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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>
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Lukas Czerner
Theodore Ts'o
parent
3abb17e82f
commit
6f9524e9e1
@ -48,7 +48,7 @@ Description:
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will have its blocks allocated out of its own unique
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preallocation pool.
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What: /sys/fs/ext4/<disk>/inode_readahead
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What: /sys/fs/ext4/<disk>/inode_readahead_blks
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Date: March 2008
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Contact: "Theodore Ts'o" <tytso@mit.edu>
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Description:
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@ -85,7 +85,14 @@ Date: June 2008
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Contact: "Theodore Ts'o" <tytso@mit.edu>
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Description:
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Tuning parameter which (if non-zero) controls the goal
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inode used by the inode allocator in p0reference to
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all other allocation hueristics. This is intended for
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inode used by the inode allocator in preference to
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all other allocation heuristics. This is intended for
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debugging use only, and should be 0 on production
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systems.
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What: /sys/fs/ext4/<disk>/max_writeback_mb_bump
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Date: September 2009
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Contact: "Theodore Ts'o" <tytso@mit.edu>
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Description:
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The maximum number of megabytes the writeback code will
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try to write out before move on to another inode.
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@ -367,12 +367,47 @@ init_itable=n The lazy itable init code will wait n times the
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minimizes the impact on the systme performance
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while file system's inode table is being initialized.
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discard Controls whether ext4 should issue discard/TRIM
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discard Controls whether ext4 should issue discard/TRIM
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nodiscard(*) commands to the underlying block device when
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blocks are freed. This is useful for SSD devices
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and sparse/thinly-provisioned LUNs, but it is off
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by default until sufficient testing has been done.
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nouid32 Disables 32-bit UIDs and GIDs. This is for
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interoperability with older kernels which only
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store and expect 16-bit values.
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resize Allows to resize filesystem to the end of the last
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existing block group, further resize has to be done
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with resize2fs either online, or offline. It can be
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used only with conjunction with remount.
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block_validity This options allows to enables/disables the in-kernel
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noblock_validity facility for tracking filesystem metadata blocks
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within internal data structures. This allows multi-
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block allocator and other routines to quickly locate
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extents which might overlap with filesystem metadata
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blocks. This option is intended for debugging
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purposes and since it negatively affects the
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performance, it is off by default.
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dioread_lock Controls whether or not ext4 should use the DIO read
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dioread_nolock locking. If the dioread_nolock option is specified
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ext4 will allocate uninitialized extent before buffer
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write and convert the extent to initialized after IO
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completes. This approach allows ext4 code to avoid
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using inode mutex, which improves scalability on high
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speed storages. However this does not work with nobh
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option and the mount will fail. Nor does it work with
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data journaling and dioread_nolock option will be
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ignored with kernel warning. Note that dioread_nolock
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code path is only used for extent-based files.
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Because of the restrictions this options comprises
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it is off by default (e.g. dioread_lock).
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i_version Enable 64-bit inode version support. This option is
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off by default.
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Data Mode
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=========
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There are 3 different data modes:
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@ -400,6 +435,176 @@ needs to be read from and written to disk at the same time where it
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outperforms all others modes. Currently ext4 does not have delayed
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allocation support if this data journalling mode is selected.
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/proc entries
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=============
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Information about mounted ext4 file systems can be found in
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/proc/fs/ext4. Each mounted filesystem will have a directory in
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/proc/fs/ext4 based on its device name (i.e., /proc/fs/ext4/hdc or
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/proc/fs/ext4/dm-0). The files in each per-device directory are shown
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in table below.
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Files in /proc/fs/ext4/<devname>
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..............................................................................
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File Content
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mb_groups details of multiblock allocator buddy cache of free blocks
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..............................................................................
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/sys entries
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============
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Information about mounted ext4 file systems can be found in
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/sys/fs/ext4. Each mounted filesystem will have a directory in
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/sys/fs/ext4 based on its device name (i.e., /sys/fs/ext4/hdc or
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/sys/fs/ext4/dm-0). The files in each per-device directory are shown
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in table below.
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Files in /sys/fs/ext4/<devname>
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(see also Documentation/ABI/testing/sysfs-fs-ext4)
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..............................................................................
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File Content
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delayed_allocation_blocks This file is read-only and shows the number of
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blocks that are dirty in the page cache, but
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which do not have their location in the
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filesystem allocated yet.
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inode_goal Tuning parameter which (if non-zero) controls
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the goal inode used by the inode allocator in
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preference to all other allocation heuristics.
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This is intended for debugging use only, and
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should be 0 on production systems.
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inode_readahead_blks Tuning parameter which controls the maximum
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number of inode table blocks that ext4's inode
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table readahead algorithm will pre-read into
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the buffer cache
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lifetime_write_kbytes This file is read-only and shows the number of
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kilobytes of data that have been written to this
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filesystem since it was created.
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max_writeback_mb_bump The maximum number of megabytes the writeback
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code will try to write out before move on to
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another inode.
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mb_group_prealloc The multiblock allocator will round up allocation
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requests to a multiple of this tuning parameter if
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the stripe size is not set in the ext4 superblock
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mb_max_to_scan The maximum number of extents the multiblock
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allocator will search to find the best extent
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mb_min_to_scan The minimum number of extents the multiblock
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allocator will search to find the best extent
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mb_order2_req Tuning parameter which controls the minimum size
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for requests (as a power of 2) where the buddy
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cache is used
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mb_stats Controls whether the multiblock allocator should
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collect statistics, which are shown during the
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unmount. 1 means to collect statistics, 0 means
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not to collect statistics
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mb_stream_req Files which have fewer blocks than this tunable
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parameter will have their blocks allocated out
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of a block group specific preallocation pool, so
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that small files are packed closely together.
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Each large file will have its blocks allocated
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out of its own unique preallocation pool.
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session_write_kbytes This file is read-only and shows the number of
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kilobytes of data that have been written to this
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filesystem since it was mounted.
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..............................................................................
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Ioctls
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======
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There is some Ext4 specific functionality which can be accessed by applications
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through the system call interfaces. The list of all Ext4 specific ioctls are
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shown in the table below.
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Table of Ext4 specific ioctls
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..............................................................................
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Ioctl Description
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EXT4_IOC_GETFLAGS Get additional attributes associated with inode.
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The ioctl argument is an integer bitfield, with
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bit values described in ext4.h. This ioctl is an
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alias for FS_IOC_GETFLAGS.
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EXT4_IOC_SETFLAGS Set additional attributes associated with inode.
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The ioctl argument is an integer bitfield, with
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bit values described in ext4.h. This ioctl is an
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alias for FS_IOC_SETFLAGS.
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EXT4_IOC_GETVERSION
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EXT4_IOC_GETVERSION_OLD
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Get the inode i_generation number stored for
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each inode. The i_generation number is normally
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changed only when new inode is created and it is
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particularly useful for network filesystems. The
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'_OLD' version of this ioctl is an alias for
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FS_IOC_GETVERSION.
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EXT4_IOC_SETVERSION
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EXT4_IOC_SETVERSION_OLD
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Set the inode i_generation number stored for
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each inode. The '_OLD' version of this ioctl
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is an alias for FS_IOC_SETVERSION.
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EXT4_IOC_GROUP_EXTEND This ioctl has the same purpose as the resize
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mount option. It allows to resize filesystem
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to the end of the last existing block group,
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further resize has to be done with resize2fs,
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either online, or offline. The argument points
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to the unsigned logn number representing the
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filesystem new block count.
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EXT4_IOC_MOVE_EXT Move the block extents from orig_fd (the one
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this ioctl is pointing to) to the donor_fd (the
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one specified in move_extent structure passed
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as an argument to this ioctl). Then, exchange
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inode metadata between orig_fd and donor_fd.
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This is especially useful for online
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defragmentation, because the allocator has the
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opportunity to allocate moved blocks better,
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ideally into one contiguous extent.
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EXT4_IOC_GROUP_ADD Add a new group descriptor to an existing or
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new group descriptor block. The new group
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descriptor is described by ext4_new_group_input
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structure, which is passed as an argument to
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this ioctl. This is especially useful in
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conjunction with EXT4_IOC_GROUP_EXTEND,
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which allows online resize of the filesystem
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to the end of the last existing block group.
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Those two ioctls combined is used in userspace
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online resize tool (e.g. resize2fs).
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EXT4_IOC_MIGRATE This ioctl operates on the filesystem itself.
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It converts (migrates) ext3 indirect block mapped
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inode to ext4 extent mapped inode by walking
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through indirect block mapping of the original
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inode and converting contiguous block ranges
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into ext4 extents of the temporary inode. Then,
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inodes are swapped. This ioctl might help, when
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migrating from ext3 to ext4 filesystem, however
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suggestion is to create fresh ext4 filesystem
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and copy data from the backup. Note, that
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filesystem has to support extents for this ioctl
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to work.
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EXT4_IOC_ALLOC_DA_BLKS Force all of the delay allocated blocks to be
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allocated to preserve application-expected ext3
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behaviour. Note that this will also start
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triggering a write of the data blocks, but this
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behaviour may change in the future as it is
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not necessary and has been done this way only
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for sake of simplicity.
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..............................................................................
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References
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==========
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