[PATCH] Documentation/filesystems/ext4.txt
This file, ext4.txt, was put together with information from Andrew Morton, Andreas Dilger, Suparna Bhattacharya, and Ted Ts'o. I copied the mount options, with the exception of "extents", from ext3.txt, so if anyone is aware of anything out-of-date, please let me know. Signed-off-by: Dave Kleikamp <shaggy@austin.ibm.com> Cc: Theodore Ts'o <tytso@mit.edu> Cc: Suparna Bhattacharya <suparna@in.ibm.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
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Dave Kleikamp
Linus Torvalds
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@ -34,6 +34,8 @@ ext2.txt
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- info, mount options and specifications for the Ext2 filesystem.
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ext3.txt
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- info, mount options and specifications for the Ext3 filesystem.
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ext4.txt
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- info, mount options and specifications for the Ext4 filesystem.
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files.txt
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- info on file management in the Linux kernel.
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fuse.txt
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@ -0,0 +1,236 @@
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Ext4 Filesystem
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===============
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This is a development version of the ext4 filesystem, an advanced level
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of the ext3 filesystem which incorporates scalability and reliability
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enhancements for supporting large filesystems (64 bit) in keeping with
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increasing disk capacities and state-of-the-art feature requirements.
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Mailing list: linux-ext4@vger.kernel.org
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1. Quick usage instructions:
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===========================
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- Grab updated e2fsprogs from
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ftp://ftp.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs-interim/
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This is a patchset on top of e2fsprogs-1.39, which can be found at
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ftp://ftp.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs/
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- It's still mke2fs -j /dev/hda1
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- mount /dev/hda1 /wherever -t ext4dev
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- To enable extents,
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mount /dev/hda1 /wherever -t ext4dev -o extents
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- The filesystem is compatible with the ext3 driver until you add a file
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which has extents (ie: `mount -o extents', then create a file).
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NOTE: The "extents" mount flag is temporary. It will soon go away and
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extents will be enabled by the "-o extents" flag to mke2fs or tune2fs
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- When comparing performance with other filesystems, remember that
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ext3/4 by default offers higher data integrity guarantees than most. So
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when comparing with a metadata-only journalling filesystem, use `mount -o
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data=writeback'. And you might as well use `mount -o nobh' too along
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with it. Making the journal larger than the mke2fs default often helps
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performance with metadata-intensive workloads.
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2. Features
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===========
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2.1 Currently available
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* ability to use filesystems > 16TB
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* extent format reduces metadata overhead (RAM, IO for access, transactions)
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* extent format more robust in face of on-disk corruption due to magics,
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* internal redunancy in tree
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2.1 Previously available, soon to be enabled by default by "mkefs.ext4":
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* dir_index and resize inode will be on by default
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* large inodes will be used by default for fast EAs, nsec timestamps, etc
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2.2 Candidate features for future inclusion
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There are several under discussion, whether they all make it in is
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partly a function of how much time everyone has to work on them:
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* improved file allocation (multi-block alloc, delayed alloc; basically done)
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* fix 32000 subdirectory limit (patch exists, needs some e2fsck work)
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* nsec timestamps for mtime, atime, ctime, create time (patch exists,
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needs some e2fsck work)
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* inode version field on disk (NFSv4, Lustre; prototype exists)
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* reduced mke2fs/e2fsck time via uninitialized groups (prototype exists)
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* journal checksumming for robustness, performance (prototype exists)
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* persistent file preallocation (e.g for streaming media, databases)
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Features like metadata checksumming have been discussed and planned for
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a bit but no patches exist yet so I'm not sure they're in the near-term
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roadmap.
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The big performance win will come with mballoc and delalloc. CFS has
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been using mballoc for a few years already with Lustre, and IBM + Bull
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did a lot of benchmarking on it. The reason it isn't in the first set of
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patches is partly a manageability issue, and partly because it doesn't
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directly affect the on-disk format (outside of much better allocation)
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so it isn't critical to get into the first round of changes. I believe
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Alex is working on a new set of patches right now.
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3. Options
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==========
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When mounting an ext4 filesystem, the following option are accepted:
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(*) == default
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extents ext4 will use extents to address file data. The
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file system will no longer be mountable by ext3.
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journal=update Update the ext4 file system's journal to the current
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format.
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journal=inum When a journal already exists, this option is ignored.
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Otherwise, it specifies the number of the inode which
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will represent the ext4 file system's journal file.
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journal_dev=devnum When the external journal device's major/minor numbers
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have changed, this option allows the user to specify
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the new journal location. The journal device is
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identified through its new major/minor numbers encoded
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in devnum.
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noload Don't load the journal on mounting.
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data=journal All data are committed into the journal prior to being
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written into the main file system.
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data=ordered (*) All data are forced directly out to the main file
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system prior to its metadata being committed to the
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journal.
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data=writeback Data ordering is not preserved, data may be written
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into the main file system after its metadata has been
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committed to the journal.
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commit=nrsec (*) Ext4 can be told to sync all its data and metadata
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every 'nrsec' seconds. The default value is 5 seconds.
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This means that if you lose your power, you will lose
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as much as the latest 5 seconds of work (your
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filesystem will not be damaged though, thanks to the
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journaling). This default value (or any low value)
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will hurt performance, but it's good for data-safety.
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Setting it to 0 will have the same effect as leaving
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it at the default (5 seconds).
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Setting it to very large values will improve
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performance.
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barrier=1 This enables/disables barriers. barrier=0 disables
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it, barrier=1 enables it.
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orlov (*) This enables the new Orlov block allocator. It is
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enabled by default.
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oldalloc This disables the Orlov block allocator and enables
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the old block allocator. Orlov should have better
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performance - we'd like to get some feedback if it's
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the contrary for you.
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user_xattr Enables Extended User Attributes. Additionally, you
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need to have extended attribute support enabled in the
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kernel configuration (CONFIG_EXT4_FS_XATTR). See the
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attr(5) manual page and http://acl.bestbits.at/ to
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learn more about extended attributes.
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nouser_xattr Disables Extended User Attributes.
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acl Enables POSIX Access Control Lists support.
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Additionally, you need to have ACL support enabled in
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the kernel configuration (CONFIG_EXT4_FS_POSIX_ACL).
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See the acl(5) manual page and http://acl.bestbits.at/
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for more information.
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noacl This option disables POSIX Access Control List
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support.
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reservation
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noreservation
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bsddf (*) Make 'df' act like BSD.
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minixdf Make 'df' act like Minix.
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check=none Don't do extra checking of bitmaps on mount.
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nocheck
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debug Extra debugging information is sent to syslog.
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errors=remount-ro(*) Remount the filesystem read-only on an error.
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errors=continue Keep going on a filesystem error.
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errors=panic Panic and halt the machine if an error occurs.
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grpid Give objects the same group ID as their creator.
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bsdgroups
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nogrpid (*) New objects have the group ID of their creator.
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sysvgroups
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resgid=n The group ID which may use the reserved blocks.
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resuid=n The user ID which may use the reserved blocks.
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sb=n Use alternate superblock at this location.
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quota
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noquota
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grpquota
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usrquota
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bh (*) ext4 associates buffer heads to data pages to
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nobh (a) cache disk block mapping information
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(b) link pages into transaction to provide
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ordering guarantees.
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"bh" option forces use of buffer heads.
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"nobh" option tries to avoid associating buffer
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heads (supported only for "writeback" mode).
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Data Mode
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---------
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There are 3 different data modes:
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* writeback mode
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In data=writeback mode, ext4 does not journal data at all. This mode provides
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a similar level of journaling as that of XFS, JFS, and ReiserFS in its default
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mode - metadata journaling. A crash+recovery can cause incorrect data to
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appear in files which were written shortly before the crash. This mode will
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typically provide the best ext4 performance.
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* ordered mode
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In data=ordered mode, ext4 only officially journals metadata, but it logically
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groups metadata and data blocks into a single unit called a transaction. When
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it's time to write the new metadata out to disk, the associated data blocks
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are written first. In general, this mode performs slightly slower than
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writeback but significantly faster than journal mode.
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* journal mode
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data=journal mode provides full data and metadata journaling. All new data is
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written to the journal first, and then to its final location.
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In the event of a crash, the journal can be replayed, bringing both data and
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metadata into a consistent state. This mode is the slowest except when data
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needs to be read from and written to disk at the same time where it
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outperforms all others modes.
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References
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==========
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kernel source: <file:fs/ext4/>
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<file:fs/jbd2/>
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programs: http://e2fsprogs.sourceforge.net/
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http://ext2resize.sourceforge.net
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useful links: http://fedoraproject.org/wiki/ext3-devel
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http://www.bullopensource.org/ext4/
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