squashfs.txt
• sysfs-pci.txt
• sysfs.txt
• sysv-fs.txt
• tmpfs.txt
• ubifs.txt
• udf.txt
• ufs.txt
• vfat.txt
• vfs.txt
• xfs.txt
• xip.txt
•