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ext4.txt
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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: [email protected]
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.
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).
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/