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-
-Ext4 Filesystem
-===============
-
-Ext4 is an 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
-
-
-1. 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
-
- - Compile and install the latest version of e2fsprogs (as of this
- writing version 1.41.3) from:
-
- http://sourceforge.net/project/showfiles.php?group_id=2406
-
- or
-
- ftp://ftp.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs/
-
- or grab the latest git repository from:
-
- git://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git
-
- - Note that it is highly important to install the mke2fs.conf file
- that comes with the e2fsprogs 1.41.x sources in /etc/mke2fs.conf. If
- you have edited the /etc/mke2fs.conf file installed on your system,
- you will need to merge your changes with the version from e2fsprogs
- 1.41.x.
-
- - 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
-
- (Note: we currently do not have tools to convert an ext4
- filesystem back to ext3; so please do not do try this on production
- filesystems.)
-
- - 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.
-
-2. Features
-===========
-
-2.1 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.
-
-2.2 Candidate features for future inclusion
-
-* Online defrag (patches available but not well tested)
-* reduced mke2fs time via lazy itable initialization in conjunction with
- the uninit_bg feature (capability to do this is available in e2fsprogs
- but a kernel thread to do lazy zeroing of unused inode table blocks
- after filesystem is first mounted is required for safety)
-
-There are several others under discussion, whether they all make it in is
-partly a function of how much time everyone has to work on them. Features like
-metadata checksumming have been discussed and planned for a bit but no patches
-exist yet so I'm not sure they're in the near-term roadmap.
-
-The big performance win will come with mballoc, delalloc and flex_bg
-grouping of bitmaps and inode tables. Some test results available here:
-
- - http://www.bullopensource.org/ext4/20080818-ffsb/ffsb-write-2.6.27-rc1.html
- - http://www.bullopensource.org/ext4/20080818-ffsb/ffsb-readwrite-2.6.27-rc1.html
-
-3. 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_dev=devnum When the external journal device's major/minor numbers
- have changed, this option allows the user to specify
- the new journal location. The journal device is
- identified through its new major/minor numbers encoded
- in devnum.
-
-norecovery Don't load the journal on mounting. Note that
-noload 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(*)> This enables/disables the use of write barriers in
-barrier(*) the jbd code. barrier=0 disables, barrier=1 enables.
-nobarrier 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. If you have extended
- attribute support enabled in the kernel configuration
- (CONFIG_EXT4_FS_XATTR), extended attribute support
- is enabled by default on mount. See the attr(5) manual
- page and http://acl.bestbits.at/ 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 and http://acl.bestbits.at/ 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 Give objects the same group ID as their creator.
-bsdgroups
-
-nogrpid (*) New objects have the group ID of their creator.
-sysvgroups
-
-resgid=n The group ID which may use the reserved blocks.
-
-resuid=n The user ID which may use the reserved blocks.
-
-sb=n Use alternate superblock at this location.
-
-quota These options are ignored by the filesystem. They
-noquota are used only by quota tools to recognize volumes
-grpquota where quota should be turned on. See documentation
-usrquota in the quota-tools package for more details
- (http://sourceforge.net/projects/linuxquota).
-
-jqfmt=<quota type> These options tell filesystem details about quota
-usrjquota=<file> so that quota information can be properly updated
-grpjquota=<file> 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(*) Many broken applications don't use fsync() when
-noauto_da_alloc 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 Controls whether ext4 should issue discard/TRIM
-nodiscard(*) 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 This options allows to enables/disables the in-kernel
-noblock_validity facility for tracking filesystem metadata blocks
- within internal data structures. This allows multi-
- block allocator and other routines to quickly locate
- extents which might overlap with filesystem metadata
- blocks. This option is intended for debugging
- purposes and since it negatively affects the
- performance, it is off by default.
-
-dioread_lock Controls whether or not ext4 should use the DIO read
-dioread_nolock 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).
-
-i_version Enable 64-bit inode version support. This option is
- off by default.
-
-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>
-..............................................................................
- File Content
- 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)
-..............................................................................
- File Content
-
- 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.
-..............................................................................
-
-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
-..............................................................................
- Ioctl Description
- 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.
-
-..............................................................................
-
-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