ext4: Documentation updates.
Some of the information in Documentation/filesystems/ext4.txt is out of date and in need of an update. Signed-off-by: Jose R. Santos <jrs@us.ibm.com> Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
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Jose R. Santos
Theodore Ts'o
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93e3270c87
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@ -13,72 +13,89 @@ Mailing list: linux-ext4@vger.kernel.org
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1. Quick usage instructions:
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===========================
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- Grab updated e2fsprogs from
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ftp://ftp.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs-interim/
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This is a patchset on top of e2fsprogs-1.39, which can be found at
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- Compile and install the latest version of e2fsprogs (as of this
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writing version 1.41) from:
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http://sourceforge.net/project/showfiles.php?group_id=2406
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or
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ftp://ftp.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs/
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- It's still mke2fs -j /dev/hda1
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or grab the latest git repository from:
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- mount /dev/hda1 /wherever -t ext4dev
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git://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git
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- To enable extents,
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- Create a new filesystem using the ext4dev filesystem type:
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mount /dev/hda1 /wherever -t ext4dev -o extents
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# mke2fs -t ext4dev /dev/hda1
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- The filesystem is compatible with the ext3 driver until you add a file
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which has extents (ie: `mount -o extents', then create a file).
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Or configure an existing ext3 filesystem to support extents and set
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the test_fs flag to indicate that it's ok for an in-development
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filesystem to touch this filesystem:
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NOTE: The "extents" mount flag is temporary. It will soon go away and
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extents will be enabled by the "-o extents" flag to mke2fs or tune2fs
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# tune2fs -O extents -E test_fs /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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# tune2fs -I 256 /dev/hda1
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(Note: we currently do not have tools to convert an ext4dev
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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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# mount -t ext4dev /dev/hda1 /wherever
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- When comparing performance with other filesystems, remember that
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ext3/4 by default offers higher data integrity guarantees than most. So
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when comparing with a metadata-only journalling filesystem, use `mount -o
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data=writeback'. And you might as well use `mount -o nobh' too along
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with it. Making the journal larger than the mke2fs default often helps
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performance with metadata-intensive workloads.
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ext3/4 by default offers higher data integrity guarantees than most.
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So when comparing with a metadata-only journalling filesystem, such
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as ext3, use `mount -o data=writeback'. And you might as well use
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`mount -o nobh' too along with it. Making the journal larger than
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the mke2fs default often helps performance with metadata-intensive
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workloads.
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2. Features
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===========
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2.1 Currently available
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* ability to use filesystems > 16TB
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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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* extent format more robust in face of on-disk corruption due to magics,
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* internal redunancy in tree
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2.1 Previously available, soon to be enabled by default by "mkefs.ext4":
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* dir_index and resize inode will be on by default
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* large inodes will be used by default for fast EAs, nsec timestamps, etc
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* improved file allocation (multi-block alloc, delayed alloc)
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* fix 32000 subdirectory limit
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* nsec timestamps for mtime, atime, ctime, create time
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* inode version field on disk (NFSv4, Lustre)
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* reduced e2fsck time via uninit_bg feature
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* journal checksumming for robustness, performance
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* persistent file preallocation (e.g for streaming media, databases)
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* ability to pack bitmaps and inode tables into larger virtual groups via the
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flex_bg feature
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* large file support
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* Inode allocation using large virtual block groups via flex_bg
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2.2 Candidate features for future inclusion
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There are several under discussion, whether they all make it in is
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partly a function of how much time everyone has to work on them:
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* Online defrag (patches available but not well tested)
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* reduced mke2fs time via lazy itable initialization in conjuction with
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the uninit_bg feature (capability to do this is available in e2fsprogs
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but a kernel thread to do lazy zeroing of unused inode table blocks
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after filesystem is first mounted is required for safety)
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* improved file allocation (multi-block alloc, delayed alloc; basically done)
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* fix 32000 subdirectory limit (patch exists, needs some e2fsck work)
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* nsec timestamps for mtime, atime, ctime, create time (patch exists,
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needs some e2fsck work)
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* inode version field on disk (NFSv4, Lustre; prototype exists)
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* reduced mke2fs/e2fsck time via uninitialized groups (prototype exists)
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* journal checksumming for robustness, performance (prototype exists)
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* persistent file preallocation (e.g for streaming media, databases)
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There are several others under discussion, whether they all make it in is
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partly a function of how much time everyone has to work on them. Features like
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metadata checksumming have been discussed and planned for a bit but no patches
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exist yet so I'm not sure they're in the near-term roadmap.
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Features like metadata checksumming have been discussed and planned for
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a bit but no patches exist yet so I'm not sure they're in the near-term
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roadmap.
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The big performance win will come with mballoc, delalloc and flex_bg
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grouping of bitmaps and inode tables. Some test results available here:
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The big performance win will come with mballoc and delalloc. CFS has
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been using mballoc for a few years already with Lustre, and IBM + Bull
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did a lot of benchmarking on it. The reason it isn't in the first set of
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patches is partly a manageability issue, and partly because it doesn't
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directly affect the on-disk format (outside of much better allocation)
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so it isn't critical to get into the first round of changes. I believe
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Alex is working on a new set of patches right now.
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- http://www.bullopensource.org/ext4/20080530/ffsb-write-2.6.26-rc2.html
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- http://www.bullopensource.org/ext4/20080530/ffsb-readwrite-2.6.26-rc2.html
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3. Options
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==========
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@ -224,7 +241,7 @@ stripe=n Number of filesystem blocks that mballoc will try
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disks * RAID chunk size in file system blocks.
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Data Mode
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---------
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=========
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There are 3 different data modes:
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* writeback mode
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@ -256,7 +273,8 @@ kernel source: <file:fs/ext4/>
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<file:fs/jbd2/>
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programs: http://e2fsprogs.sourceforge.net/
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http://ext2resize.sourceforge.net
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useful links: http://fedoraproject.org/wiki/ext3-devel
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http://www.bullopensource.org/ext4/
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http://ext4.wiki.kernel.org/index.php/Main_Page
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http://fedoraproject.org/wiki/Features/Ext4
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