ext4: update ext4 documentation
Add documentation for mount options and ioctls to
Documentation/filesystem/ext4.txt, which has not been udpated for some
time. Also add for ext4 sysfs tunables to the
Documentation/ABI/testing/sysfs-fs-ext4 file, and fix a few
typographical errors in that file.
https://bugzilla.kernel.org/show_bug.cgi?id=9423
Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
diff --git a/Documentation/filesystems/ext4.txt b/Documentation/filesystems/ext4.txt
index 6ab9442..6b05046 100644
--- a/Documentation/filesystems/ext4.txt
+++ b/Documentation/filesystems/ext4.txt
@@ -367,12 +367,47 @@
minimizes the impact on the systme performance
while file system's inode table is being initialized.
-discard Controls whether ext4 should issue discard/TRIM
+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.
+
+resize 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. It can be
+ used only with conjunction with remount.
+
+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 nobh
+ option and the mount will fail. Nor does it 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).
+
+i_version Enable 64-bit inode version support. This option is
+ off by default.
+
Data Mode
=========
There are 3 different data modes:
@@ -400,6 +435,176 @@
outperforms all others modes. Currently ext4 does not have delayed
allocation support if this data journalling mode is selected.
+/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.
+..............................................................................
+
+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.
+..............................................................................
+
References
==========