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Diffstat (limited to 'Documentation/filesystems/ext4.txt')
| -rw-r--r-- | Documentation/filesystems/ext4.txt | 378 |
1 files changed, 314 insertions, 64 deletions
diff --git a/Documentation/filesystems/ext4.txt b/Documentation/filesystems/ext4.txt index 97882df0486..919a3293aaa 100644 --- a/Documentation/filesystems/ext4.txt +++ b/Documentation/filesystems/ext4.txt @@ -2,7 +2,7 @@ Ext4 Filesystem =============== -Ext4 is an an advanced level of the ext3 filesystem which incorporates +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. @@ -68,12 +68,12 @@ Note: More extensive information for getting started with ext4 can be '-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,nobh' 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. + 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 =========== @@ -97,7 +97,7 @@ Note: More extensive information for getting started with ext4 can be * Inode allocation using large virtual block groups via flex_bg * delayed allocation * large block (up to pagesize) support -* efficent new ordered mode in JBD2 and ext4(avoid using buffer head to force +* 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 @@ -106,7 +106,7 @@ 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 conjuction with +* 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) @@ -144,23 +144,23 @@ journal_async_commit Commit block can be written to disk without waiting mount the device. This will enable 'journal_checksum' internally. -journal=update Update the ext4 file system's journal to the current - format. - +journal_path=path journal_dev=devnum When the external journal device's major/minor numbers - have changed, this option allows the user to specify + have changed, these options allow the user to specify the new journal location. The journal device is - identified through its new major/minor numbers encoded - in devnum. + identified through either its new major/minor numbers + encoded in devnum, or via a path to the device. -noload Don't load the journal on mounting. Note that - if the filesystem was not unmounted cleanly, +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. + 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 @@ -196,45 +196,31 @@ nobarrier This also requires an IO stack which can support also be used to enable or disable barriers, for consistency with other ext4 mount options. -inode_readahead=n This tuning parameter controls the maximum +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. -orlov (*) This enables the new Orlov block allocator. It is - enabled by default. - -oldalloc This disables the Orlov block allocator and enables - the old block allocator. Orlov should have better - performance - we'd like to get some feedback if it's - the contrary for you. - -user_xattr Enables Extended User Attributes. Additionally, you - need to have extended attribute support enabled in the - kernel configuration (CONFIG_EXT4_FS_XATTR). See the - attr(5) manual page and http://acl.bestbits.at/ to - learn more about extended attributes. - -nouser_xattr Disables Extended User Attributes. - -acl Enables POSIX Access Control Lists support. - Additionally, you need to have ACL support enabled in - the kernel configuration (CONFIG_EXT4_FS_POSIX_ACL). - See the acl(5) manual page and http://acl.bestbits.at/ - for more information. +nouser_xattr Disables Extended User Attributes. See the + attr(5) manual page and http://acl.bestbits.at/ + for more information about extended attributes. noacl This option disables POSIX Access Control List - support. - -reservation - -noreservation + support. If ACL support is enabled in the kernel + configuration (CONFIG_EXT4_FS_POSIX_ACL), ACL is + enabled by default on mount. See the acl(5) manual + page and http://acl.bestbits.at/ for more information + about acl. bsddf (*) Make 'df' act like BSD. minixdf Make 'df' act like Minix. debug Extra debugging information is sent to syslog. +abort Simulate the effects of calling ext4_abort() for + debugging purposes. This is normally used while + remounting a filesystem which is already mounted. + errors=remount-ro Remount the filesystem read-only on an error. errors=continue Keep going on a filesystem error. errors=panic Panic and halt the machine if an error occurs. @@ -259,26 +245,33 @@ resuid=n The user ID which may use the reserved blocks. sb=n Use alternate superblock at this location. -quota -noquota -grpquota -usrquota +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). -bh (*) ext4 associates buffer heads to data pages to -nobh (a) cache disk block mapping information - (b) link pages into transaction to provide - ordering guarantees. - "bh" option forces use of buffer heads. - "nobh" option tries to avoid associating buffer - heads (supported only for "writeback" mode). +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 (*) Deferring block allocation until write-out time. -nodelalloc Disable delayed allocation. Blocks are allocation - when data is copied from user to page cache. + +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 @@ -294,7 +287,7 @@ max_batch_time=usec Maximum amount of time ext4 should wait for amount of time (on average) that it takes to finish committing a transaction. Call this time the "commit time". If the time that the - transactoin has been running is less than 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 @@ -310,7 +303,7 @@ min_batch_time=usec This parameter sets the commit time (as fast disks, at the cost of increasing latency. journal_ioprio=prio The I/O priority (from 0 to 7, where 0 is the - highest priorty) which should be used for I/O + 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 @@ -328,12 +321,71 @@ noauto_da_alloc replacing existing files via patterns such as journal commit, in the default data=ordered mode, the data blocks of the new file are forced to disk before the rename() operation is - commited. This provides roughly the same level + committed. This provides roughly the same level of guarantees as ext3, and avoids the "zero-length" problem that can happen when a system crashes before the delayed allocation blocks are forced to disk. +noinit_itable Do not initialize any uninitialized inode table + blocks in the background. This feature may be + used by installation CD's so that the install + process can complete as quickly as possible; the + inode table initialization process would then be + deferred until the next time the file system + is unmounted. + +init_itable=n The lazy itable init code will wait n times the + number of milliseconds it took to zero out the + previous block group's inode table. This + minimizes the impact on the system performance + while file system's inode table is being initialized. + +discard Controls whether ext4 should issue discard/TRIM +nodiscard(*) commands to the underlying block device when + blocks are freed. This is useful for SSD devices + and sparse/thinly-provisioned LUNs, but it is off + by default until sufficient testing has been done. + +nouid32 Disables 32-bit UIDs and GIDs. This is for + interoperability with older kernels which only + store and expect 16-bit values. + +block_validity This options allows to enables/disables the in-kernel +noblock_validity facility for tracking filesystem metadata blocks + within internal data structures. This allows multi- + block allocator and other routines to quickly locate + extents which might overlap with filesystem metadata + blocks. This option is intended for debugging + purposes and since it negatively affects the + performance, it is off by default. + +dioread_lock Controls whether or not ext4 should use the DIO read +dioread_nolock locking. If the dioread_nolock option is specified + ext4 will allocate uninitialized extent before buffer + write and convert the extent to initialized after IO + completes. This approach allows ext4 code to avoid + using inode mutex, which improves scalability on high + speed storages. However this does not work with + data journaling and dioread_nolock option will be + ignored with kernel warning. Note that dioread_nolock + code path is only used for extent-based files. + Because of the restrictions this options comprises + it is off by default (e.g. dioread_lock). + +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. + Data Mode ========= There are 3 different data modes: @@ -358,8 +410,206 @@ 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. Curently ext4 does not have delayed -allocation support if this data journalling mode is selected. +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 ========== |