OpenCloudOS-Kernel/Documentation/admin-guide/ext4.rst

.. 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)
* Case-insensitive file name lookups
* file-based encryption support (fscrypt)
* file-based verity support (fsverity)

[1] Filesystems with a block size of 1k may see a limit imposed by the
directory hash tree having a maximum depth of two.

case-insensitive file name lookups
======================================================

The case-insensitive file name lookup feature is supported on a
per-directory basis, allowing the user to mix case-insensitive and
case-sensitive directories in the same filesystem.  It is enabled by
flipping the +F inode attribute of an empty directory.  The
case-insensitive string match operation is only defined when we know how
text in encoded in a byte sequence.  For that reason, in order to enable
case-insensitive directories, the filesystem must have the
casefold feature, which stores the filesystem-wide encoding
model used.  By default, the charset adopted is the latest version of
Unicode (12.1.0, by the time of this writing), encoded in the UTF-8
form.  The comparison algorithm is implemented by normalizing the
strings to the Canonical decomposition form, as defined by Unicode,
followed by a byte per byte comparison.

The case-awareness is name-preserving on the disk, meaning that the file
name provided by userspace is a byte-per-byte match to what is actually
written in the disk.  The Unicode normalization format used by the
kernel is thus an internal representation, and not exposed to the
userspace nor to the disk, with the important exception of disk hashes,
used on large case-insensitive directories with DX feature.  On DX
directories, the hash must be calculated using the casefolded version of
the filename, meaning that the normalization format used actually has an
impact on where the directory entry is stored.

When we change from viewing filenames as opaque byte sequences to seeing
them as encoded strings we need to address what happens when a program
tries to create a file with an invalid name.  The Unicode subsystem
within the kernel leaves the decision of what to do in this case to the
filesystem, which select its preferred behavior by enabling/disabling
the strict mode.  When Ext4 encounters one of those strings and the
filesystem did not require strict mode, it falls back to considering the
entire string as an opaque byte sequence, which still allows the user to
operate on that file, but the case-insensitive lookups won't work.

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	(*)
        This setting limits the maximum age of the running transaction to
        '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
        metadata changes (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.  Note that due to
        delayed allocation even older data can be lost on power failure since
        writeback of those data begins only after time set in
        /proc/sys/vm/dirty_expire_centisecs.

  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.

  inlinecrypt
        When possible, encrypt/decrypt the contents of encrypted files using the
        blk-crypto framework rather than filesystem-layer encryption. This
        allows the use of inline encryption hardware. The on-disk format is
        unaffected. For more details, see
        Documentation/block/inline-encryption.rst.

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_inode_prealloc
        The maximum length of per-inode ext4_prealloc_space list.

  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
======

Ext4 implements various ioctls which can be used by applications to access
ext4-specific functionality. An incomplete list of these ioctls is shown in the
table below. This list includes truly ext4-specific ioctls (``EXT4_IOC_*``) as
well as ioctls that may have been ext4-specific originally but are now supported
by some other filesystem(s) too (``FS_IOC_*``).

Table of Ext4 ioctls

  FS_IOC_GETFLAGS
        Get additional attributes associated with inode.  The ioctl argument is
        an integer bitfield, with bit values described in ext4.h.

  FS_IOC_SETFLAGS
        Set additional attributes associated with inode.  The ioctl argument is
        an integer bitfield, with bit values described in ext4.h.

  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:	https://fedoraproject.org/wiki/ext3-devel
		http://www.bullopensource.org/ext4/
		http://ext4.wiki.kernel.org/index.php/Main_Page
		https://fedoraproject.org/wiki/Features/Ext4