401 lines
16 KiB
Plaintext
401 lines
16 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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* lift 32000 subdirectory limit imposed by i_links_count[1]
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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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[1] Filesystems with a block size of 1k may see a limit imposed by the
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directory hash tree having a maximum depth of two.
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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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norecovery Don't load the journal on mounting. Note that
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noload 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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barrier(*) the jbd code. barrier=0 disables, barrier=1 enables.
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nobarrier 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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The mount options "barrier" and "nobarrier" can
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also be used to enable or disable barriers, for
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consistency with other ext4 mount options.
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inode_readahead_blks=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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abort Simulate the effects of calling ext4_abort() for
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debugging purposes. This is normally used while
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remounting a filesystem which is already mounted.
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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 These options are ignored by the filesystem. They
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noquota are used only by quota tools to recognize volumes
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grpquota where quota should be turned on. See documentation
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usrquota in the quota-tools package for more details
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(http://sourceforge.net/projects/linuxquota).
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jqfmt=<quota type> These options tell filesystem details about quota
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usrjquota=<file> so that quota information can be properly updated
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grpjquota=<file> during journal replay. They replace the above
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quota options. See documentation in the quota-tools
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package for more details
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(http://sourceforge.net/projects/linuxquota).
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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 (*) Defer block allocation until just before ext4
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writes out the block(s) in question. This
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allows ext4 to better allocation decisions
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more efficiently.
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nodelalloc Disable delayed allocation. Blocks are allocated
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when the data is copied from userspace to the
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page cache, either via the write(2) system call
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or when an mmap'ed page which was previously
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unallocated is written for the first time.
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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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transaction 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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auto_da_alloc(*) Many broken applications don't use fsync() when
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noauto_da_alloc replacing existing files via patterns such as
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fd = open("foo.new")/write(fd,..)/close(fd)/
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rename("foo.new", "foo"), or worse yet,
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fd = open("foo", O_TRUNC)/write(fd,..)/close(fd).
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If auto_da_alloc is enabled, ext4 will detect
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the replace-via-rename and replace-via-truncate
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patterns and force that any delayed allocation
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blocks are allocated such that at the next
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journal commit, in the default data=ordered
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mode, the data blocks of the new file are forced
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to disk before the rename() operation is
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committed. This provides roughly the same level
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of guarantees as ext3, and avoids the
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"zero-length" problem that can happen when a
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system crashes before the delayed allocation
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blocks are forced to disk.
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discard Controls whether ext4 should issue discard/TRIM
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nodiscard(*) commands to the underlying block device when
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blocks are freed. This is useful for SSD devices
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and sparse/thinly-provisioned LUNs, but it is off
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by default until sufficient testing has been done.
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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. Currently 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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