Documentation/filesystems/ext4.txt - third_party/linux - Git at Google


 Ext4 Filesystem
 ===============

 This is a development version of the ext4 filesystem, 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


 1. Quick usage instructions:
 ===========================

   - Grab updated e2fsprogs from
     ftp://ftp.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs-interim/
     This is a patchset on top of e2fsprogs-1.39, which can be found at
     ftp://ftp.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs/

   - It's still mke2fs -j /dev/hda1

   - mount /dev/hda1 /wherever -t ext4dev

   - To enable extents,

 	mount /dev/hda1 /wherever -t ext4dev -o extents

   - The filesystem is compatible with the ext3 driver until you add a file
     which has extents (ie: `mount -o extents', then create a file).

     NOTE: The "extents" mount flag is temporary.  It will soon go away and
     extents will be enabled by the "-o extents" flag to mke2fs or tune2fs

   - When comparing performance with other filesystems, remember that
     ext3/4 by default offers higher data integrity guarantees than most.  So
     when comparing with a metadata-only journalling filesystem, use `mount -o
     data=writeback'.  And you might as well use `mount -o nobh' too along
     with it.  Making the journal larger than the mke2fs default often helps
     performance with metadata-intensive workloads.

 2. Features
 ===========

 2.1 Currently available

 * ability to use filesystems > 16TB
 * extent format reduces metadata overhead (RAM, IO for access, transactions)
 * extent format more robust in face of on-disk corruption due to magics,
 * internal redunancy in tree

 2.1 Previously available, soon to be enabled by default by "mkefs.ext4":

 * dir_index and resize inode will be on by default
 * large inodes will be used by default for fast EAs, nsec timestamps, etc

 2.2 Candidate features for future inclusion

 There are several under discussion, whether they all make it in is
 partly a function of how much time everyone has to work on them:

 * improved file allocation (multi-block alloc, delayed alloc; basically done)
 * fix 32000 subdirectory limit (patch exists, needs some e2fsck work)
 * nsec timestamps for mtime, atime, ctime, create time (patch exists,
   needs some e2fsck work)
 * inode version field on disk (NFSv4, Lustre; prototype exists)
 * reduced mke2fs/e2fsck time via uninitialized groups (prototype exists)
 * journal checksumming for robustness, performance (prototype exists)
 * persistent file preallocation (e.g for streaming media, databases)

 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 and delalloc.  CFS has
 been using mballoc for a few years already with Lustre, and IBM + Bull
 did a lot of benchmarking on it.  The reason it isn't in the first set of
 patches is partly a manageability issue, and partly because it doesn't
 directly affect the on-disk format (outside of much better allocation)
 so it isn't critical to get into the first round of changes.  I believe
 Alex is working on a new set of patches right now.

 3. Options
 ==========

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

 extents		(*)	ext4 will use extents to address file data.  The
 			file system will no longer be mountable by ext3.

 noextents		ext4 will not use extents for newly created files

 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=inum		When a journal already exists, this option is ignored.
 			Otherwise, it specifies the number of the inode which
 			will represent the ext4 file system's journal file.

 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.

 noload			Don't load the journal on mounting.

 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=1		This enables/disables barriers.  barrier=0 disables
 			it, barrier=1 enables it.

 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.

 check=none		Don't do extra checking of bitmaps on mount.
 nocheck

 debug			Extra debugging information is sent to syslog.

 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.

 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
 noquota
 grpquota
 usrquota

 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).

 mballoc		(*)	Use the multiple block allocator for block allocation
 nomballoc		disabled multiple block allocator for block allocation.
 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.

 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 and 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.

 References
 ==========

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

 programs:	http://e2fsprogs.sourceforge.net/
 		http://ext2resize.sourceforge.net

 useful links:	http://fedoraproject.org/wiki/ext3-devel
 		http://www.bullopensource.org/ext4/

	Ext4 Filesystem
	===============

	This is a development version of the ext4 filesystem, 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


	1. Quick usage instructions:
	===========================

	- Grab updated e2fsprogs from
	ftp://ftp.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs-interim/
	This is a patchset on top of e2fsprogs-1.39, which can be found at
	ftp://ftp.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs/

	- It's still mke2fs -j /dev/hda1

	- mount /dev/hda1 /wherever -t ext4dev

	- To enable extents,

	mount /dev/hda1 /wherever -t ext4dev -o extents

	- The filesystem is compatible with the ext3 driver until you add a file
	which has extents (ie: `mount -o extents', then create a file).

	NOTE: The "extents" mount flag is temporary. It will soon go away and
	extents will be enabled by the "-o extents" flag to mke2fs or tune2fs

	- When comparing performance with other filesystems, remember that
	ext3/4 by default offers higher data integrity guarantees than most. So
	when comparing with a metadata-only journalling filesystem, use `mount -o
	data=writeback'. And you might as well use `mount -o nobh' too along
	with it. Making the journal larger than the mke2fs default often helps
	performance with metadata-intensive workloads.

	2. Features
	===========

	2.1 Currently available

	* ability to use filesystems > 16TB
	* extent format reduces metadata overhead (RAM, IO for access, transactions)
	* extent format more robust in face of on-disk corruption due to magics,
	* internal redunancy in tree

	2.1 Previously available, soon to be enabled by default by "mkefs.ext4":

	* dir_index and resize inode will be on by default
	* large inodes will be used by default for fast EAs, nsec timestamps, etc

	2.2 Candidate features for future inclusion

	There are several under discussion, whether they all make it in is
	partly a function of how much time everyone has to work on them:

	* improved file allocation (multi-block alloc, delayed alloc; basically done)
	* fix 32000 subdirectory limit (patch exists, needs some e2fsck work)
	* nsec timestamps for mtime, atime, ctime, create time (patch exists,
	needs some e2fsck work)
	* inode version field on disk (NFSv4, Lustre; prototype exists)
	* reduced mke2fs/e2fsck time via uninitialized groups (prototype exists)
	* journal checksumming for robustness, performance (prototype exists)
	* persistent file preallocation (e.g for streaming media, databases)

	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 and delalloc. CFS has
	been using mballoc for a few years already with Lustre, and IBM + Bull
	did a lot of benchmarking on it. The reason it isn't in the first set of
	patches is partly a manageability issue, and partly because it doesn't
	directly affect the on-disk format (outside of much better allocation)
	so it isn't critical to get into the first round of changes. I believe
	Alex is working on a new set of patches right now.

	3. Options
	==========

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

	extents (*) ext4 will use extents to address file data. The
	file system will no longer be mountable by ext3.

	noextents ext4 will not use extents for newly created files

	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=inum When a journal already exists, this option is ignored.
	Otherwise, it specifies the number of the inode which
	will represent the ext4 file system's journal file.

	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.

	noload Don't load the journal on mounting.

	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=1 This enables/disables barriers. barrier=0 disables
	it, barrier=1 enables it.

	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.

	check=none Don't do extra checking of bitmaps on mount.
	nocheck

	debug Extra debugging information is sent to syslog.

	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.

	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
	noquota
	grpquota
	usrquota

	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).

	mballoc (*) Use the multiple block allocator for block allocation
	nomballoc disabled multiple block allocator for block allocation.
	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.

	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 and 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.

	References
	==========

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

	programs: http://e2fsprogs.sourceforge.net/
	http://ext2resize.sourceforge.net

	useful links: http://fedoraproject.org/wiki/ext3-devel
	http://www.bullopensource.org/ext4/