19f5946001
Fix various typos in documentation txts. Signed-off-by: Matt LaPlante <kernel1@cyberdogtech.com> Signed-off-by: Jiri Kosina <jkosina@suse.cz>
376 lines
15 KiB
Plaintext
376 lines
15 KiB
Plaintext
|
|
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@vger.kernel.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.
|
|
|
|
noload Don't load the journal on mounting. Note that
|
|
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=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.
|
|
|
|
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
|
|
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).
|
|
|
|
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 (*) Deferring block allocation until write-out time.
|
|
nodelalloc Disable delayed allocation. Blocks are allocation
|
|
when data is copied from user to page cache.
|
|
|
|
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.
|
|
|
|
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
|