.. SPDX-License-Identifier: GPL-2.0 ========================ext4 General Information======================== Ext4 is an advanced level of the ext3 filesystem which incorporatesscalability and reliability enhancements for supporting large filesystems(64 bit) in keeping with increasing disk capacities and state-of-the-artfeature requirements. Mailing list:	linux-ext4@vger.kernel.orgWeb site:	http://ext4.wiki.kernel.org  Quick usage instructions======================== Note: More extensive information for getting started with ext4 can befound 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 thedirectory hash tree having a maximum depth of two. case-insensitive file name lookups====================================================== The case-insensitive file name lookup feature is supported on aper-directory basis, allowing the user to mix case-insensitive andcase-sensitive directories in the same filesystem.  It is enabled byflipping the +F inode attribute of an empty directory.  Thecase-insensitive string match operation is only defined when we know howtext in encoded in a byte sequence.  For that reason, in order to enablecase-insensitive directories, the filesystem must have thecasefold feature, which stores the filesystem-wide encodingmodel used.  By default, the charset adopted is the latest version ofUnicode (12.1.0, by the time of this writing), encoded in the UTF-8form.  The comparison algorithm is implemented by normalizing thestrings 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 filename provided by userspace is a byte-per-byte match to what is actuallywritten in the disk.  The Unicode normalization format used by thekernel is thus an internal representation, and not exposed to theuserspace nor to the disk, with the important exception of disk hashes,used on large case-insensitive directories with DX feature.  On DXdirectories, the hash must be calculated using the casefolded version ofthe filename, meaning that the normalization format used actually has animpact on where the directory entry is stored. When we change from viewing filenames as opaque byte sequences to seeingthem as encoded strings we need to address what happens when a programtries to create a file with an invalid name.  The Unicode subsystemwithin the kernel leaves the decision of what to do in this case to thefilesystem, which select its preferred behavior by enabling/disablingthe strict mode.  When Ext4 encounters one of those strings and thefilesystem did not require strict mode, it falls back to considering theentire string as an opaque byte sequence, which still allows the user tooperate 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.   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.   data_err=abort        Abort the journal if an error occurs in a file data buffer.   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.rst.  Note that this option is        incompatible with data=journal.   inlinecrypt        When possible, encrypt/decrypt the contents of encrypted files using        inline encryption hardware rather than the CPU. For more details, see        Documentation/filesystems/fscrypt.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 and JFS 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 shownin 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 shownin 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

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