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@@ -1,6 +1,8 @@
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+.. SPDX-License-Identifier: GPL-2.0
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-Ext4 Filesystem
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-===============
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+========================
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+General Information
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+========================
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Ext4 is an advanced level of the ext3 filesystem which incorporates
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scalability and reliability enhancements for supporting large filesystems
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@@ -11,18 +13,18 @@ Mailing list: linux-ext4@vger.kernel.org
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Web site: http://ext4.wiki.kernel.org
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-1. Quick usage instructions:
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-===========================
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+Quick usage instructions
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+========================
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Note: More extensive information for getting started with ext4 can be
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- found at the ext4 wiki site at the URL:
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- http://ext4.wiki.kernel.org/index.php/Ext4_Howto
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+found at the ext4 wiki site at the URL:
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+http://ext4.wiki.kernel.org/index.php/Ext4_Howto
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- Compile and install the latest version of e2fsprogs (as of this
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writing version 1.41.3) from:
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http://sourceforge.net/project/showfiles.php?group_id=2406
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-
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+
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or
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https://www.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs/
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@@ -37,16 +39,16 @@ Note: More extensive information for getting started with ext4 can be
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you will need to merge your changes with the version from e2fsprogs
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1.41.x.
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- - Create a new filesystem using the ext4 filesystem type:
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+ - Create a new filesystem using the ext4 filesystem type:::
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- # mke2fs -t ext4 /dev/hda1
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+ # mke2fs -t ext4 /dev/hda1
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- Or to configure an existing ext3 filesystem to support extents:
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+ Or to configure an existing ext3 filesystem to support extents:::
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# tune2fs -O extents /dev/hda1
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If the filesystem was created with 128 byte inodes, it can be
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- converted to use 256 byte for greater efficiency via:
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+ converted to use 256 byte for greater efficiency via:::
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# tune2fs -I 256 /dev/hda1
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@@ -54,7 +56,7 @@ Note: More extensive information for getting started with ext4 can be
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filesystem back to ext3; so please do not do try this on production
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filesystems.)
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- - Mounting:
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+ - Mounting:::
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# mount -t ext4 /dev/hda1 /wherever
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@@ -75,10 +77,11 @@ Note: More extensive information for getting started with ext4 can be
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the filesystem with a large journal can also be helpful for
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metadata-intensive workloads.
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-2. Features
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-===========
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+Features
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+========
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-2.1 Currently available
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+Currently Available
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+-------------------
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* ability to use filesystems > 16TB (e2fsprogs support not available yet)
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* extent format reduces metadata overhead (RAM, IO for access, transactions)
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@@ -103,7 +106,8 @@ Note: More extensive information for getting started with ext4 can be
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[1] Filesystems with a block size of 1k may see a limit imposed by the
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directory hash tree having a maximum depth of two.
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-2.2 Candidate features for future inclusion
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+Candidate Features for Future Inclusion
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+---------------------------------------
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* online defrag (patches available but not well tested)
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* reduced mke2fs time via lazy itable initialization in conjunction with
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@@ -122,12 +126,15 @@ grouping of bitmaps and inode tables. Some test results available here:
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- http://www.bullopensource.org/ext4/20080818-ffsb/ffsb-write-2.6.27-rc1.html
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- http://www.bullopensource.org/ext4/20080818-ffsb/ffsb-readwrite-2.6.27-rc1.html
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-3. Options
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-==========
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+Options
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+=======
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When mounting an ext4 filesystem, the following option are accepted:
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(*) == default
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+======================= =======================================================
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+Mount Option Description
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+======================= =======================================================
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ro Mount filesystem read only. Note that ext4 will
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replay the journal (and thus write to the
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partition) even when mounted "read only". The
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@@ -387,33 +394,38 @@ i_version Enable 64-bit inode version support. This option is
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dax Use direct access (no page cache). See
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Documentation/filesystems/dax.txt. Note that
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this option is incompatible with data=journal.
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+======================= =======================================================
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Data Mode
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=========
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There are 3 different data modes:
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* writeback mode
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-In data=writeback mode, ext4 does not journal data at all. This mode provides
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-a similar level of journaling as that of XFS, JFS, and ReiserFS in its default
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-mode - metadata journaling. A crash+recovery can cause incorrect data to
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-appear in files which were written shortly before the crash. This mode will
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-typically provide the best ext4 performance.
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+
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+ In data=writeback mode, ext4 does not journal data at all. This mode provides
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+ a similar level of journaling as that of XFS, JFS, and ReiserFS in its default
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+ mode - metadata journaling. A crash+recovery can cause incorrect data to
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+ appear in files which were written shortly before the crash. This mode will
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+ typically provide the best ext4 performance.
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* ordered mode
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-In data=ordered mode, ext4 only officially journals metadata, but it logically
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-groups metadata information related to data changes with the data blocks into a
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-single unit called a transaction. When it's time to write the new metadata
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-out to disk, the associated data blocks are written first. In general,
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-this mode performs slightly slower than writeback but significantly faster than journal mode.
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+
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+ In data=ordered mode, ext4 only officially journals metadata, but it logically
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+ groups metadata information related to data changes with the data blocks into
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+ a single unit called a transaction. When it's time to write the new metadata
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+ out to disk, the associated data blocks are written first. In general, this
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+ mode performs slightly slower than writeback but significantly faster than
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+ journal mode.
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* journal mode
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-data=journal mode provides full data and metadata journaling. All new data is
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-written to the journal first, and then to its final location.
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-In the event of a crash, the journal can be replayed, bringing both data and
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-metadata into a consistent state. This mode is the slowest except when data
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-needs to be read from and written to disk at the same time where it
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-outperforms all others modes. Enabling this mode will disable delayed
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-allocation and O_DIRECT support.
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+
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+ data=journal mode provides full data and metadata journaling. All new data is
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+ written to the journal first, and then to its final location. In the event of
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+ a crash, the journal can be replayed, bringing both data and metadata into a
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+ consistent state. This mode is the slowest except when data needs to be read
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+ from and written to disk at the same time where it outperforms all others
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+ modes. Enabling this mode will disable delayed allocation and O_DIRECT
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+ support.
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/proc entries
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=============
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@@ -425,10 +437,12 @@ Information about mounted ext4 file systems can be found in
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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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+
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+================ =======
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File Content
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+================ =======
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mb_groups details of multiblock allocator buddy cache of free blocks
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-..............................................................................
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+================ =======
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/sys entries
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============
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@@ -439,28 +453,30 @@ Information about mounted ext4 file systems can be found in
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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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+Files in /sys/fs/ext4/<devname>:
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+
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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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+============================= =================================================
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+File Content
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+============================= =================================================
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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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+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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+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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+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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@@ -508,7 +524,7 @@ Files in /sys/fs/ext4/<devname>
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in the file system. If there is not enough space
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for the reserved space when mounting the file
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mount will _not_ fail.
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-..............................................................................
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+============================= =================================================
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Ioctls
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======
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@@ -518,8 +534,10 @@ 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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+
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+============================= =================================================
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+Ioctl Description
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+============================= =================================================
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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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@@ -610,8 +628,7 @@ Table of Ext4 specific ioctls
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normal user by accident.
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The data blocks of the previous boot loader
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will be associated with the given inode.
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-
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-..............................................................................
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+============================= =================================================
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References
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==========
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Darrick J. Wong