ext4

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 This article contains information about computer software currently in development.
The content may change as the software development progresses.
ext4
Developer Mingming Cao, Andreas Dilger, Alex Tomas, Dave Kleikamp, Theodore Ts'o, Eric Sandeen, Sam Naghshineh, others
Full name Fourth extended file system
Introduced Stable: Yet to be released
Unstable: October 10, 2006 (Linux 2.6.19)
Partition identifier 0x83 (MBR)
EBD0A0A2-B9E5-4433-87C0-68B6B72699C7 (GPT)
Structures
Directory contents Table, H tree
File allocation bitmap (free space), table (metadata)
Bad blocks Table
Limits
Max file size 1 EiB
Max number of files Set at filesystem creation
Max filename length 256
Max volume size 1 EiB
Allowed characters in filenames All bytes except NUL and '/'
Features
Dates recorded modification (mtime), attribute modification (ctime), access (atime), delete (dtime), create (crtime)
Date range December 14, 1901 - April, 25 2514
Date resolution Nanosecond
Forks No
Attributes extents, noextents, mballoc, delalloc, data=journal, data=ordered, data=writeback, commit=nrsec, orlov, oldalloc, user_xattr, nouser_xattr, acl, noacl, bsddf, minixdf, bh, nobh
File system permissions POSIX
Transparent compression No
Transparent encryption No
Supported operating systems Linux

The ext4, or fourth extended filesystem is a journaled file system in development, designed as a backwards-compatible replacement of the ext3 filesystem.

Contents

[edit] History

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Ext4 began as a fork of ext3. The developers of ext3 had designed a set of new features for ext3. Some of those were implemented in ext3 but others were not, so that only newer versions would be able to read the file system structure created with the new features. Patches to implement those features were proposed[1] on the Linux kernel development mailing list, but some developers argued that it would be better to keep ext3 stable, and continue the development in a forked version to be called ext4.[2] The ext3 developers argued that maintaining a separate code base for the new file system would be harder than a shared code base for both file systems. But arguments for forking, including ones by Linus Torvalds[3], convinced the ext3 developers to start a new file system.

On June 28 2006 Theodore Ts'o, the ext3 maintainer, announced[4] the new plan of development. A preliminary development snapshot of ext4 was included in version 2.6.19 of the Linux kernel which was released on November 29, 2006.

The file system is currently marked as developmental and is titled "ext4dev".[5] It is considered unstable and so is not recommended for use in production environments, since data corruption is possible in this early stage of development.

[edit] Features

[edit] Large filesystem

The ext4 filesystem can support volumes with sizes up to 1 exabyte. Since version 2.6.25, it also supports files as large as the file system.

[edit] Extents

Extents are introduced to map a range of contiguous physical blocks into a single descriptor. A single extent can map up to 128MiB of contiguous space with a 4KiB block size.[6]

[edit] Backward compatibility

The ext4 filesystem is backward compatible with ext3, making it possible to mount an ext3 filesystem as ext4 (using the “ext4dev” filesystem type, “ext4” after final release).

[edit] Forward compatibility

The ext4 file system is forward compatible with ext3, that is, it can be mounted as an ext3 partition (using “ext3” as the filesystem type when mounting). However, if the ext4 partition uses extents (a major new feature of ext4), then the ability to mount the file system as ext3 is lost. Extents were enabled by default in the 2.6.23 kernel. Previously, the “extents” option was explicitly required (e.g. mount /dev/sda1 /mnt/point -t ext4dev -o extents).

[edit] Persistent pre-allocation

The ext4 filesystem allows for pre-allocation of on disk space for a file. The current methodology for this on most file systems is to write the file full of 0's to reserve the space when the file is created (although XFS has an ioctl to allow for true pre-allocation as well). This method would no longer be required for ext4; instead, a new preallocate() system call was added for use by filesystems, including ext4 and XFS, that have this capability. The space allocated for files such as these would be guaranteed and would likely be contiguous. This has applications for media streaming and databases.

[edit] Delayed allocation

Main article: Allocate-on-flush

Ext4 uses a "delayed allocation" feature to delay block allocation as long as possible. During this delay pending writes will only change the free space counter. This improves performance and reduces fragmentation by virtue of improving block allocation decisions based on the actual file size.

[edit] Break 32000 subdirectory limit

In ext3 the number of subdirectories that a directory can contain is limited to 32000. This limit is lifted in ext4. To allow for continued performance given the possibility of much larger directories, Htree indexes (a specialized version of a Btree) is turned on by default in ext4. This feature is implemented in 2.6.23. Htree is also available in ext3 when the dir_index feature is enabled.

[edit] Journal checksumming

Ext4 uses checksums in the journal to improve reliability, since the journal is one of the most used files of the disk. This feature has a side benefit; it can safely avoid a disk IO wait during the journaling process, improving performance slightly. The technique of journal checksumming was inspired by research from Wisconsin on IRON File Systems (Section 6, called "transaction checksums") [7].

[edit] Online defragmentation

Ext4 will eventually also have an online defragmenter. Even with the various techniques used to avoid it, a long lived file system does tend to become fragmented over time. Ext4 has a tool which can defragment individual files or entire file systems.

[edit] Faster file system checking

In ext4, unallocated block groups and sections of the inode table are marked as such. This enables e2fsck to skip them entirely on a check and greatly reduce the time it takes to check a file system of the size ext4 is built to support. This feature is implemented in version 2.6.24 of the Linux kernel.

[edit] Multiblock allocator

ext4 will have a multiblock allocator. This allows many blocks to be allocated to a file in single operation and therefore a better decision can be made on finding a chunk of free space where all the blocks can fit. The mutiblock allocator is active when using O_DIRECT or if delayed allocation is on. This allows the file to have many dirty blocks submitted for writes at the same time, unlike the existing kernel mechanism of submitting each block to the filesystem separately for allocation.

[edit] Improved timestamps

As computers become faster in general and specifically Linux becomes used more for mission critical applications, the granularity of second-based timestamps becomes insufficient. To solve this, ext4 will have timestamps measured in nanoseconds. This feature is currently implemented in 2.6.23. In addition, 2 bits of the expanded timestamp field are added to the most significant bits of the seconds field of the timestamps to defer the year 2038 problem for an additional 500 years. Support for date-created timestamps is added in ext4.[citation needed]

[edit] References

  1. ^ LKML: Mingming Cao: [RFC 0/13] extents and 48bit ext3
  2. ^ LKML: Jeff Garzik: Re: [RFC 0/13] extents and 48bit ext3
  3. ^ LKML: Linus Torvalds: Re: [Ext2-devel] [RFC 0/13] extents and 48bit ext3
  4. ^ LKML: "Theodore Ts'o": Proposal and plan for ext2/3 future development work
  5. ^ OLS 2007: Ext4 Paper. Retrieved on 2007-08-10.
  6. ^ Ext4 overview. Retrieved on 2008-01-15.
  7. ^ IRON File Systems

[edit] See also

[edit] External links