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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.rst. 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_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 |