--- /dev/null
+.. SPDX-License-Identifier: GPL-2.0
+
+========================
+ext4 General Information
+========================
+
+Ext4 is an advanced level of the ext3 filesystem which incorporates
+scalability and reliability enhancements for supporting large filesystems
+(64 bit) in keeping with increasing disk capacities and state-of-the-art
+feature requirements.
+
+Mailing list: linux-ext4@vger.kernel.org
+Web site: http://ext4.wiki.kernel.org
+
+
+Quick usage instructions
+========================
+
+Note: More extensive information for getting started with ext4 can be
+found at the ext4 wiki site at the URL:
+http://ext4.wiki.kernel.org/index.php/Ext4_Howto
+
+ - The latest version of e2fsprogs can be found at:
+
+ https://www.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs/
+
+ or
+
+ http://sourceforge.net/project/showfiles.php?group_id=2406
+
+ or grab the latest git repository from:
+
+ https://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git
+
+ - Create a new filesystem using the ext4 filesystem type:
+
+ # mke2fs -t ext4 /dev/hda1
+
+ Or to configure an existing ext3 filesystem to support extents:
+
+ # tune2fs -O extents /dev/hda1
+
+ If the filesystem was created with 128 byte inodes, it can be
+ converted to use 256 byte for greater efficiency via:
+
+ # tune2fs -I 256 /dev/hda1
+
+ - Mounting:
+
+ # mount -t ext4 /dev/hda1 /wherever
+
+ - When comparing performance with other filesystems, it's always
+ important to try multiple workloads; very often a subtle change in a
+ workload parameter can completely change the ranking of which
+ filesystems do well compared to others. When comparing versus ext3,
+ note that ext4 enables write barriers by default, while ext3 does
+ not enable write barriers by default. So it is useful to use
+ explicitly specify whether barriers are enabled or not when via the
+ '-o barriers=[0|1]' mount option for both ext3 and ext4 filesystems
+ for a fair comparison. When tuning ext3 for best benchmark numbers,
+ it is often worthwhile to try changing the data journaling mode; '-o
+ data=writeback' can be faster for some workloads. (Note however that
+ running mounted with data=writeback can potentially leave stale data
+ exposed in recently written files in case of an unclean shutdown,
+ which could be a security exposure in some situations.) Configuring
+ the filesystem with a large journal can also be helpful for
+ metadata-intensive workloads.
+
+Features
+========
+
+Currently Available
+-------------------
+
+* ability to use filesystems > 16TB (e2fsprogs support not available yet)
+* extent format reduces metadata overhead (RAM, IO for access, transactions)
+* extent format more robust in face of on-disk corruption due to magics,
+* internal redundancy in tree
+* improved file allocation (multi-block alloc)
+* lift 32000 subdirectory limit imposed by i_links_count[1]
+* nsec timestamps for mtime, atime, ctime, create time
+* inode version field on disk (NFSv4, Lustre)
+* reduced e2fsck time via uninit_bg feature
+* journal checksumming for robustness, performance
+* persistent file preallocation (e.g for streaming media, databases)
+* ability to pack bitmaps and inode tables into larger virtual groups via the
+ flex_bg feature
+* large file support
+* inode allocation using large virtual block groups via flex_bg
+* delayed allocation
+* large block (up to pagesize) support
+* efficient new ordered mode in JBD2 and ext4 (avoid using buffer head to force
+ the ordering)
+
+[1] Filesystems with a block size of 1k may see a limit imposed by the
+directory hash tree having a maximum depth of two.
+
+Options
+=======
+
+When mounting an ext4 filesystem, the following option are accepted:
+(*) == default
+
+ ro
+ Mount filesystem read only. Note that ext4 will replay the journal (and
+ thus write to the partition) even when mounted "read only". The mount
+ options "ro,noload" can be used to prevent writes to the filesystem.
+
+ journal_checksum
+ Enable checksumming of the journal transactions. This will allow the
+ recovery code in e2fsck and the kernel to detect corruption in the
+ kernel. It is a compatible change and will be ignored by older
+ kernels.
+
+ journal_async_commit
+ Commit block can be written to disk without waiting for descriptor
+ blocks. If enabled older kernels cannot mount the device. This will
+ enable 'journal_checksum' internally.
+
+ journal_path=path, journal_dev=devnum
+ When the external journal device's major/minor numbers have changed,
+ these options allow the user to specify the new journal location. The
+ journal device is identified through either its new major/minor numbers
+ encoded in devnum, or via a path to the device.
+
+ norecovery, noload
+ Don't load the journal on mounting. Note that if the filesystem was
+ not unmounted cleanly, skipping the journal replay will lead to the
+ filesystem containing inconsistencies that can lead to any number of
+ problems.
+
+ data=journal
+ All data are committed into the journal prior to being written into the
+ main file system. Enabling this mode will disable delayed allocation
+ and O_DIRECT support.
+
+ data=ordered (*)
+ All data are forced directly out to the main file system prior to its
+ metadata being committed to the journal.
+
+ data=writeback
+ Data ordering is not preserved, data may be written into the main file
+ system after its metadata has been committed to the journal.
+
+ commit=nrsec (*)
+ Ext4 can be told to sync all its data and metadata every 'nrsec'
+ seconds. The default value is 5 seconds. This means that if you lose
+ your power, you will lose as much as the latest 5 seconds of work (your
+ filesystem will not be damaged though, thanks to the journaling). This
+ default value (or any low value) will hurt performance, but it's good
+ for data-safety. Setting it to 0 will have the same effect as leaving
+ it at the default (5 seconds). Setting it to very large values will
+ improve performance.
+
+ barrier=<0|1(*)>, barrier(*), nobarrier
+ This enables/disables the use of write barriers in the jbd code.
+ barrier=0 disables, barrier=1 enables. This also requires an IO stack
+ which can support barriers, and if jbd gets an error on a barrier
+ write, it will disable again with a warning. Write barriers enforce
+ proper on-disk ordering of journal commits, making volatile disk write
+ caches safe to use, at some performance penalty. If your disks are
+ battery-backed in one way or another, disabling barriers may safely
+ improve performance. The mount options "barrier" and "nobarrier" can
+ also be used to enable or disable barriers, for consistency with other
+ ext4 mount options.
+
+ inode_readahead_blks=n
+ This tuning parameter controls the maximum number of inode table blocks
+ that ext4's inode table readahead algorithm will pre-read into the
+ buffer cache. The default value is 32 blocks.
+
+ nouser_xattr
+ Disables Extended User Attributes. See the attr(5) manual page for
+ more information about extended attributes.
+
+ noacl
+ This option disables POSIX Access Control List support. If ACL support
+ is enabled in the kernel configuration (CONFIG_EXT4_FS_POSIX_ACL), ACL
+ is enabled by default on mount. See the acl(5) manual page for more
+ information about acl.
+
+ bsddf (*)
+ Make 'df' act like BSD.
+
+ minixdf
+ Make 'df' act like Minix.
+
+ debug
+ Extra debugging information is sent to syslog.
+
+ abort
+ Simulate the effects of calling ext4_abort() for debugging purposes.
+ This is normally used while remounting a filesystem which is already
+ mounted.
+
+ errors=remount-ro
+ Remount the filesystem read-only on an error.
+
+ errors=continue
+ Keep going on a filesystem error.
+
+ errors=panic
+ Panic and halt the machine if an error occurs. (These mount options
+ override the errors behavior specified in the superblock, which can be
+ configured using tune2fs)
+
+ data_err=ignore(*)
+ Just print an error message if an error occurs in a file data buffer in
+ ordered mode.
+ data_err=abort
+ Abort the journal if an error occurs in a file data buffer in ordered
+ mode.
+
+ grpid | bsdgroups
+ New objects have the group ID of their parent.
+
+ nogrpid (*) | sysvgroups
+ New objects have the group ID of their creator.
+
+ resgid=n
+ The group ID which may use the reserved blocks.
+
+ resuid=n
+ The user ID which may use the reserved blocks.
+
+ sb=
+ Use alternate superblock at this location.
+
+ quota, noquota, grpquota, usrquota
+ These options are ignored by the filesystem. They are used only by
+ quota tools to recognize volumes where quota should be turned on. See
+ documentation in the quota-tools package for more details
+ (http://sourceforge.net/projects/linuxquota).
+
+ jqfmt=<quota type>, usrjquota=<file>, grpjquota=<file>
+ These options tell filesystem details about quota so that quota
+ information can be properly updated during journal replay. They replace
+ the above quota options. See documentation in the quota-tools package
+ for more details (http://sourceforge.net/projects/linuxquota).
+
+ stripe=n
+ Number of filesystem blocks that mballoc will try to use for allocation
+ size and alignment. For RAID5/6 systems this should be the number of
+ data disks * RAID chunk size in file system blocks.
+
+ delalloc (*)
+ Defer block allocation until just before ext4 writes out the block(s)
+ in question. This allows ext4 to better allocation decisions more
+ efficiently.
+
+ nodelalloc
+ Disable delayed allocation. Blocks are allocated when the data is
+ copied from userspace to the page cache, either via the write(2) system
+ call or when an mmap'ed page which was previously unallocated is
+ written for the first time.
+
+ max_batch_time=usec
+ Maximum amount of time ext4 should wait for additional filesystem
+ operations to be batch together with a synchronous write operation.
+ Since a synchronous write operation is going to force a commit and then
+ a wait for the I/O complete, it doesn't cost much, and can be a huge
+ throughput win, we wait for a small amount of time to see if any other
+ transactions can piggyback on the synchronous write. The algorithm
+ used is designed to automatically tune for the speed of the disk, by
+ measuring the amount of time (on average) that it takes to finish
+ committing a transaction. Call this time the "commit time". If the
+ time that the transaction has been running is less than the commit
+ time, ext4 will try sleeping for the commit time to see if other
+ operations will join the transaction. The commit time is capped by
+ the max_batch_time, which defaults to 15000us (15ms). This
+ optimization can be turned off entirely by setting max_batch_time to 0.
+
+ min_batch_time=usec
+ This parameter sets the commit time (as described above) to be at least
+ min_batch_time. It defaults to zero microseconds. Increasing this
+ parameter may improve the throughput of multi-threaded, synchronous
+ workloads on very fast disks, at the cost of increasing latency.
+
+ journal_ioprio=prio
+ The I/O priority (from 0 to 7, where 0 is the highest priority) which
+ should be used for I/O operations submitted by kjournald2 during a
+ commit operation. This defaults to 3, which is a slightly higher
+ priority than the default I/O priority.
+
+ auto_da_alloc(*), noauto_da_alloc
+ Many broken applications don't use fsync() when replacing existing
+ files via patterns such as fd = open("foo.new")/write(fd,..)/close(fd)/
+ rename("foo.new", "foo"), or worse yet, fd = open("foo",
+ O_TRUNC)/write(fd,..)/close(fd). If auto_da_alloc is enabled, ext4
+ will detect the replace-via-rename and replace-via-truncate patterns
+ and force that any delayed allocation blocks are allocated such that at
+ the next journal commit, in the default data=ordered mode, the data
+ blocks of the new file are forced to disk before the rename() operation
+ is committed. This provides roughly the same level of guarantees as
+ ext3, and avoids the "zero-length" problem that can happen when a
+ system crashes before the delayed allocation blocks are forced to disk.
+
+ noinit_itable
+ Do not initialize any uninitialized inode table blocks in the
+ background. This feature may be used by installation CD's so that the
+ install process can complete as quickly as possible; the inode table
+ initialization process would then be deferred until the next time the
+ file system is unmounted.
+
+ init_itable=n
+ The lazy itable init code will wait n times the number of milliseconds
+ it took to zero out the previous block group's inode table. This
+ minimizes the impact on the system performance while file system's
+ inode table is being initialized.
+
+ discard, nodiscard(*)
+ Controls whether ext4 should issue discard/TRIM commands to the
+ underlying block device when blocks are freed. This is useful for SSD
+ devices and sparse/thinly-provisioned LUNs, but it is off by default
+ until sufficient testing has been done.
+
+ nouid32
+ Disables 32-bit UIDs and GIDs. This is for interoperability with
+ older kernels which only store and expect 16-bit values.
+
+ block_validity(*), noblock_validity
+ These options enable or disable the in-kernel facility for tracking
+ filesystem metadata blocks within internal data structures. This
+ allows multi- block allocator and other routines to notice bugs or
+ corrupted allocation bitmaps which cause blocks to be allocated which
+ overlap with filesystem metadata blocks.
+
+ dioread_lock, dioread_nolock
+ Controls whether or not ext4 should use the DIO read locking. If the
+ dioread_nolock option is specified ext4 will allocate uninitialized
+ extent before buffer write and convert the extent to initialized after
+ IO completes. This approach allows ext4 code to avoid using inode
+ mutex, which improves scalability on high speed storages. However this
+ does not work with data journaling and dioread_nolock option will be
+ ignored with kernel warning. Note that dioread_nolock code path is only
+ used for extent-based files. Because of the restrictions this options
+ comprises it is off by default (e.g. dioread_lock).
+
+ max_dir_size_kb=n
+ This limits the size of directories so that any attempt to expand them
+ beyond the specified limit in kilobytes will cause an ENOSPC error.
+ This is useful in memory constrained environments, where a very large
+ directory can cause severe performance problems or even provoke the Out
+ Of Memory killer. (For example, if there is only 512mb memory
+ available, a 176mb directory may seriously cramp the system's style.)
+
+ i_version
+ Enable 64-bit inode version support. This option is off by default.
+
+ dax
+ Use direct access (no page cache). See
+ Documentation/filesystems/dax.txt. Note that this option is
+ incompatible with data=journal.
+
+Data Mode
+=========
+There are 3 different data modes:
+
+* writeback mode
+
+ In data=writeback mode, ext4 does not journal data at all. This mode provides
+ a similar level of journaling as that of XFS, JFS, and ReiserFS in its default
+ mode - metadata journaling. A crash+recovery can cause incorrect data to
+ appear in files which were written shortly before the crash. This mode will
+ typically provide the best ext4 performance.
+
+* ordered mode
+
+ In data=ordered mode, ext4 only officially journals metadata, but it logically
+ groups metadata information related to data changes with the data blocks into
+ a single unit called a transaction. When it's time to write the new metadata
+ out to disk, the associated data blocks are written first. In general, this
+ mode performs slightly slower than writeback but significantly faster than
+ journal mode.
+
+* journal mode
+
+ data=journal mode provides full data and metadata journaling. All new data is
+ written to the journal first, and then to its final location. In the event of
+ a crash, the journal can be replayed, bringing both data and metadata into a
+ consistent state. This mode is the slowest except when data needs to be read
+ from and written to disk at the same time where it outperforms all others
+ modes. Enabling this mode will disable delayed allocation and O_DIRECT
+ support.
+
+/proc entries
+=============
+
+Information about mounted ext4 file systems can be found in
+/proc/fs/ext4. Each mounted filesystem will have a directory in
+/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
java
ras
bcache
+ ext4
pm/index
thunderbolt
LSM/index
'The kernel development community', 'manual'),
('filesystems/index', 'filesystems.tex', 'Linux Filesystems API',
'The kernel development community', 'manual'),
+ ('admin-guide/ext4', 'ext4-admin-guide.tex', 'ext4 Administration Guide',
+ 'ext4 Community', 'manual'),
('filesystems/ext4/index', 'ext4.tex', 'ext4 Filesystem',
'ext4 Filesystem Developers', 'manual'),
('gpu/index', 'gpu.tex', 'Linux GPU Driver Developer\'s Guide',
+++ /dev/null
-.. SPDX-License-Identifier: GPL-2.0
-
-========================
-General Information
-========================
-
-Ext4 is an advanced level of the ext3 filesystem which incorporates
-scalability and reliability enhancements for supporting large filesystems
-(64 bit) in keeping with increasing disk capacities and state-of-the-art
-feature requirements.
-
-Mailing list: linux-ext4@vger.kernel.org
-Web site: http://ext4.wiki.kernel.org
-
-
-Quick usage instructions
-========================
-
-Note: More extensive information for getting started with ext4 can be
-found at the ext4 wiki site at the URL:
-http://ext4.wiki.kernel.org/index.php/Ext4_Howto
-
- - The latest version of e2fsprogs can be found at:
-
- https://www.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs/
-
- or
-
- http://sourceforge.net/project/showfiles.php?group_id=2406
-
- or grab the latest git repository from:
-
- https://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git
-
- - Create a new filesystem using the ext4 filesystem type:
-
- # mke2fs -t ext4 /dev/hda1
-
- Or to configure an existing ext3 filesystem to support extents:
-
- # tune2fs -O extents /dev/hda1
-
- If the filesystem was created with 128 byte inodes, it can be
- converted to use 256 byte for greater efficiency via:
-
- # tune2fs -I 256 /dev/hda1
-
- - Mounting:
-
- # mount -t ext4 /dev/hda1 /wherever
-
- - When comparing performance with other filesystems, it's always
- important to try multiple workloads; very often a subtle change in a
- workload parameter can completely change the ranking of which
- filesystems do well compared to others. When comparing versus ext3,
- note that ext4 enables write barriers by default, while ext3 does
- not enable write barriers by default. So it is useful to use
- explicitly specify whether barriers are enabled or not when via the
- '-o barriers=[0|1]' mount option for both ext3 and ext4 filesystems
- for a fair comparison. When tuning ext3 for best benchmark numbers,
- it is often worthwhile to try changing the data journaling mode; '-o
- data=writeback' can be faster for some workloads. (Note however that
- running mounted with data=writeback can potentially leave stale data
- exposed in recently written files in case of an unclean shutdown,
- which could be a security exposure in some situations.) Configuring
- the filesystem with a large journal can also be helpful for
- metadata-intensive workloads.
-
-Features
-========
-
-Currently Available
--------------------
-
-* ability to use filesystems > 16TB (e2fsprogs support not available yet)
-* extent format reduces metadata overhead (RAM, IO for access, transactions)
-* extent format more robust in face of on-disk corruption due to magics,
-* internal redundancy in tree
-* improved file allocation (multi-block alloc)
-* lift 32000 subdirectory limit imposed by i_links_count[1]
-* nsec timestamps for mtime, atime, ctime, create time
-* inode version field on disk (NFSv4, Lustre)
-* reduced e2fsck time via uninit_bg feature
-* journal checksumming for robustness, performance
-* persistent file preallocation (e.g for streaming media, databases)
-* ability to pack bitmaps and inode tables into larger virtual groups via the
- flex_bg feature
-* large file support
-* inode allocation using large virtual block groups via flex_bg
-* delayed allocation
-* large block (up to pagesize) support
-* efficient new ordered mode in JBD2 and ext4 (avoid using buffer head to force
- the ordering)
-
-[1] Filesystems with a block size of 1k may see a limit imposed by the
-directory hash tree having a maximum depth of two.
-
-Options
-=======
-
-When mounting an ext4 filesystem, the following option are accepted:
-(*) == default
-
- ro
- Mount filesystem read only. Note that ext4 will replay the journal (and
- thus write to the partition) even when mounted "read only". The mount
- options "ro,noload" can be used to prevent writes to the filesystem.
-
- journal_checksum
- Enable checksumming of the journal transactions. This will allow the
- recovery code in e2fsck and the kernel to detect corruption in the
- kernel. It is a compatible change and will be ignored by older
- kernels.
-
- journal_async_commit
- Commit block can be written to disk without waiting for descriptor
- blocks. If enabled older kernels cannot mount the device. This will
- enable 'journal_checksum' internally.
-
- journal_path=path, journal_dev=devnum
- When the external journal device's major/minor numbers have changed,
- these options allow the user to specify the new journal location. The
- journal device is identified through either its new major/minor numbers
- encoded in devnum, or via a path to the device.
-
- norecovery, noload
- Don't load the journal on mounting. Note that if the filesystem was
- not unmounted cleanly, skipping the journal replay will lead to the
- filesystem containing inconsistencies that can lead to any number of
- problems.
-
- data=journal
- All data are committed into the journal prior to being written into the
- main file system. Enabling this mode will disable delayed allocation
- and O_DIRECT support.
-
- data=ordered (*)
- All data are forced directly out to the main file system prior to its
- metadata being committed to the journal.
-
- data=writeback
- Data ordering is not preserved, data may be written into the main file
- system after its metadata has been committed to the journal.
-
- commit=nrsec (*)
- Ext4 can be told to sync all its data and metadata every 'nrsec'
- seconds. The default value is 5 seconds. This means that if you lose
- your power, you will lose as much as the latest 5 seconds of work (your
- filesystem will not be damaged though, thanks to the journaling). This
- default value (or any low value) will hurt performance, but it's good
- for data-safety. Setting it to 0 will have the same effect as leaving
- it at the default (5 seconds). Setting it to very large values will
- improve performance.
-
- barrier=<0|1(*)>, barrier(*), nobarrier
- This enables/disables the use of write barriers in the jbd code.
- barrier=0 disables, barrier=1 enables. This also requires an IO stack
- which can support barriers, and if jbd gets an error on a barrier
- write, it will disable again with a warning. Write barriers enforce
- proper on-disk ordering of journal commits, making volatile disk write
- caches safe to use, at some performance penalty. If your disks are
- battery-backed in one way or another, disabling barriers may safely
- improve performance. The mount options "barrier" and "nobarrier" can
- also be used to enable or disable barriers, for consistency with other
- ext4 mount options.
-
- inode_readahead_blks=n
- This tuning parameter controls the maximum number of inode table blocks
- that ext4's inode table readahead algorithm will pre-read into the
- buffer cache. The default value is 32 blocks.
-
- nouser_xattr
- Disables Extended User Attributes. See the attr(5) manual page for
- more information about extended attributes.
-
- noacl
- This option disables POSIX Access Control List support. If ACL support
- is enabled in the kernel configuration (CONFIG_EXT4_FS_POSIX_ACL), ACL
- is enabled by default on mount. See the acl(5) manual page for more
- information about acl.
-
- bsddf (*)
- Make 'df' act like BSD.
-
- minixdf
- Make 'df' act like Minix.
-
- debug
- Extra debugging information is sent to syslog.
-
- abort
- Simulate the effects of calling ext4_abort() for debugging purposes.
- This is normally used while remounting a filesystem which is already
- mounted.
-
- errors=remount-ro
- Remount the filesystem read-only on an error.
-
- errors=continue
- Keep going on a filesystem error.
-
- errors=panic
- Panic and halt the machine if an error occurs. (These mount options
- override the errors behavior specified in the superblock, which can be
- configured using tune2fs)
-
- data_err=ignore(*)
- Just print an error message if an error occurs in a file data buffer in
- ordered mode.
- data_err=abort
- Abort the journal if an error occurs in a file data buffer in ordered
- mode.
-
- grpid | bsdgroups
- New objects have the group ID of their parent.
-
- nogrpid (*) | sysvgroups
- New objects have the group ID of their creator.
-
- resgid=n
- The group ID which may use the reserved blocks.
-
- resuid=n
- The user ID which may use the reserved blocks.
-
- sb=
- Use alternate superblock at this location.
-
- quota, noquota, grpquota, usrquota
- These options are ignored by the filesystem. They are used only by
- quota tools to recognize volumes where quota should be turned on. See
- documentation in the quota-tools package for more details
- (http://sourceforge.net/projects/linuxquota).
-
- jqfmt=<quota type>, usrjquota=<file>, grpjquota=<file>
- These options tell filesystem details about quota so that quota
- information can be properly updated during journal replay. They replace
- the above quota options. See documentation in the quota-tools package
- for more details (http://sourceforge.net/projects/linuxquota).
-
- stripe=n
- Number of filesystem blocks that mballoc will try to use for allocation
- size and alignment. For RAID5/6 systems this should be the number of
- data disks * RAID chunk size in file system blocks.
-
- delalloc (*)
- Defer block allocation until just before ext4 writes out the block(s)
- in question. This allows ext4 to better allocation decisions more
- efficiently.
-
- nodelalloc
- Disable delayed allocation. Blocks are allocated when the data is
- copied from userspace to the page cache, either via the write(2) system
- call or when an mmap'ed page which was previously unallocated is
- written for the first time.
-
- max_batch_time=usec
- Maximum amount of time ext4 should wait for additional filesystem
- operations to be batch together with a synchronous write operation.
- Since a synchronous write operation is going to force a commit and then
- a wait for the I/O complete, it doesn't cost much, and can be a huge
- throughput win, we wait for a small amount of time to see if any other
- transactions can piggyback on the synchronous write. The algorithm
- used is designed to automatically tune for the speed of the disk, by
- measuring the amount of time (on average) that it takes to finish
- committing a transaction. Call this time the "commit time". If the
- time that the transaction has been running is less than the commit
- time, ext4 will try sleeping for the commit time to see if other
- operations will join the transaction. The commit time is capped by
- the max_batch_time, which defaults to 15000us (15ms). This
- optimization can be turned off entirely by setting max_batch_time to 0.
-
- min_batch_time=usec
- This parameter sets the commit time (as described above) to be at least
- min_batch_time. It defaults to zero microseconds. Increasing this
- parameter may improve the throughput of multi-threaded, synchronous
- workloads on very fast disks, at the cost of increasing latency.
-
- journal_ioprio=prio
- The I/O priority (from 0 to 7, where 0 is the highest priority) which
- should be used for I/O operations submitted by kjournald2 during a
- commit operation. This defaults to 3, which is a slightly higher
- priority than the default I/O priority.
-
- auto_da_alloc(*), noauto_da_alloc
- Many broken applications don't use fsync() when replacing existing
- files via patterns such as fd = open("foo.new")/write(fd,..)/close(fd)/
- rename("foo.new", "foo"), or worse yet, fd = open("foo",
- O_TRUNC)/write(fd,..)/close(fd). If auto_da_alloc is enabled, ext4
- will detect the replace-via-rename and replace-via-truncate patterns
- and force that any delayed allocation blocks are allocated such that at
- the next journal commit, in the default data=ordered mode, the data
- blocks of the new file are forced to disk before the rename() operation
- is committed. This provides roughly the same level of guarantees as
- ext3, and avoids the "zero-length" problem that can happen when a
- system crashes before the delayed allocation blocks are forced to disk.
-
- noinit_itable
- Do not initialize any uninitialized inode table blocks in the
- background. This feature may be used by installation CD's so that the
- install process can complete as quickly as possible; the inode table
- initialization process would then be deferred until the next time the
- file system is unmounted.
-
- init_itable=n
- The lazy itable init code will wait n times the number of milliseconds
- it took to zero out the previous block group's inode table. This
- minimizes the impact on the system performance while file system's
- inode table is being initialized.
-
- discard, nodiscard(*)
- Controls whether ext4 should issue discard/TRIM commands to the
- underlying block device when blocks are freed. This is useful for SSD
- devices and sparse/thinly-provisioned LUNs, but it is off by default
- until sufficient testing has been done.
-
- nouid32
- Disables 32-bit UIDs and GIDs. This is for interoperability with
- older kernels which only store and expect 16-bit values.
-
- block_validity(*), noblock_validity
- These options enable or disable the in-kernel facility for tracking
- filesystem metadata blocks within internal data structures. This
- allows multi- block allocator and other routines to notice bugs or
- corrupted allocation bitmaps which cause blocks to be allocated which
- overlap with filesystem metadata blocks.
-
- dioread_lock, dioread_nolock
- Controls whether or not ext4 should use the DIO read locking. If the
- dioread_nolock option is specified ext4 will allocate uninitialized
- extent before buffer write and convert the extent to initialized after
- IO completes. This approach allows ext4 code to avoid using inode
- mutex, which improves scalability on high speed storages. However this
- does not work with data journaling and dioread_nolock option will be
- ignored with kernel warning. Note that dioread_nolock code path is only
- used for extent-based files. Because of the restrictions this options
- comprises it is off by default (e.g. dioread_lock).
-
- max_dir_size_kb=n
- This limits the size of directories so that any attempt to expand them
- beyond the specified limit in kilobytes will cause an ENOSPC error.
- This is useful in memory constrained environments, where a very large
- directory can cause severe performance problems or even provoke the Out
- Of Memory killer. (For example, if there is only 512mb memory
- available, a 176mb directory may seriously cramp the system's style.)
-
- i_version
- Enable 64-bit inode version support. This option is off by default.
-
- dax
- Use direct access (no page cache). See
- Documentation/filesystems/dax.txt. Note that this option is
- incompatible with data=journal.
-
-Data Mode
-=========
-There are 3 different data modes:
-
-* writeback mode
-
- In data=writeback mode, ext4 does not journal data at all. This mode provides
- a similar level of journaling as that of XFS, JFS, and ReiserFS in its default
- mode - metadata journaling. A crash+recovery can cause incorrect data to
- appear in files which were written shortly before the crash. This mode will
- typically provide the best ext4 performance.
-
-* ordered mode
-
- In data=ordered mode, ext4 only officially journals metadata, but it logically
- groups metadata information related to data changes with the data blocks into
- a single unit called a transaction. When it's time to write the new metadata
- out to disk, the associated data blocks are written first. In general, this
- mode performs slightly slower than writeback but significantly faster than
- journal mode.
-
-* journal mode
-
- data=journal mode provides full data and metadata journaling. All new data is
- written to the journal first, and then to its final location. In the event of
- a crash, the journal can be replayed, bringing both data and metadata into a
- consistent state. This mode is the slowest except when data needs to be read
- from and written to disk at the same time where it outperforms all others
- modes. Enabling this mode will disable delayed allocation and O_DIRECT
- support.
-
-/proc entries
-=============
-
-Information about mounted ext4 file systems can be found in
-/proc/fs/ext4. Each mounted filesystem will have a directory in
-/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
:maxdepth: 5
:numbered:
- ext4
ondisk/index
projects / openwrt / staging / blogic.git / commitdiff