OpenCloudOS-Kernel/Documentation/filesystems/ext4.txt

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=<0|1(*)>	This enables/disables the use of write barriers in
			the jbd code.  barrier=0 disables, barrier=1 enables.
			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.

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/