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+.. SPDX-License-Identifier: GPL-2.0
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+
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+========================
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+ext4 General Information
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+========================
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+
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+Ext4 is 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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+
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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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+
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+
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+Quick usage instructions
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+========================
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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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+
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+ - The latest version of e2fsprogs can be found at:
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+
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+ https://www.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs/
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+
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+ or
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+
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+ http://sourceforge.net/project/showfiles.php?group_id=2406
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+
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+ or grab the latest git repository from:
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+
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+ https://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git
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+
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+ - Create a new filesystem using the ext4 filesystem type:
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+
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+ # mke2fs -t ext4 /dev/hda1
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+
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+ Or to configure an existing ext3 filesystem to support extents:
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+
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+ # tune2fs -O extents /dev/hda1
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+
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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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+
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+ # tune2fs -I 256 /dev/hda1
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+
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+ - Mounting:
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+
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+ # mount -t ext4 /dev/hda1 /wherever
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+
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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' can be faster for some workloads. (Note however that
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+ running mounted with data=writeback can potentially leave stale data
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+ exposed in recently written files in case of an unclean shutdown,
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+ which could be a security exposure in some situations.) Configuring
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+ the filesystem with a large journal can also be helpful for
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+ metadata-intensive workloads.
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+
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+Features
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+========
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+
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+Currently Available
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+-------------------
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+
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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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+* efficient new ordered mode in JBD2 and ext4 (avoid using buffer head to force
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+ the ordering)
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+
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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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+
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+Options
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+=======
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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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+
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+ ro
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+ Mount filesystem read only. Note that ext4 will replay the journal (and
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+ thus write to the partition) even when mounted "read only". The mount
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+ options "ro,noload" can be used to prevent writes to the filesystem.
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+
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+ journal_checksum
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+ Enable checksumming of the journal transactions. This will allow the
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+ recovery code in e2fsck and the kernel to detect corruption in the
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+ kernel. It is a compatible change and will be ignored by older
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+ kernels.
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+
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+ journal_async_commit
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+ Commit block can be written to disk without waiting for descriptor
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+ blocks. If enabled older kernels cannot mount the device. This will
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+ enable 'journal_checksum' internally.
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+
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+ journal_path=path, journal_dev=devnum
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+ When the external journal device's major/minor numbers have changed,
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+ these options allow the user to specify the new journal location. The
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+ journal device is identified through either its new major/minor numbers
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+ encoded in devnum, or via a path to the device.
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+
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+ norecovery, noload
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+ Don't load the journal on mounting. Note that if the filesystem was
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+ not unmounted cleanly, skipping the journal replay will lead to the
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+ filesystem containing inconsistencies that can lead to any number of
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+ problems.
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+
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+ data=journal
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+ All data are committed into the journal prior to being written into the
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+ main file system. Enabling this mode will disable delayed allocation
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+ and O_DIRECT support.
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+
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+ data=ordered (*)
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+ All data are forced directly out to the main file system prior to its
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+ metadata being committed to the journal.
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+
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+ data=writeback
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+ Data ordering is not preserved, data may be written into the main file
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+ system after its metadata has been committed to the journal.
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+
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+ commit=nrsec (*)
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+ Ext4 can be told to sync all its data and metadata every 'nrsec'
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+ seconds. The default value is 5 seconds. This means that if you lose
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+ your power, you will lose as much as the latest 5 seconds of work (your
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+ filesystem will not be damaged though, thanks to the journaling). This
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+ default value (or any low value) will hurt performance, but it's good
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+ for data-safety. Setting it to 0 will have the same effect as leaving
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+ it at the default (5 seconds). Setting it to very large values will
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+ improve performance.
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+
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+ barrier=<0|1(*)>, barrier(*), nobarrier
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+ This enables/disables the use of write barriers in the jbd code.
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+ barrier=0 disables, barrier=1 enables. This also requires an IO stack
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+ which can support barriers, and if jbd gets an error on a barrier
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+ write, it will disable again with a warning. Write barriers enforce
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+ proper on-disk ordering of journal commits, making volatile disk write
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+ caches safe to use, at some performance penalty. If your disks are
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+ battery-backed in one way or another, disabling barriers may safely
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+ improve performance. The mount options "barrier" and "nobarrier" can
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+ also be used to enable or disable barriers, for consistency with other
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+ ext4 mount options.
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+
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+ inode_readahead_blks=n
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+ This tuning parameter controls the maximum number of inode table blocks
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+ that ext4's inode table readahead algorithm will pre-read into the
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+ buffer cache. The default value is 32 blocks.
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+
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+ nouser_xattr
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+ Disables Extended User Attributes. See the attr(5) manual page for
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+ more information about extended attributes.
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+
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+ noacl
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+ This option disables POSIX Access Control List support. If ACL support
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+ is enabled in the kernel configuration (CONFIG_EXT4_FS_POSIX_ACL), ACL
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+ is enabled by default on mount. See the acl(5) manual page for more
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+ information about acl.
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+
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+ bsddf (*)
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+ Make 'df' act like BSD.
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+
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+ minixdf
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+ Make 'df' act like Minix.
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+
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+ debug
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+ Extra debugging information is sent to syslog.
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+
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+ abort
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+ Simulate the effects of calling ext4_abort() for debugging purposes.
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+ This is normally used while remounting a filesystem which is already
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+ mounted.
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+
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+ errors=remount-ro
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+ Remount the filesystem read-only on an error.
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+
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+ errors=continue
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+ Keep going on a filesystem error.
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+
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+ errors=panic
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+ Panic and halt the machine if an error occurs. (These mount options
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+ override the errors behavior specified in the superblock, which can be
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+ configured using tune2fs)
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+
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+ data_err=ignore(*)
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+ Just print an error message if an error occurs in a file data buffer in
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+ ordered mode.
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+ data_err=abort
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+ Abort the journal if an error occurs in a file data buffer in ordered
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+ mode.
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+
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+ grpid | bsdgroups
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+ New objects have the group ID of their parent.
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+
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+ nogrpid (*) | sysvgroups
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+ New objects have the group ID of their creator.
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+
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+ resgid=n
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+ The group ID which may use the reserved blocks.
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+
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+ resuid=n
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+ The user ID which may use the reserved blocks.
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+
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+ sb=
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+ Use alternate superblock at this location.
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+
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+ quota, noquota, grpquota, usrquota
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+ These options are ignored by the filesystem. They are used only by
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+ quota tools to recognize volumes where quota should be turned on. See
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+ documentation in the quota-tools package for more details
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+ (http://sourceforge.net/projects/linuxquota).
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+
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+ jqfmt=<quota type>, usrjquota=<file>, grpjquota=<file>
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+ These options tell filesystem details about quota so that quota
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+ information can be properly updated during journal replay. They replace
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+ the above quota options. See documentation in the quota-tools package
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+ for more details (http://sourceforge.net/projects/linuxquota).
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+
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+ stripe=n
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+ Number of filesystem blocks that mballoc will try to use for allocation
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+ size and alignment. For RAID5/6 systems this should be the number of
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+ data disks * RAID chunk size in file system blocks.
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+
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+ delalloc (*)
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+ Defer block allocation until just before ext4 writes out the block(s)
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+ in question. This allows ext4 to better allocation decisions more
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+ efficiently.
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+
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+ nodelalloc
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+ Disable delayed allocation. Blocks are allocated when the data is
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+ copied from userspace to the page cache, either via the write(2) system
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+ call or when an mmap'ed page which was previously unallocated is
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+ written for the first time.
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+
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+ max_batch_time=usec
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+ Maximum amount of time ext4 should wait for additional filesystem
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+ operations to be batch together with a synchronous write operation.
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+ Since a synchronous write operation is going to force a commit and then
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+ a wait for the I/O complete, it doesn't cost much, and can be a huge
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+ throughput win, we wait for a small amount of time to see if any other
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+ transactions can piggyback on the synchronous write. The algorithm
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+ used is designed to automatically tune for the speed of the disk, by
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+ measuring the amount of time (on average) that it takes to finish
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+ committing a transaction. Call this time the "commit time". If the
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+ time that the transaction has been running is less than the commit
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+ time, ext4 will try sleeping for the commit time to see if other
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+ operations will join the transaction. The commit time is capped by
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+ the max_batch_time, which defaults to 15000us (15ms). This
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+ optimization can be turned off entirely by setting max_batch_time to 0.
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+
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+ min_batch_time=usec
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+ This parameter sets the commit time (as described above) to be at least
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+ min_batch_time. It defaults to zero microseconds. Increasing this
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+ parameter may improve the throughput of multi-threaded, synchronous
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+ workloads on very fast disks, at the cost of increasing latency.
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+
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+ journal_ioprio=prio
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+ The I/O priority (from 0 to 7, where 0 is the highest priority) which
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+ should be used for I/O operations submitted by kjournald2 during a
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+ commit operation. This defaults to 3, which is a slightly higher
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+ priority than the default I/O priority.
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+
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+ auto_da_alloc(*), noauto_da_alloc
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+ Many broken applications don't use fsync() when replacing existing
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+ files via patterns such as fd = open("foo.new")/write(fd,..)/close(fd)/
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+ rename("foo.new", "foo"), or worse yet, fd = open("foo",
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+ O_TRUNC)/write(fd,..)/close(fd). If auto_da_alloc is enabled, ext4
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+ will detect the replace-via-rename and replace-via-truncate patterns
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+ and force that any delayed allocation blocks are allocated such that at
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+ the next journal commit, in the default data=ordered mode, the data
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+ blocks of the new file are forced to disk before the rename() operation
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+ is committed. This provides roughly the same level of guarantees as
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+ ext3, and avoids the "zero-length" problem that can happen when a
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+ system crashes before the delayed allocation blocks are forced to disk.
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+
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+ noinit_itable
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+ Do not initialize any uninitialized inode table blocks in the
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+ background. This feature may be used by installation CD's so that the
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+ install process can complete as quickly as possible; the inode table
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+ initialization process would then be deferred until the next time the
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+ file system is unmounted.
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+
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+ init_itable=n
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+ The lazy itable init code will wait n times the number of milliseconds
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+ it took to zero out the previous block group's inode table. This
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+ minimizes the impact on the system performance while file system's
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+ inode table is being initialized.
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+
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+ discard, nodiscard(*)
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+ Controls whether ext4 should issue discard/TRIM commands to the
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+ underlying block device when blocks are freed. This is useful for SSD
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+ devices and sparse/thinly-provisioned LUNs, but it is off by default
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+ until sufficient testing has been done.
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+
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+ nouid32
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+ Disables 32-bit UIDs and GIDs. This is for interoperability with
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+ older kernels which only store and expect 16-bit values.
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+
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+ block_validity(*), noblock_validity
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+ These options enable or disable the in-kernel facility for tracking
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+ filesystem metadata blocks within internal data structures. This
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+ allows multi- block allocator and other routines to notice bugs or
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+ corrupted allocation bitmaps which cause blocks to be allocated which
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+ overlap with filesystem metadata blocks.
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+
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+ dioread_lock, dioread_nolock
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+ Controls whether or not ext4 should use the DIO read locking. If the
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+ dioread_nolock option is specified ext4 will allocate uninitialized
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+ extent before buffer write and convert the extent to initialized after
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+ IO completes. This approach allows ext4 code to avoid using inode
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+ mutex, which improves scalability on high speed storages. However this
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+ does not work with data journaling and dioread_nolock option will be
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+ ignored with kernel warning. Note that dioread_nolock code path is only
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+ used for extent-based files. Because of the restrictions this options
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+ comprises it is off by default (e.g. dioread_lock).
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+
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+ max_dir_size_kb=n
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+ This limits the size of directories so that any attempt to expand them
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+ beyond the specified limit in kilobytes will cause an ENOSPC error.
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+ This is useful in memory constrained environments, where a very large
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+ directory can cause severe performance problems or even provoke the Out
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+ Of Memory killer. (For example, if there is only 512mb memory
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+ available, a 176mb directory may seriously cramp the system's style.)
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+
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+ i_version
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+ Enable 64-bit inode version support. This option is off by default.
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+
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+ dax
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+ Use direct access (no page cache). See
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+ Documentation/filesystems/dax.txt. Note that this option is
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+ incompatible with data=journal.
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+
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+Data Mode
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+=========
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+There are 3 different data modes:
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+
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+* writeback mode
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+
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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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+
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+* ordered mode
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+
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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
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+ a 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, this
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+ mode performs slightly slower than writeback but significantly faster than
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+ journal mode.
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+
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+* journal mode
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+
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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. In the event of
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+ a crash, the journal can be replayed, bringing both data and metadata into a
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+ consistent state. This mode is the slowest except when data needs to be read
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+ from and written to disk at the same time where it outperforms all others
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+ modes. Enabling this mode will disable delayed allocation and O_DIRECT
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+ support.
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+
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+/proc entries
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+=============
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+
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+Information about mounted ext4 file systems can be found in
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+/proc/fs/ext4. Each mounted filesystem will have a directory in
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+/proc/fs/ext4 based on its device name (i.e., /proc/fs/ext4/hdc or
|
|
|
+/proc/fs/ext4/dm-0). The files in each per-device directory are shown
|
|
|
+in table below.
|
|
|
+
|
|
|
+Files in /proc/fs/ext4/<devname>
|
|
|
+
|
|
|
+ mb_groups
|
|
|
+ details of multiblock allocator buddy cache of free blocks
|
|
|
+
|
|
|
+/sys entries
|
|
|
+============
|
|
|
+
|
|
|
+Information about mounted ext4 file systems can be found in
|
|
|
+/sys/fs/ext4. Each mounted filesystem will have a directory in
|
|
|
+/sys/fs/ext4 based on its device name (i.e., /sys/fs/ext4/hdc or
|
|
|
+/sys/fs/ext4/dm-0). The files in each per-device directory are shown
|
|
|
+in table below.
|
|
|
+
|
|
|
+Files in /sys/fs/ext4/<devname>:
|
|
|
+
|
|
|
+(see also Documentation/ABI/testing/sysfs-fs-ext4)
|
|
|
+
|
|
|
+ delayed_allocation_blocks
|
|
|
+ This file is read-only and shows the number of blocks that are dirty in
|
|
|
+ the page cache, but which do not have their location in the filesystem
|
|
|
+ allocated yet.
|
|
|
+
|
|
|
+ inode_goal
|
|
|
+ Tuning parameter which (if non-zero) controls the goal inode used by
|
|
|
+ the inode allocator in preference to all other allocation heuristics.
|
|
|
+ This is intended for debugging use only, and should be 0 on production
|
|
|
+ systems.
|
|
|
+
|
|
|
+ inode_readahead_blks
|
|
|
+ Tuning parameter which controls the maximum number of inode table
|
|
|
+ blocks that ext4's inode table readahead algorithm will pre-read into
|
|
|
+ the buffer cache.
|
|
|
+
|
|
|
+ lifetime_write_kbytes
|
|
|
+ This file is read-only and shows the number of kilobytes of data that
|
|
|
+ have been written to this filesystem since it was created.
|
|
|
+
|
|
|
+ max_writeback_mb_bump
|
|
|
+ The maximum number of megabytes the writeback code will try to write
|
|
|
+ out before move on to another inode.
|
|
|
+
|
|
|
+ mb_group_prealloc
|
|
|
+ The multiblock allocator will round up allocation requests to a
|
|
|
+ multiple of this tuning parameter if the stripe size is not set in the
|
|
|
+ ext4 superblock
|
|
|
+
|
|
|
+ mb_max_to_scan
|
|
|
+ The maximum number of extents the multiblock allocator will search to
|
|
|
+ find the best extent.
|
|
|
+
|
|
|
+ mb_min_to_scan
|
|
|
+ The minimum number of extents the multiblock allocator will search to
|
|
|
+ find the best extent.
|
|
|
+
|
|
|
+ mb_order2_req
|
|
|
+ Tuning parameter which controls the minimum size for requests (as a
|
|
|
+ power of 2) where the buddy cache is used.
|
|
|
+
|
|
|
+ mb_stats
|
|
|
+ Controls whether the multiblock allocator should collect statistics,
|
|
|
+ which are shown during the unmount. 1 means to collect statistics, 0
|
|
|
+ means not to collect statistics.
|
|
|
+
|
|
|
+ mb_stream_req
|
|
|
+ Files which have fewer blocks than this tunable parameter will have
|
|
|
+ their blocks allocated out of a block group specific preallocation
|
|
|
+ pool, so that small files are packed closely together. Each large file
|
|
|
+ will have its blocks allocated out of its own unique preallocation
|
|
|
+ pool.
|
|
|
+
|
|
|
+ session_write_kbytes
|
|
|
+ This file is read-only and shows the number of kilobytes of data that
|
|
|
+ have been written to this filesystem since it was mounted.
|
|
|
+
|
|
|
+ reserved_clusters
|
|
|
+ This is RW file and contains number of reserved clusters in the file
|
|
|
+ system which will be used in the specific situations to avoid costly
|
|
|
+ zeroout, unexpected ENOSPC, or possible data loss. The default is 2% or
|
|
|
+ 4096 clusters, whichever is smaller and this can be changed however it
|
|
|
+ can never exceed number of clusters in the file system. If there is not
|
|
|
+ enough space for the reserved space when mounting the file mount will
|
|
|
+ _not_ fail.
|
|
|
+
|
|
|
+Ioctls
|
|
|
+======
|
|
|
+
|
|
|
+There is some Ext4 specific functionality which can be accessed by applications
|
|
|
+through the system call interfaces. The list of all Ext4 specific ioctls are
|
|
|
+shown in the table below.
|
|
|
+
|
|
|
+Table of Ext4 specific ioctls
|
|
|
+
|
|
|
+ EXT4_IOC_GETFLAGS
|
|
|
+ Get additional attributes associated with inode. The ioctl argument is
|
|
|
+ an integer bitfield, with bit values described in ext4.h. This ioctl is
|
|
|
+ an alias for FS_IOC_GETFLAGS.
|
|
|
+
|
|
|
+ EXT4_IOC_SETFLAGS
|
|
|
+ Set additional attributes associated with inode. The ioctl argument is
|
|
|
+ an integer bitfield, with bit values described in ext4.h. This ioctl is
|
|
|
+ an alias for FS_IOC_SETFLAGS.
|
|
|
+
|
|
|
+ EXT4_IOC_GETVERSION, EXT4_IOC_GETVERSION_OLD
|
|
|
+ Get the inode i_generation number stored for each inode. The
|
|
|
+ i_generation number is normally changed only when new inode is created
|
|
|
+ and it is particularly useful for network filesystems. The '_OLD'
|
|
|
+ version of this ioctl is an alias for FS_IOC_GETVERSION.
|
|
|
+
|
|
|
+ EXT4_IOC_SETVERSION, EXT4_IOC_SETVERSION_OLD
|
|
|
+ Set the inode i_generation number stored for each inode. The '_OLD'
|
|
|
+ version of this ioctl is an alias for FS_IOC_SETVERSION.
|
|
|
+
|
|
|
+ EXT4_IOC_GROUP_EXTEND
|
|
|
+ This ioctl has the same purpose as the resize mount option. It allows
|
|
|
+ to resize filesystem to the end of the last existing block group,
|
|
|
+ further resize has to be done with resize2fs, either online, or
|
|
|
+ offline. The argument points to the unsigned logn number representing
|
|
|
+ the filesystem new block count.
|
|
|
+
|
|
|
+ EXT4_IOC_MOVE_EXT
|
|
|
+ Move the block extents from orig_fd (the one this ioctl is pointing to)
|
|
|
+ to the donor_fd (the one specified in move_extent structure passed as
|
|
|
+ an argument to this ioctl). Then, exchange inode metadata between
|
|
|
+ orig_fd and donor_fd. This is especially useful for online
|
|
|
+ defragmentation, because the allocator has the opportunity to allocate
|
|
|
+ moved blocks better, ideally into one contiguous extent.
|
|
|
+
|
|
|
+ EXT4_IOC_GROUP_ADD
|
|
|
+ Add a new group descriptor to an existing or new group descriptor
|
|
|
+ block. The new group descriptor is described by ext4_new_group_input
|
|
|
+ structure, which is passed as an argument to this ioctl. This is
|
|
|
+ especially useful in conjunction with EXT4_IOC_GROUP_EXTEND, which
|
|
|
+ allows online resize of the filesystem to the end of the last existing
|
|
|
+ block group. Those two ioctls combined is used in userspace online
|
|
|
+ resize tool (e.g. resize2fs).
|
|
|
+
|
|
|
+ EXT4_IOC_MIGRATE
|
|
|
+ This ioctl operates on the filesystem itself. It converts (migrates)
|
|
|
+ ext3 indirect block mapped inode to ext4 extent mapped inode by walking
|
|
|
+ through indirect block mapping of the original inode and converting
|
|
|
+ contiguous block ranges into ext4 extents of the temporary inode. Then,
|
|
|
+ inodes are swapped. This ioctl might help, when migrating from ext3 to
|
|
|
+ ext4 filesystem, however suggestion is to create fresh ext4 filesystem
|
|
|
+ and copy data from the backup. Note, that filesystem has to support
|
|
|
+ extents for this ioctl to work.
|
|
|
+
|
|
|
+ EXT4_IOC_ALLOC_DA_BLKS
|
|
|
+ Force all of the delay allocated blocks to be allocated to preserve
|
|
|
+ application-expected ext3 behaviour. Note that this will also start
|
|
|
+ triggering a write of the data blocks, but this behaviour may change in
|
|
|
+ the future as it is not necessary and has been done this way only for
|
|
|
+ sake of simplicity.
|
|
|
+
|
|
|
+ EXT4_IOC_RESIZE_FS
|
|
|
+ Resize the filesystem to a new size. The number of blocks of resized
|
|
|
+ filesystem is passed in via 64 bit integer argument. The kernel
|
|
|
+ allocates bitmaps and inode table, the userspace tool thus just passes
|
|
|
+ the new number of blocks.
|
|
|
+
|
|
|
+ EXT4_IOC_SWAP_BOOT
|
|
|
+ Swap i_blocks and associated attributes (like i_blocks, i_size,
|
|
|
+ i_flags, ...) from the specified inode with inode EXT4_BOOT_LOADER_INO
|
|
|
+ (#5). This is typically used to store a boot loader in a secure part of
|
|
|
+ the filesystem, where it can't be changed by a normal user by accident.
|
|
|
+ The data blocks of the previous boot loader will be associated with the
|
|
|
+ given inode.
|
|
|
+
|
|
|
+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
|