From: Darrick J. Wong <darrick.wong@xxxxxxxxxx> Move Documentation/filesystems/ext4.txt into Documentation/filesystems/ext4/ext4.rst in preparation for adding more ext4 documentation. Note that the documentation isn't in rst format yet, but as it's not linked from anywhere it won't cause build errors. Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx> --- Documentation/filesystems/ext4.txt | 627 ------------------------------- Documentation/filesystems/ext4/ext4.rst | 627 +++++++++++++++++++++++++++++++ 2 files changed, 627 insertions(+), 627 deletions(-) delete mode 100644 Documentation/filesystems/ext4.txt create mode 100644 Documentation/filesystems/ext4/ext4.rst diff --git a/Documentation/filesystems/ext4.txt b/Documentation/filesystems/ext4.txt deleted file mode 100644 index 7f628b9f7c4b..000000000000 --- a/Documentation/filesystems/ext4.txt +++ /dev/null @@ -1,627 +0,0 @@ - -Ext4 Filesystem -=============== - -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@xxxxxxxxxxxxxxx -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 - - https://www.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_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 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. 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 New objects have the group ID of their parent. -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(*) These options enable or disable the in-kernel -noblock_validity 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 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). - -max_dir_size_kb=n This limits the size of directories so that any - attempt to expand them beyond the specified - limit in kilobytes will cause an ENOSPC error. - This is useful in memory constrained - environments, where a very large directory can - cause severe performance problems or even - provoke the Out Of Memory killer. (For example, - if there is only 512mb memory available, a 176mb - directory may seriously cramp the system's style.) - -i_version Enable 64-bit inode version support. This option is - off by default. - -dax Use direct access (no page cache). See - Documentation/filesystems/dax.txt. Note that - this option is incompatible with data=journal. - -Data Mode -========= -There are 3 different data modes: - -* writeback mode -In data=writeback mode, ext4 does not journal data at all. This mode provides -a similar level of journaling as that of XFS, JFS, and ReiserFS in its default -mode - metadata journaling. A crash+recovery can cause incorrect data to -appear in files which were written shortly before the crash. This mode will -typically provide the best ext4 performance. - -* ordered mode -In data=ordered mode, ext4 only officially journals metadata, but it logically -groups metadata information related to data changes with the data blocks into a -single unit called a transaction. When it's time to write the new metadata -out to disk, the associated data blocks are written first. In general, -this mode performs slightly slower than writeback but significantly faster than journal mode. - -* journal mode -data=journal mode provides full data and metadata journaling. All new data is -written to the journal first, and then to its final location. -In the event of a crash, the journal can be replayed, bringing both data and -metadata into a consistent state. This mode is the slowest except when data -needs to be read from and written to disk at the same time where it -outperforms all others modes. Enabling this mode will disable delayed -allocation and O_DIRECT support. - -/proc entries -============= - -Information about mounted ext4 file systems can be found in -/proc/fs/ext4. Each mounted filesystem will have a directory in -/proc/fs/ext4 based on its device name (i.e., /proc/fs/ext4/hdc or -/proc/fs/ext4/dm-0). The files in each per-device directory are shown -in table below. - -Files in /proc/fs/ext4/<devname> -.............................................................................. - 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. - - reserved_clusters This is RW file and contains number of reserved - clusters in the file system which will be used - in the specific situations to avoid costly - zeroout, unexpected ENOSPC, or possible data - loss. The default is 2% or 4096 clusters, - whichever is smaller and this can be changed - however it can never exceed number of clusters - in the file system. If there is not enough space - for the reserved space when mounting the file - mount will _not_ fail. -.............................................................................. - -Ioctls -====== - -There is some Ext4 specific functionality which can be accessed by applications -through the system call interfaces. The list of all Ext4 specific ioctls are -shown in the table below. - -Table of Ext4 specific ioctls -.............................................................................. - 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. - - EXT4_IOC_SWAP_BOOT Swap i_blocks and associated attributes - (like i_blocks, i_size, i_flags, ...) from - the specified inode with inode - EXT4_BOOT_LOADER_INO (#5). This is typically - used to store a boot loader in a secure part of - the filesystem, where it can't be changed by a - normal user by accident. - The data blocks of the previous boot loader - will be associated with the given inode. - -.............................................................................. - -References -========== - -kernel source: <file:fs/ext4/> - <file:fs/jbd2/> - -programs: http://e2fsprogs.sourceforge.net/ - -useful links: http://fedoraproject.org/wiki/ext3-devel - http://www.bullopensource.org/ext4/ - http://ext4.wiki.kernel.org/index.php/Main_Page - http://fedoraproject.org/wiki/Features/Ext4 diff --git a/Documentation/filesystems/ext4/ext4.rst b/Documentation/filesystems/ext4/ext4.rst new file mode 100644 index 000000000000..7f628b9f7c4b --- /dev/null +++ b/Documentation/filesystems/ext4/ext4.rst @@ -0,0 +1,627 @@ + +Ext4 Filesystem +=============== + +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@xxxxxxxxxxxxxxx +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 + + https://www.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_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 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. 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 New objects have the group ID of their parent. +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(*) These options enable or disable the in-kernel +noblock_validity 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 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). + +max_dir_size_kb=n This limits the size of directories so that any + attempt to expand them beyond the specified + limit in kilobytes will cause an ENOSPC error. + This is useful in memory constrained + environments, where a very large directory can + cause severe performance problems or even + provoke the Out Of Memory killer. (For example, + if there is only 512mb memory available, a 176mb + directory may seriously cramp the system's style.) + +i_version Enable 64-bit inode version support. This option is + off by default. + +dax Use direct access (no page cache). See + Documentation/filesystems/dax.txt. Note that + this option is incompatible with data=journal. + +Data Mode +========= +There are 3 different data modes: + +* writeback mode +In data=writeback mode, ext4 does not journal data at all. This mode provides +a similar level of journaling as that of XFS, JFS, and ReiserFS in its default +mode - metadata journaling. A crash+recovery can cause incorrect data to +appear in files which were written shortly before the crash. This mode will +typically provide the best ext4 performance. + +* ordered mode +In data=ordered mode, ext4 only officially journals metadata, but it logically +groups metadata information related to data changes with the data blocks into a +single unit called a transaction. When it's time to write the new metadata +out to disk, the associated data blocks are written first. In general, +this mode performs slightly slower than writeback but significantly faster than journal mode. + +* journal mode +data=journal mode provides full data and metadata journaling. All new data is +written to the journal first, and then to its final location. +In the event of a crash, the journal can be replayed, bringing both data and +metadata into a consistent state. This mode is the slowest except when data +needs to be read from and written to disk at the same time where it +outperforms all others modes. Enabling this mode will disable delayed +allocation and O_DIRECT support. + +/proc entries +============= + +Information about mounted ext4 file systems can be found in +/proc/fs/ext4. Each mounted filesystem will have a directory in +/proc/fs/ext4 based on its device name (i.e., /proc/fs/ext4/hdc or +/proc/fs/ext4/dm-0). The files in each per-device directory are shown +in table below. + +Files in /proc/fs/ext4/<devname> +.............................................................................. + 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. + + reserved_clusters This is RW file and contains number of reserved + clusters in the file system which will be used + in the specific situations to avoid costly + zeroout, unexpected ENOSPC, or possible data + loss. The default is 2% or 4096 clusters, + whichever is smaller and this can be changed + however it can never exceed number of clusters + in the file system. If there is not enough space + for the reserved space when mounting the file + mount will _not_ fail. +.............................................................................. + +Ioctls +====== + +There is some Ext4 specific functionality which can be accessed by applications +through the system call interfaces. The list of all Ext4 specific ioctls are +shown in the table below. + +Table of Ext4 specific ioctls +.............................................................................. + 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. + + EXT4_IOC_SWAP_BOOT Swap i_blocks and associated attributes + (like i_blocks, i_size, i_flags, ...) from + the specified inode with inode + EXT4_BOOT_LOADER_INO (#5). This is typically + used to store a boot loader in a secure part of + the filesystem, where it can't be changed by a + normal user by accident. + The data blocks of the previous boot loader + will be associated with the given inode. + +.............................................................................. + +References +========== + +kernel source: <file:fs/ext4/> + <file:fs/jbd2/> + +programs: http://e2fsprogs.sourceforge.net/ + +useful links: http://fedoraproject.org/wiki/ext3-devel + http://www.bullopensource.org/ext4/ + http://ext4.wiki.kernel.org/index.php/Main_Page + http://fedoraproject.org/wiki/Features/Ext4
