mirror of
https://github.com/torvalds/linux.git
synced 2025-11-04 02:30:34 +02:00
0a790fe438
Introduces the case-insensitive features on ext4 for system administrators. Explain the minimum of design decisions that are important for sysadmins wanting to enable this feature. Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk> Signed-off-by: Theodore Ts'o <tytso@mit.edu>
612 lines
27 KiB
ReStructuredText
612 lines
27 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
|
|
|
|
========================
|
|
ext4 General Information
|
|
========================
|
|
|
|
Ext4 is an advanced level of the ext3 filesystem which incorporates
|
|
scalability and reliability enhancements for supporting large filesystems
|
|
(64 bit) in keeping with increasing disk capacities and state-of-the-art
|
|
feature requirements.
|
|
|
|
Mailing list: linux-ext4@vger.kernel.org
|
|
Web site: http://ext4.wiki.kernel.org
|
|
|
|
|
|
Quick usage instructions
|
|
========================
|
|
|
|
Note: More extensive information for getting started with ext4 can be
|
|
found at the ext4 wiki site at the URL:
|
|
http://ext4.wiki.kernel.org/index.php/Ext4_Howto
|
|
|
|
- The latest version of e2fsprogs can be found at:
|
|
|
|
https://www.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs/
|
|
|
|
or
|
|
|
|
http://sourceforge.net/project/showfiles.php?group_id=2406
|
|
|
|
or grab the latest git repository from:
|
|
|
|
https://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git
|
|
|
|
- Create a new filesystem using the ext4 filesystem type:
|
|
|
|
# mke2fs -t ext4 /dev/hda1
|
|
|
|
Or to configure an existing ext3 filesystem to support extents:
|
|
|
|
# tune2fs -O extents /dev/hda1
|
|
|
|
If the filesystem was created with 128 byte inodes, it can be
|
|
converted to use 256 byte for greater efficiency via:
|
|
|
|
# tune2fs -I 256 /dev/hda1
|
|
|
|
- Mounting:
|
|
|
|
# mount -t ext4 /dev/hda1 /wherever
|
|
|
|
- When comparing performance with other filesystems, it's always
|
|
important to try multiple workloads; very often a subtle change in a
|
|
workload parameter can completely change the ranking of which
|
|
filesystems do well compared to others. When comparing versus ext3,
|
|
note that ext4 enables write barriers by default, while ext3 does
|
|
not enable write barriers by default. So it is useful to use
|
|
explicitly specify whether barriers are enabled or not when via the
|
|
'-o barriers=[0|1]' mount option for both ext3 and ext4 filesystems
|
|
for a fair comparison. When tuning ext3 for best benchmark numbers,
|
|
it is often worthwhile to try changing the data journaling mode; '-o
|
|
data=writeback' can be faster for some workloads. (Note however that
|
|
running mounted with data=writeback can potentially leave stale data
|
|
exposed in recently written files in case of an unclean shutdown,
|
|
which could be a security exposure in some situations.) Configuring
|
|
the filesystem with a large journal can also be helpful for
|
|
metadata-intensive workloads.
|
|
|
|
Features
|
|
========
|
|
|
|
Currently Available
|
|
-------------------
|
|
|
|
* ability to use filesystems > 16TB (e2fsprogs support not available yet)
|
|
* extent format reduces metadata overhead (RAM, IO for access, transactions)
|
|
* extent format more robust in face of on-disk corruption due to magics,
|
|
* internal redundancy in tree
|
|
* improved file allocation (multi-block alloc)
|
|
* lift 32000 subdirectory limit imposed by i_links_count[1]
|
|
* nsec timestamps for mtime, atime, ctime, create time
|
|
* inode version field on disk (NFSv4, Lustre)
|
|
* reduced e2fsck time via uninit_bg feature
|
|
* journal checksumming for robustness, performance
|
|
* persistent file preallocation (e.g for streaming media, databases)
|
|
* ability to pack bitmaps and inode tables into larger virtual groups via the
|
|
flex_bg feature
|
|
* large file support
|
|
* inode allocation using large virtual block groups via flex_bg
|
|
* delayed allocation
|
|
* large block (up to pagesize) support
|
|
* efficient new ordered mode in JBD2 and ext4 (avoid using buffer head to force
|
|
the ordering)
|
|
* Case-insensitive file name lookups
|
|
|
|
[1] Filesystems with a block size of 1k may see a limit imposed by the
|
|
directory hash tree having a maximum depth of two.
|
|
|
|
case-insensitive file name lookups
|
|
======================================================
|
|
|
|
The case-insensitive file name lookup feature is supported on a
|
|
per-directory basis, allowing the user to mix case-insensitive and
|
|
case-sensitive directories in the same filesystem. It is enabled by
|
|
flipping the +F inode attribute of an empty directory. The
|
|
case-insensitive string match operation is only defined when we know how
|
|
text in encoded in a byte sequence. For that reason, in order to enable
|
|
case-insensitive directories, the filesystem must have the
|
|
casefold feature, which stores the filesystem-wide encoding
|
|
model used. By default, the charset adopted is the latest version of
|
|
Unicode (12.1.0, by the time of this writing), encoded in the UTF-8
|
|
form. The comparison algorithm is implemented by normalizing the
|
|
strings to the Canonical decomposition form, as defined by Unicode,
|
|
followed by a byte per byte comparison.
|
|
|
|
The case-awareness is name-preserving on the disk, meaning that the file
|
|
name provided by userspace is a byte-per-byte match to what is actually
|
|
written in the disk. The Unicode normalization format used by the
|
|
kernel is thus an internal representation, and not exposed to the
|
|
userspace nor to the disk, with the important exception of disk hashes,
|
|
used on large case-insensitive directories with DX feature. On DX
|
|
directories, the hash must be calculated using the casefolded version of
|
|
the filename, meaning that the normalization format used actually has an
|
|
impact on where the directory entry is stored.
|
|
|
|
When we change from viewing filenames as opaque byte sequences to seeing
|
|
them as encoded strings we need to address what happens when a program
|
|
tries to create a file with an invalid name. The Unicode subsystem
|
|
within the kernel leaves the decision of what to do in this case to the
|
|
filesystem, which select its preferred behavior by enabling/disabling
|
|
the strict mode. When Ext4 encounters one of those strings and the
|
|
filesystem did not require strict mode, it falls back to considering the
|
|
entire string as an opaque byte sequence, which still allows the user to
|
|
operate on that file, but the case-insensitive lookups won't work.
|
|
|
|
Options
|
|
=======
|
|
|
|
When mounting an ext4 filesystem, the following option are accepted:
|
|
(*) == default
|
|
|
|
ro
|
|
Mount filesystem read only. Note that ext4 will replay the journal (and
|
|
thus write to the partition) even when mounted "read only". The mount
|
|
options "ro,noload" can be used to prevent writes to the filesystem.
|
|
|
|
journal_checksum
|
|
Enable checksumming of the journal transactions. This will allow the
|
|
recovery code in e2fsck and the kernel to detect corruption in the
|
|
kernel. It is a compatible change and will be ignored by older
|
|
kernels.
|
|
|
|
journal_async_commit
|
|
Commit block can be written to disk without waiting for descriptor
|
|
blocks. If enabled older kernels cannot mount the device. This will
|
|
enable 'journal_checksum' internally.
|
|
|
|
journal_path=path, journal_dev=devnum
|
|
When the external journal device's major/minor numbers have changed,
|
|
these options allow the user to specify the new journal location. The
|
|
journal device is identified through either its new major/minor numbers
|
|
encoded in devnum, or via a path to the device.
|
|
|
|
norecovery, noload
|
|
Don't load the journal on mounting. Note that if the filesystem was
|
|
not unmounted cleanly, skipping the journal replay will lead to the
|
|
filesystem containing inconsistencies that can lead to any number of
|
|
problems.
|
|
|
|
data=journal
|
|
All data are committed into the journal prior to being written into the
|
|
main file system. Enabling this mode will disable delayed allocation
|
|
and O_DIRECT support.
|
|
|
|
data=ordered (*)
|
|
All data are forced directly out to the main file system prior to its
|
|
metadata being committed to the journal.
|
|
|
|
data=writeback
|
|
Data ordering is not preserved, data may be written into the main file
|
|
system after its metadata has been committed to the journal.
|
|
|
|
commit=nrsec (*)
|
|
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(*)>, barrier(*), nobarrier
|
|
This enables/disables the use of write barriers in the jbd code.
|
|
barrier=0 disables, barrier=1 enables. This also requires an IO stack
|
|
which can support barriers, and if jbd gets an error on a barrier
|
|
write, it will disable again with a warning. Write barriers enforce
|
|
proper on-disk ordering of journal commits, making volatile disk write
|
|
caches safe to use, at some performance penalty. If your disks are
|
|
battery-backed in one way or another, disabling barriers may safely
|
|
improve performance. The mount options "barrier" and "nobarrier" can
|
|
also be used to enable or disable barriers, for consistency with other
|
|
ext4 mount options.
|
|
|
|
inode_readahead_blks=n
|
|
This tuning parameter controls the maximum number of inode table blocks
|
|
that ext4's inode table readahead algorithm will pre-read into the
|
|
buffer cache. The default value is 32 blocks.
|
|
|
|
nouser_xattr
|
|
Disables Extended User Attributes. See the attr(5) manual page for
|
|
more information about extended attributes.
|
|
|
|
noacl
|
|
This option disables POSIX Access Control List support. If ACL support
|
|
is enabled in the kernel configuration (CONFIG_EXT4_FS_POSIX_ACL), ACL
|
|
is enabled by default on mount. See the acl(5) manual page for more
|
|
information about acl.
|
|
|
|
bsddf (*)
|
|
Make 'df' act like BSD.
|
|
|
|
minixdf
|
|
Make 'df' act like Minix.
|
|
|
|
debug
|
|
Extra debugging information is sent to syslog.
|
|
|
|
abort
|
|
Simulate the effects of calling ext4_abort() for debugging purposes.
|
|
This is normally used while remounting a filesystem which is already
|
|
mounted.
|
|
|
|
errors=remount-ro
|
|
Remount the filesystem read-only on an error.
|
|
|
|
errors=continue
|
|
Keep going on a filesystem error.
|
|
|
|
errors=panic
|
|
Panic and halt the machine if an error occurs. (These mount options
|
|
override the errors behavior specified in the superblock, which can be
|
|
configured using tune2fs)
|
|
|
|
data_err=ignore(*)
|
|
Just print an error message if an error occurs in a file data buffer in
|
|
ordered mode.
|
|
data_err=abort
|
|
Abort the journal if an error occurs in a file data buffer in ordered
|
|
mode.
|
|
|
|
grpid | bsdgroups
|
|
New objects have the group ID of their parent.
|
|
|
|
nogrpid (*) | sysvgroups
|
|
New objects have the group ID of their creator.
|
|
|
|
resgid=n
|
|
The group ID which may use the reserved blocks.
|
|
|
|
resuid=n
|
|
The user ID which may use the reserved blocks.
|
|
|
|
sb=
|
|
Use alternate superblock at this location.
|
|
|
|
quota, noquota, grpquota, usrquota
|
|
These options are ignored by the filesystem. They are used only by
|
|
quota tools to recognize volumes where quota should be turned on. See
|
|
documentation in the quota-tools package for more details
|
|
(http://sourceforge.net/projects/linuxquota).
|
|
|
|
jqfmt=<quota type>, usrjquota=<file>, grpjquota=<file>
|
|
These options tell filesystem details about quota so that quota
|
|
information can be properly updated during journal replay. They replace
|
|
the above quota options. See documentation in the quota-tools package
|
|
for more details (http://sourceforge.net/projects/linuxquota).
|
|
|
|
stripe=n
|
|
Number of filesystem blocks that mballoc will try to use for allocation
|
|
size and alignment. For RAID5/6 systems this should be the number of
|
|
data disks * RAID chunk size in file system blocks.
|
|
|
|
delalloc (*)
|
|
Defer block allocation until just before ext4 writes out the block(s)
|
|
in question. This allows ext4 to better allocation decisions more
|
|
efficiently.
|
|
|
|
nodelalloc
|
|
Disable delayed allocation. Blocks are allocated when the data is
|
|
copied from userspace to the page cache, either via the write(2) system
|
|
call or when an mmap'ed page which was previously unallocated is
|
|
written for the first time.
|
|
|
|
max_batch_time=usec
|
|
Maximum amount of time ext4 should wait for additional filesystem
|
|
operations to be batch together with a synchronous write operation.
|
|
Since a synchronous write operation is going to force a commit and then
|
|
a wait for the I/O complete, it doesn't cost much, and can be a huge
|
|
throughput win, we wait for a small amount of time to see if any other
|
|
transactions can piggyback on the synchronous write. The algorithm
|
|
used is designed to automatically tune for the speed of the disk, by
|
|
measuring the amount of time (on average) that it takes to finish
|
|
committing a transaction. Call this time the "commit time". If the
|
|
time that the transaction has been running is less than the commit
|
|
time, ext4 will try sleeping for the commit time to see if other
|
|
operations will join the transaction. The commit time is capped by
|
|
the max_batch_time, which defaults to 15000us (15ms). This
|
|
optimization can be turned off entirely by setting max_batch_time to 0.
|
|
|
|
min_batch_time=usec
|
|
This parameter sets the commit time (as described above) to be at least
|
|
min_batch_time. It defaults to zero microseconds. Increasing this
|
|
parameter may improve the throughput of multi-threaded, synchronous
|
|
workloads on very fast disks, at the cost of increasing latency.
|
|
|
|
journal_ioprio=prio
|
|
The I/O priority (from 0 to 7, where 0 is the highest priority) which
|
|
should be used for I/O operations submitted by kjournald2 during a
|
|
commit operation. This defaults to 3, which is a slightly higher
|
|
priority than the default I/O priority.
|
|
|
|
auto_da_alloc(*), noauto_da_alloc
|
|
Many broken applications don't use fsync() when replacing existing
|
|
files via patterns such as fd = open("foo.new")/write(fd,..)/close(fd)/
|
|
rename("foo.new", "foo"), or worse yet, fd = open("foo",
|
|
O_TRUNC)/write(fd,..)/close(fd). If auto_da_alloc is enabled, ext4
|
|
will detect the replace-via-rename and replace-via-truncate patterns
|
|
and force that any delayed allocation blocks are allocated such that at
|
|
the next journal commit, in the default data=ordered mode, the data
|
|
blocks of the new file are forced to disk before the rename() operation
|
|
is committed. This provides roughly the same level of guarantees as
|
|
ext3, and avoids the "zero-length" problem that can happen when a
|
|
system crashes before the delayed allocation blocks are forced to disk.
|
|
|
|
noinit_itable
|
|
Do not initialize any uninitialized inode table blocks in the
|
|
background. This feature may be used by installation CD's so that the
|
|
install process can complete as quickly as possible; the inode table
|
|
initialization process would then be deferred until the next time the
|
|
file system is unmounted.
|
|
|
|
init_itable=n
|
|
The lazy itable init code will wait n times the number of milliseconds
|
|
it took to zero out the previous block group's inode table. This
|
|
minimizes the impact on the system performance while file system's
|
|
inode table is being initialized.
|
|
|
|
discard, nodiscard(*)
|
|
Controls whether ext4 should issue discard/TRIM commands to the
|
|
underlying block device when blocks are freed. This is useful for SSD
|
|
devices and sparse/thinly-provisioned LUNs, but it is off by default
|
|
until sufficient testing has been done.
|
|
|
|
nouid32
|
|
Disables 32-bit UIDs and GIDs. This is for interoperability with
|
|
older kernels which only store and expect 16-bit values.
|
|
|
|
block_validity(*), noblock_validity
|
|
These options enable or disable the in-kernel facility for tracking
|
|
filesystem metadata blocks within internal data structures. This
|
|
allows multi- block allocator and other routines to notice bugs or
|
|
corrupted allocation bitmaps which cause blocks to be allocated which
|
|
overlap with filesystem metadata blocks.
|
|
|
|
dioread_lock, dioread_nolock
|
|
Controls whether or not ext4 should use the DIO read locking. If the
|
|
dioread_nolock option is specified ext4 will allocate uninitialized
|
|
extent before buffer write and convert the extent to initialized after
|
|
IO completes. This approach allows ext4 code to avoid using inode
|
|
mutex, which improves scalability on high speed storages. However this
|
|
does not work with data journaling and dioread_nolock option will be
|
|
ignored with kernel warning. Note that dioread_nolock code path is only
|
|
used for extent-based files. Because of the restrictions this options
|
|
comprises it is off by default (e.g. dioread_lock).
|
|
|
|
max_dir_size_kb=n
|
|
This limits the size of directories so that any attempt to expand them
|
|
beyond the specified limit in kilobytes will cause an ENOSPC error.
|
|
This is useful in memory constrained environments, where a very large
|
|
directory can cause severe performance problems or even provoke the Out
|
|
Of Memory killer. (For example, if there is only 512mb memory
|
|
available, a 176mb directory may seriously cramp the system's style.)
|
|
|
|
i_version
|
|
Enable 64-bit inode version support. This option is off by default.
|
|
|
|
dax
|
|
Use direct access (no page cache). See
|
|
Documentation/filesystems/dax.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>
|
|
|
|
mb_groups
|
|
details of multiblock allocator buddy cache of free blocks
|
|
|
|
/sys entries
|
|
============
|
|
|
|
Information about mounted ext4 file systems can be found in
|
|
/sys/fs/ext4. Each mounted filesystem will have a directory in
|
|
/sys/fs/ext4 based on its device name (i.e., /sys/fs/ext4/hdc or
|
|
/sys/fs/ext4/dm-0). The files in each per-device directory are shown
|
|
in table below.
|
|
|
|
Files in /sys/fs/ext4/<devname>:
|
|
|
|
(see also Documentation/ABI/testing/sysfs-fs-ext4)
|
|
|
|
delayed_allocation_blocks
|
|
This file is read-only and shows the number of blocks that are dirty in
|
|
the page cache, but which do not have their location in the filesystem
|
|
allocated yet.
|
|
|
|
inode_goal
|
|
Tuning parameter which (if non-zero) controls the goal inode used by
|
|
the inode allocator in preference to all other allocation heuristics.
|
|
This is intended for debugging use only, and should be 0 on production
|
|
systems.
|
|
|
|
inode_readahead_blks
|
|
Tuning parameter which controls the maximum number of inode table
|
|
blocks that ext4's inode table readahead algorithm will pre-read into
|
|
the buffer cache.
|
|
|
|
lifetime_write_kbytes
|
|
This file is read-only and shows the number of kilobytes of data that
|
|
have been written to this filesystem since it was created.
|
|
|
|
max_writeback_mb_bump
|
|
The maximum number of megabytes the writeback code will try to write
|
|
out before move on to another inode.
|
|
|
|
mb_group_prealloc
|
|
The multiblock allocator will round up allocation requests to a
|
|
multiple of this tuning parameter if the stripe size is not set in the
|
|
ext4 superblock
|
|
|
|
mb_max_to_scan
|
|
The maximum number of extents the multiblock allocator will search to
|
|
find the best extent.
|
|
|
|
mb_min_to_scan
|
|
The minimum number of extents the multiblock allocator will search to
|
|
find the best extent.
|
|
|
|
mb_order2_req
|
|
Tuning parameter which controls the minimum size for requests (as a
|
|
power of 2) where the buddy cache is used.
|
|
|
|
mb_stats
|
|
Controls whether the multiblock allocator should collect statistics,
|
|
which are shown during the unmount. 1 means to collect statistics, 0
|
|
means not to collect statistics.
|
|
|
|
mb_stream_req
|
|
Files which have fewer blocks than this tunable parameter will have
|
|
their blocks allocated out of a block group specific preallocation
|
|
pool, so that small files are packed closely together. Each large file
|
|
will have its blocks allocated out of its own unique preallocation
|
|
pool.
|
|
|
|
session_write_kbytes
|
|
This file is read-only and shows the number of kilobytes of data that
|
|
have been written to this filesystem since it was mounted.
|
|
|
|
reserved_clusters
|
|
This is RW file and contains number of reserved clusters in the file
|
|
system which will be used in the specific situations to avoid costly
|
|
zeroout, unexpected ENOSPC, or possible data loss. The default is 2% or
|
|
4096 clusters, whichever is smaller and this can be changed however it
|
|
can never exceed number of clusters in the file system. If there is not
|
|
enough space for the reserved space when mounting the file mount will
|
|
_not_ fail.
|
|
|
|
Ioctls
|
|
======
|
|
|
|
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
|
|
|
|
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
|