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83982b6f47
This mount option is largely superfluous, and in fact the way it was implemented was buggy; if a filesystem which did not have the extents feature flag was mounted -o extents, the filesystem would attempt to create and use extents-based file even though the extents feature flag was not eabled. The simplest thing to do is to nuke the mount option entirely. It's not all that useful to force the non-creation of new extent-based files if the filesystem can support it. Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
352 lines
14 KiB
Plaintext
352 lines
14 KiB
Plaintext
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Ext4 Filesystem
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===============
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Ext4 is an an advanced level of the ext3 filesystem which incorporates
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scalability and reliability enhancements for supporting large filesystems
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(64 bit) in keeping with increasing disk capacities and state-of-the-art
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feature requirements.
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Mailing list: linux-ext4@vger.kernel.org
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Web site: http://ext4.wiki.kernel.org
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1. Quick usage instructions:
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===========================
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Note: More extensive information for getting started with ext4 can be
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found at the ext4 wiki site at the URL:
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http://ext4.wiki.kernel.org/index.php/Ext4_Howto
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- Compile and install the latest version of e2fsprogs (as of this
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writing version 1.41.3) from:
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http://sourceforge.net/project/showfiles.php?group_id=2406
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or
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ftp://ftp.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs/
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or grab the latest git repository from:
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git://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git
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- Note that it is highly important to install the mke2fs.conf file
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that comes with the e2fsprogs 1.41.x sources in /etc/mke2fs.conf. If
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you have edited the /etc/mke2fs.conf file installed on your system,
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you will need to merge your changes with the version from e2fsprogs
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1.41.x.
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- Create a new filesystem using the ext4 filesystem type:
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# mke2fs -t ext4 /dev/hda1
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Or to configure an existing ext3 filesystem to support extents:
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# tune2fs -O extents /dev/hda1
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If the filesystem was created with 128 byte inodes, it can be
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converted to use 256 byte for greater efficiency via:
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# tune2fs -I 256 /dev/hda1
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(Note: we currently do not have tools to convert an ext4
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filesystem back to ext3; so please do not do try this on production
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filesystems.)
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- Mounting:
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# mount -t ext4 /dev/hda1 /wherever
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- When comparing performance with other filesystems, it's always
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important to try multiple workloads; very often a subtle change in a
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workload parameter can completely change the ranking of which
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filesystems do well compared to others. When comparing versus ext3,
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note that ext4 enables write barriers by default, while ext3 does
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not enable write barriers by default. So it is useful to use
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explicitly specify whether barriers are enabled or not when via the
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'-o barriers=[0|1]' mount option for both ext3 and ext4 filesystems
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for a fair comparison. When tuning ext3 for best benchmark numbers,
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it is often worthwhile to try changing the data journaling mode; '-o
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data=writeback,nobh' can be faster for some workloads. (Note
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however that running mounted with data=writeback can potentially
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leave stale data exposed in recently written files in case of an
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unclean shutdown, which could be a security exposure in some
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situations.) Configuring the filesystem with a large journal can
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also be helpful for 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 (e2fsprogs support not available yet)
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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 redundancy in tree
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* improved file allocation (multi-block alloc)
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* fix 32000 subdirectory limit
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* nsec timestamps for mtime, atime, ctime, create time
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* inode version field on disk (NFSv4, Lustre)
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* reduced e2fsck time via uninit_bg feature
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* journal checksumming for robustness, performance
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* persistent file preallocation (e.g for streaming media, databases)
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* ability to pack bitmaps and inode tables into larger virtual groups via the
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flex_bg feature
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* large file support
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* Inode allocation using large virtual block groups via flex_bg
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* delayed allocation
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* large block (up to pagesize) support
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* efficent new ordered mode in JBD2 and ext4(avoid using buffer head to force
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the ordering)
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2.2 Candidate features for future inclusion
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* Online defrag (patches available but not well tested)
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* reduced mke2fs time via lazy itable initialization in conjuction with
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the uninit_bg feature (capability to do this is available in e2fsprogs
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but a kernel thread to do lazy zeroing of unused inode table blocks
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after filesystem is first mounted is required for safety)
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There are several others 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. Features like
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metadata checksumming have been discussed and planned for a bit but no patches
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exist yet so I'm not sure they're in the near-term roadmap.
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The big performance win will come with mballoc, delalloc and flex_bg
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grouping of bitmaps and inode tables. Some test results available here:
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- http://www.bullopensource.org/ext4/20080818-ffsb/ffsb-write-2.6.27-rc1.html
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- http://www.bullopensource.org/ext4/20080818-ffsb/ffsb-readwrite-2.6.27-rc1.html
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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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ro Mount filesystem read only. Note that ext4 will
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replay the journal (and thus write to the
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partition) even when mounted "read only". The
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mount options "ro,noload" can be used to prevent
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writes to the filesystem.
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journal_checksum Enable checksumming of the journal transactions.
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This will allow the recovery code in e2fsck and the
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kernel to detect corruption in the kernel. It is a
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compatible change and will be ignored by older kernels.
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journal_async_commit Commit block can be written to disk without waiting
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for descriptor blocks. If enabled older kernels cannot
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mount the device. This will enable 'journal_checksum'
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internally.
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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_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. Note that
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if the filesystem was not unmounted cleanly,
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skipping the journal replay will lead to the
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filesystem containing inconsistencies that can
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lead to any number of problems.
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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=<0|1(*)> This enables/disables the use of write barriers in
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the jbd code. barrier=0 disables, barrier=1 enables.
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This also requires an IO stack which can support
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barriers, and if jbd gets an error on a barrier
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write, it will disable again with a warning.
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Write barriers enforce proper on-disk ordering
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of journal commits, making volatile disk write caches
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safe to use, at some performance penalty. If
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your disks are battery-backed in one way or another,
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disabling barriers may safely improve performance.
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inode_readahead=n This tuning parameter controls the maximum
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number of inode table blocks that ext4's inode
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table readahead algorithm will pre-read into
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the buffer cache. The default value is 32 blocks.
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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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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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(These mount options override the errors behavior
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specified in the superblock, which can be configured
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using tune2fs)
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data_err=ignore(*) Just print an error message if an error occurs
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in a file data buffer in ordered mode.
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data_err=abort Abort the journal if an error occurs in a file
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data buffer in ordered mode.
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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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stripe=n Number of filesystem blocks that mballoc will try
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to use for allocation size and alignment. For RAID5/6
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systems this should be the number of data
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disks * RAID chunk size in file system blocks.
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delalloc (*) Deferring block allocation until write-out time.
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nodelalloc Disable delayed allocation. Blocks are allocation
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when data is copied from user to page cache.
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max_batch_time=usec Maximum amount of time ext4 should wait for
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additional filesystem operations to be batch
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together with a synchronous write operation.
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Since a synchronous write operation is going to
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force a commit and then a wait for the I/O
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complete, it doesn't cost much, and can be a
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huge throughput win, we wait for a small amount
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of time to see if any other transactions can
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piggyback on the synchronous write. The
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algorithm used is designed to automatically tune
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for the speed of the disk, by measuring the
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amount of time (on average) that it takes to
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finish committing a transaction. Call this time
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the "commit time". If the time that the
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transactoin has been running is less than the
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commit time, ext4 will try sleeping for the
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commit time to see if other operations will join
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the transaction. The commit time is capped by
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the max_batch_time, which defaults to 15000us
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(15ms). This optimization can be turned off
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entirely by setting max_batch_time to 0.
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min_batch_time=usec This parameter sets the commit time (as
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described above) to be at least min_batch_time.
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It defaults to zero microseconds. Increasing
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this parameter may improve the throughput of
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multi-threaded, synchronous workloads on very
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fast disks, at the cost of increasing latency.
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journal_ioprio=prio The I/O priority (from 0 to 7, where 0 is the
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highest priorty) which should be used for I/O
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operations submitted by kjournald2 during a
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commit operation. This defaults to 3, which is
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a slightly higher priority than the default I/O
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priority.
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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 information related to data changes with the data blocks into a
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single unit called a transaction. When it's time to write the new metadata
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out to disk, the associated data blocks are written first. In general,
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this mode performs slightly slower than 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. Curently ext4 does not have delayed
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allocation support if this data journalling mode is selected.
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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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useful links: http://fedoraproject.org/wiki/ext3-devel
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http://www.bullopensource.org/ext4/
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http://ext4.wiki.kernel.org/index.php/Main_Page
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http://fedoraproject.org/wiki/Features/Ext4
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