mirror of https://gitee.com/openkylin/linux.git
237 lines
8.4 KiB
Plaintext
237 lines
8.4 KiB
Plaintext
|
|
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.
|
|
|
|
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/
|