ext4: Documentation updates.

1. Quick usage instructions:

===========================

  - Grab updated e2fsprogs from

    ftp://ftp.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs-interim/

    This is a patchset on top of e2fsprogs-1.39, which can be found at

  - Compile and install the latest version of e2fsprogs (as of this

    writing version 1.41) from:

    http://sourceforge.net/project/showfiles.php?group_id=2406

	or

    ftp://ftp.kernel.org/pub/linux/kernel/people/tytso/e2fsprogs/

  - It's still mke2fs -j /dev/hda1

	or grab the latest git repository from:

    git://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git

  - Create a new filesystem using the ext4dev filesystem type:

    	# mke2fs -t ext4dev /dev/hda1

  - mount /dev/hda1 /wherever -t ext4dev

    Or configure an existing ext3 filesystem to support extents and set

    the test_fs flag to indicate that it's ok for an in-development

    filesystem to touch this filesystem:

  - To enable extents,

	# tune2fs -O extents -E test_fs /dev/hda1

	mount /dev/hda1 /wherever -t ext4dev -o extents

    If the filesystem was created with 128 byte inodes, it can be

    converted to use 256 byte for greater efficiency via:

  - The filesystem is compatible with the ext3 driver until you add a file

    which has extents (ie: `mount -o extents', then create a file).

        # tune2fs -I 256 /dev/hda1

    NOTE: The "extents" mount flag is temporary.  It will soon go away and

    extents will be enabled by the "-o extents" flag to mke2fs or tune2fs

    (Note: we currently do not have tools to convert an ext4dev

    filesystem back to ext3; so please do not do try this on production

    filesystems.)

  - Mounting:

	# mount -t ext4dev /dev/hda1 /wherever

  - When comparing performance with other filesystems, remember that

    ext3/4 by default offers higher data integrity guarantees than most.  So

    when comparing with a metadata-only journalling filesystem, use `mount -o

    data=writeback'.  And you might as well use `mount -o nobh' too along

    with it.  Making the journal larger than the mke2fs default often helps

    performance with metadata-intensive workloads.

    ext3/4 by default offers higher data integrity guarantees than most.

    So when comparing with a metadata-only journalling filesystem, such

    as ext3, use `mount -o data=writeback'.  And you might as well use

    `mount -o nobh' too along with it.  Making the journal larger than

    the mke2fs default often helps performance with metadata-intensive

    workloads.

2. Features

===========

2.1 Currently available

* ability to use filesystems > 16TB

* 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 redunancy in tree

2.1 Previously available, soon to be enabled by default by "mkefs.ext4":

* dir_index and resize inode will be on by default

* large inodes will be used by default for fast EAs, nsec timestamps, etc

* improved file allocation (multi-block alloc, delayed alloc)

* fix 32000 subdirectory limit

* 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

2.2 Candidate features for future inclusion

There are several under discussion, whether they all make it in is

partly a function of how much time everyone has to work on them:

* Online defrag (patches available but not well tested)

* reduced mke2fs time via lazy itable initialization in conjuction 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)

* improved file allocation (multi-block alloc, delayed alloc; basically done)

* fix 32000 subdirectory limit (patch exists, needs some e2fsck work)

* nsec timestamps for mtime, atime, ctime, create time (patch exists,

  needs some e2fsck work)

* inode version field on disk (NFSv4, Lustre; prototype exists)

* reduced mke2fs/e2fsck time via uninitialized groups (prototype exists)

* journal checksumming for robustness, performance (prototype exists)

* persistent file preallocation (e.g for streaming media, databases)

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.

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:

The big performance win will come with mballoc and delalloc.  CFS has

been using mballoc for a few years already with Lustre, and IBM + Bull

did a lot of benchmarking on it.  The reason it isn't in the first set of

patches is partly a manageability issue, and partly because it doesn't

directly affect the on-disk format (outside of much better allocation)

so it isn't critical to get into the first round of changes.  I believe

Alex is working on a new set of patches right now.

 - http://www.bullopensource.org/ext4/20080530/ffsb-write-2.6.26-rc2.html

 - http://www.bullopensource.org/ext4/20080530/ffsb-readwrite-2.6.26-rc2.html

3. Options

==========

			disks *  RAID chunk size in file system blocks.

Data Mode

---------

=========

There are 3 different data modes:

* writeback mode

		<file:fs/jbd2/>

programs:	http://e2fsprogs.sourceforge.net/

		http://ext2resize.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