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linux/Documentation/filesystems/ext4.txt
Theodore Ts'o df981d03ee ext4: add max_dir_size_kb mount option 
Very large directories can cause significant performance problems, or
perhaps even invoke the OOM killer, if the process is running in a
highly constrained memory environment (whether it is VM's with a small
amount of memory or in a small memory cgroup).

So it is useful, in cloud server/data center environments, to be able
to set a filesystem-wide cap on the maximum size of a directory, to
ensure that directories never get larger than a sane size.  We do this
via a new mount option, max_dir_size_kb.  If there is an attempt to
grow the directory larger than max_dir_size_kb, the system call will
return ENOSPC instead.

Google-Bug-Id: 6863013

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
2012-08-17 09:48:17 -04:00

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Ext4 Filesystem
===============
Ext4 is an 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
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
ftp://ftp.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_dev=devnum When the external journal device's major/minor numbers
have changed, this option allows the user to specify
the new journal location. The journal device is
identified through its new major/minor numbers encoded
in devnum.
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. If you have extended
attribute support enabled in the kernel configuration
(CONFIG_EXT4_FS_XATTR), extended attribute support
is enabled by default on mount. 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. 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.
(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 Give objects the same group ID as their creator.
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 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:
* 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.
..............................................................................
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.
..............................................................................
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
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