614 lines
26 KiB
ReStructuredText
614 lines
26 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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========================
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General Information
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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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Mailing list: linux-ext4@vger.kernel.org
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Web site: http://ext4.wiki.kernel.org
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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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- The latest version of e2fsprogs can be found at:
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https://www.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs/
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or
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http://sourceforge.net/project/showfiles.php?group_id=2406
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or grab the latest git repository from:
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https://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git
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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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- 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' 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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Features
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========
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Currently Available
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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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[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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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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======================= =======================================================
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Mount Option Description
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======================= =======================================================
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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_path=path
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journal_dev=devnum When the external journal device's major/minor numbers
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have changed, these options allow the user to specify
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the new journal location. The journal device is
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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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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. Enabling
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this mode will disable delayed allocation and
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O_DIRECT support.
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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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nouser_xattr Disables Extended User Attributes. See the
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attr(5) manual page for more information about
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extended attributes.
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noacl This option disables POSIX Access Control List
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support. If ACL support is enabled in the kernel
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configuration (CONFIG_EXT4_FS_POSIX_ACL), ACL is
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enabled by default on mount. See the acl(5) manual
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page for more information about acl.
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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 New objects have the group ID of their parent.
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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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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 priority) 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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noinit_itable Do not initialize any uninitialized inode table
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blocks in the background. This feature may be
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used by installation CD's so that the install
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process can complete as quickly as possible; the
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inode table initialization process would then be
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deferred until the next time the file system
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is unmounted.
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init_itable=n The lazy itable init code will wait n times the
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number of milliseconds it took to zero out the
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previous block group's inode table. This
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minimizes the impact on the system performance
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while file system's inode table is being initialized.
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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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nouid32 Disables 32-bit UIDs and GIDs. This is for
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interoperability with older kernels which only
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store and expect 16-bit values.
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block_validity(*) These options enable or disable the in-kernel
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noblock_validity facility for tracking filesystem metadata blocks
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within internal data structures. This allows multi-
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block allocator and other routines to notice
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bugs or corrupted allocation bitmaps which cause
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blocks to be allocated which overlap with
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filesystem metadata blocks.
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dioread_lock Controls whether or not ext4 should use the DIO read
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dioread_nolock locking. If the dioread_nolock option is specified
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ext4 will allocate uninitialized extent before buffer
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write and convert the extent to initialized after IO
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completes. This approach allows ext4 code to avoid
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using inode mutex, which improves scalability on high
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speed storages. However this does not work with
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data journaling and dioread_nolock option will be
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ignored with kernel warning. Note that dioread_nolock
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code path is only used for extent-based files.
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Because of the restrictions this options comprises
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it is off by default (e.g. dioread_lock).
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max_dir_size_kb=n This limits the size of directories so that any
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attempt to expand them beyond the specified
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limit in kilobytes will cause an ENOSPC error.
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This is useful in memory constrained
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environments, where a very large directory can
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cause severe performance problems or even
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provoke the Out Of Memory killer. (For example,
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if there is only 512mb memory available, a 176mb
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directory may seriously cramp the system's style.)
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i_version Enable 64-bit inode version support. This option is
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off by default.
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dax Use direct access (no page cache). See
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Documentation/filesystems/dax.txt. Note that
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this option is 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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* 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
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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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* 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. 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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/proc entries
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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
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/proc/fs/ext4/dm-0). The files in each per-device directory are shown
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in table below.
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Files in /proc/fs/ext4/<devname>
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================ =======
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File Content
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================ =======
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mb_groups details of multiblock allocator buddy cache of free blocks
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================ =======
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/sys entries
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============
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Information about mounted ext4 file systems can be found in
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/sys/fs/ext4. Each mounted filesystem will have a directory in
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/sys/fs/ext4 based on its device name (i.e., /sys/fs/ext4/hdc or
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/sys/fs/ext4/dm-0). The files in each per-device directory are shown
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in table below.
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Files in /sys/fs/ext4/<devname>:
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(see also Documentation/ABI/testing/sysfs-fs-ext4)
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============================= =================================================
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File Content
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============================= =================================================
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delayed_allocation_blocks This file is read-only and shows the number of
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blocks that are dirty in the page cache, but
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which do not have their location in the
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filesystem allocated yet.
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inode_goal Tuning parameter which (if non-zero) controls
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the goal inode used by the inode allocator in
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preference to all other allocation heuristics.
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This is intended for debugging use only, and
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should be 0 on production systems.
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inode_readahead_blks Tuning parameter which 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
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lifetime_write_kbytes This file is read-only and shows the number of
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kilobytes of data that have been written to this
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filesystem since it was created.
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max_writeback_mb_bump The maximum number of megabytes the writeback
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code will try to write out before move on to
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another inode.
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mb_group_prealloc The multiblock allocator will round up allocation
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requests to a multiple of this tuning parameter if
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the stripe size is not set in the ext4 superblock
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mb_max_to_scan The maximum number of extents the multiblock
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allocator will search to find the best extent
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mb_min_to_scan The minimum number of extents the multiblock
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allocator will search to find the best extent
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mb_order2_req Tuning parameter which controls the minimum size
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for requests (as a power of 2) where the buddy
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cache is used
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mb_stats Controls whether the multiblock allocator should
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collect statistics, which are shown during the
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unmount. 1 means to collect statistics, 0 means
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not to collect statistics
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mb_stream_req Files which have fewer blocks than this tunable
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parameter will have their blocks allocated out
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of a block group specific preallocation pool, so
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that small files are packed closely together.
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Each large file will have its blocks allocated
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out of its own unique preallocation pool.
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session_write_kbytes This file is read-only and shows the number of
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kilobytes of data that have been written to this
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filesystem since it was mounted.
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reserved_clusters This is RW file and contains number of reserved
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clusters in the file system which will be used
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in the specific situations to avoid costly
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zeroout, unexpected ENOSPC, or possible data
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loss. The default is 2% or 4096 clusters,
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whichever is smaller and this can be changed
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however it can never exceed number of clusters
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in the file system. If there is not enough space
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for the reserved space when mounting the file
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mount will _not_ fail.
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============================= =================================================
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Ioctls
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======
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There is some Ext4 specific functionality which can be accessed by applications
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through the system call interfaces. The list of all Ext4 specific ioctls are
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shown in the table below.
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Table of Ext4 specific ioctls
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============================= =================================================
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Ioctl Description
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============================= =================================================
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EXT4_IOC_GETFLAGS Get additional attributes associated with inode.
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The ioctl argument is an integer bitfield, with
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bit values described in ext4.h. This ioctl is an
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alias for FS_IOC_GETFLAGS.
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EXT4_IOC_SETFLAGS Set additional attributes associated with inode.
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The ioctl argument is an integer bitfield, with
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bit values described in ext4.h. This ioctl is an
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alias for FS_IOC_SETFLAGS.
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EXT4_IOC_GETVERSION
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EXT4_IOC_GETVERSION_OLD
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Get the inode i_generation number stored for
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each inode. The i_generation number is normally
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changed only when new inode is created and it is
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particularly useful for network filesystems. The
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'_OLD' version of this ioctl is an alias for
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FS_IOC_GETVERSION.
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EXT4_IOC_SETVERSION
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EXT4_IOC_SETVERSION_OLD
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Set the inode i_generation number stored for
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each inode. The '_OLD' version of this ioctl
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is an alias for FS_IOC_SETVERSION.
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EXT4_IOC_GROUP_EXTEND This ioctl has the same purpose as the resize
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mount option. It allows to resize filesystem
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to the end of the last existing block group,
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further resize has to be done with resize2fs,
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either online, or offline. The argument points
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to the unsigned logn number representing the
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filesystem new block count.
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EXT4_IOC_MOVE_EXT Move the block extents from orig_fd (the one
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this ioctl is pointing to) to the donor_fd (the
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one specified in move_extent structure passed
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as an argument to this ioctl). Then, exchange
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inode metadata between orig_fd and donor_fd.
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This is especially useful for online
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defragmentation, because the allocator has the
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opportunity to allocate moved blocks better,
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ideally into one contiguous extent.
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EXT4_IOC_GROUP_ADD Add a new group descriptor to an existing or
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new group descriptor block. The new group
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descriptor is described by ext4_new_group_input
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structure, which is passed as an argument to
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this ioctl. This is especially useful in
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conjunction with EXT4_IOC_GROUP_EXTEND,
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which allows online resize of the filesystem
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to the end of the last existing block group.
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Those two ioctls combined is used in userspace
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online resize tool (e.g. resize2fs).
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EXT4_IOC_MIGRATE This ioctl operates on the filesystem itself.
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It converts (migrates) ext3 indirect block mapped
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inode to ext4 extent mapped inode by walking
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through indirect block mapping of the original
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inode and converting contiguous block ranges
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into ext4 extents of the temporary inode. Then,
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inodes are swapped. This ioctl might help, when
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migrating from ext3 to ext4 filesystem, however
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suggestion is to create fresh ext4 filesystem
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and copy data from the backup. Note, that
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filesystem has to support extents for this ioctl
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to work.
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EXT4_IOC_ALLOC_DA_BLKS Force all of the delay allocated blocks to be
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allocated to preserve application-expected ext3
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behaviour. Note that this will also start
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triggering a write of the data blocks, but this
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behaviour may change in the future as it is
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not necessary and has been done this way only
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for sake of simplicity.
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EXT4_IOC_RESIZE_FS Resize the filesystem to a new size. The number
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of blocks of resized filesystem is passed in via
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64 bit integer argument. The kernel allocates
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bitmaps and inode table, the userspace tool thus
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just passes the new number of blocks.
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EXT4_IOC_SWAP_BOOT Swap i_blocks and associated attributes
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(like i_blocks, i_size, i_flags, ...) from
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the specified inode with inode
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EXT4_BOOT_LOADER_INO (#5). This is typically
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used to store a boot loader in a secure part of
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the filesystem, where it can't be changed by a
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normal user by accident.
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The data blocks of the previous boot loader
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will be associated with the given inode.
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============================= =================================================
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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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