Friday, August 7, 2009

Linux File system

The extended file system or ext was invented in April 1992 and the first file system created specifically for the Linux operating system. It was designed by Rémy Card to overcome certain limitations of the Minix file system. It is the first of the extended file systems. It was superseded by both ext2 and xiafs, between which there was a competition, which ext2 won because of its long-term viability.

The ext2 or second extended filesystem is a file system for the Linux kernel. It was initially designed by Rémy Card as a replacement for the extended file system (ext).
ext2 was the default filesystem in several Linux distributions, including Debian and Red Hat Linux, until supplanted more recently by ext3, which is almost completely compatible with ext2 and is a journaling file system. ext2 is still the filesystem of choice for flash-based storage media (such as SD cards, SSDs, and USB flash drives) since its lack of a journal minimizes the number of writes. Flash devices have only a limited number of write cycles.

FileSystem Limits:

The reason for some limits of the ext2-file system are the file format of the data and the operating system's kernel. Mostly these factors will be determined once when the file system is built. They depend on the block size and the ratio of the number of blocks and inodes. In Linux the block size is limited by the architecture page size. There are also many userspace programs that can't handle files larger than 2 GB. The limit of sublevel-directories is about 32768. If the number of files in a directory exceeds 10000 to 15000 files, the user will normally be warned that operations can last for a long time unless directory indexing is enabled. The theoretical limit on the number of files in a directory is 1.3 × 1020, although this is not relevant for practical situations.
Block size: 1 KiB 2 KiB 4 KiB 8 KiB
max. file size: 16 GiB 256 GiB 4 TiB 64 TiB
max. filesystem size: 4* TiB 8 TiB 16 TiB 32 TiB

The ext3 or third extended filesystem is a journaled file system that is commonly used by the Linux kernel. It is the default file system for many popular Linux distributions. Stephen Tweedie first revealed that he was working on extending ext2 in Journaling the Linux ext2fs Filesystem in 1998[2] paper and later in a February 1999 kernel mailing list posting[3] and the filesystem was merged with the mainline Linux kernel in November 2001 from 2.4.15 onward.[4] Its main advantage over ext2 is journaling which improves reliability and eliminates the need to check the file system after an unclean shutdown. Its successor is ext4.


Although its performance (speed) is less attractive than competing Linux filesystems such as JFS, ReiserFS and XFS, it has a significant advantage in that it allows in-place upgrades from the ext2 file system without having to back up and restore data. Ext3 also uses less CPU power than ReiserFS and XFS.[5] It is also considered safer than the other Linux file systems due to its relative simplicity and wider testing base.

The ext3 file system adds, over its predecessor:

  • A Journaling file system
  • Online file system growth
  • Htree indexing for larger directories. An HTree is a specialized version of a B-tree
Without these, any ext3 file system is also a valid ext2 file system.
This has allowed well-tested and mature file system maintenance utilities for maintaining and repairing ext2 file systems to also be used with ext3 without major changes. The ext2 and ext3 file systems share the same standard set of utilities, e2fsprogs, which includes a fsck tool. The close relationship also makes conversion between the two file systems (both forward to ext3 and backward to ext2) straightforward.
While in some contexts the lack of "modern" filesystem features such as dynamic inode allocation and extents could be considered a disadvantage

Journaling Levels:
There are three levels of journaling available in the Linux implementation of ext3:

Journal (lowest risk):

Both metadata and file contents are written to the journal before being committed to the main file system. Because the journal is relatively continuous on disk, this can improve performance in some circumstances. In other cases, performance gets worse because the data must be written twice - once to the journal, and once to the main part of the filesystem.[8]

Ordered (medium risk)
Only metadata is journaled; file contents are not, but it's guaranteed that file contents are written to disk before associated metadata is marked as committed in the journal. This is the default on many Linux distributions. If there is a power outage or kernel panic while a file is being written or appended to, the journal will indicate the new file or appended data has not been "committed", so it will be purged by the cleanup process. (Thus appends and new files have the same level of integrity protection as the "journaled" level.) However, files being overwritten can be corrupted because the original version of the file is not stored. Thus it's possible to end up with a file in an intermediate state between new and old, without enough information to restore either one or the other (the new data never made it to disk completely, and the old data is not stored anywhere). Even worse, the intermediate state might intersperse old and new data, because the order of the write is left up to the disk's hardware.

Writeback (highest risk):
Only metadata is journaled; file contents are not. The contents might be written before or after the journal is updated. As a result, files modified right before a crash can become corrupted. For example, a file being appended to may be marked in the journal as being larger than it actually is, causing garbage at the end. Older versions of files could also appear unexpectedly after a journal recovery. The lack of synchronization between data and journal is faster in many cases. XFS and JFS use this level of journaling, but ensure that any "garbage" due to unwritten data is zeroed out on reboot.

Size Limit

Block size Max file size Max filesystem size
1 KiB 16 GiB <2 href="" title="Tebibyte">TiB
2 KiB 256 GiB <4 href="" title="Tebibyte">TiB
4 KiB 2 TiB <8 href="" title="Tebibyte">TiB
8 KiB 2 TiB <16 href="" title="Tebibyte">TiB


The ext4 or fourth extended filesystem is a journaling file system developed as the successor to ext3. It was born as a series of backward compatible extensions to add 64-bit storage limits and other performance improvements to ext3.[1] However, other Linux kernel developers opposed accepting extensions to ext3 for stability reasons.

1. Large File system
The ext4 filesystem can support volumes with sizes up to 1 exbibyte and files with sizes up to 16 tebibytes.

2. Extents
Extents are introduced to replace the traditional block mapping scheme used by ext2/3 filesystems. An extent is a range of contiguous physical blocks, improving large file performance and reducing fragmentation. A single extent in ext4 can map up to 128MiB of contiguous space with a 4KiB block size.[1] There can be 4 extents stored in the Inode. When there are more than 4 extents to a file, the rest of the extents are indexed in an Htree.

3. Backward Compatibility
The ext4 filesystem is backward compatible with ext3 and ext2, making it possible to mount ext3 and ext2 filesystems as ext4. This will already slightly improve performance, because certain new features of ext4 can also be used with ext3 and ext2, such as the new block allocation algorithm. The ext3 file system is partially forward compatible with ext4, that is, an ext4 filesystem 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.

4.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 0s to reserve the space when the file is created. This method would no longer be required for ext4; instead, a new fallocate() system call was added to the linux kernel 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.

5. Delayed allocation
Ext4 uses a filesystem performance technique called allocate-on-flush, also known as delayed allocation. It consists of delaying block allocation until the data is going to be written to the disk, unlike some other file systems, which may allocate the necessary blocks before that step. This improves performance and reduces fragmentation by improving block allocation decisions based on the actual file size.

6.Break 32,000 subdirectory limit
In ext3 the number of subdirectories that a directory can contain is limited to 32,000. This limit has been raised to 64,000 in ext4, and with the "dir_nlink" feature it can go beyond this (although it will stop increasing the link count on the parent). To allow for continued performance given the possibility of much larger directories, Htree indexes (a specialized version of a B-tree) are turned on by default in ext4. This feature is implemented in Linux kernel 2.6.23. Htree is also available in ext3 when the dir_index feature is enabled.

7.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 I/O wait during the journaling process, improving performance slightly. The technique of journal checksumming was inspired by a research paper from the University of Wisconsin titled IRON File Systems (specifically, section 6, called "transaction checksums").

8. Online defragmentation
There are a number of proposals for an online defragmenter, but that support is not yet included in the mainline kernel. Even with the various techniques used to avoid fragmentation, a long lived file system does tend to become fragmented over time. Ext4 will have a tool which can defragment individual files or entire file systems.

9.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.

10.Multiblock allocator
Ext4 allocates multiple blocks for a file in a single operation, which reduces fragmentation by attempting to choose contiguous blocks on the disk. The multiblock 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.

11.Improved Timestamps
As computers become faster in general and as Linux becomes used more for mission critical applications, the granularity of second-based timestamps becomes insufficient. To solve this, ext4 provides timestamps measured in nanoseconds. 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 approximately an additional 204 years. Ext4 also adds support for date-created timestamps. But, as Theodore Ts'o points out, while it is easy to add an extra creation-date field in the inode (thus technically enabling support for date-created timestamps in ext4), it is more difficult to modify or add the necessary system calls, like stat() (which would probably require a new version), and the various libraries that depend on them (like glibc). These changes would require coordination of many projects. So, even if ext4 developers implement initial support for creation-date timestamps, this feature will not be available to user programs for now.

Delayed allocation and potential data loss Delayed allocation poses some additional risk of data loss in cases where the system crashes before all of the data has been written to the disk. The typical scenario in which this might occur is a program replacing the contents of a file without forcing a write to the disk with fsync. Problems can arise if the system crashes before the actual write occurs. In this situation, users of ext3 have come to expect that the disk will hold either the old version or the new version of the file following the crash. However, the ext4 code in the Linux kernel version 2.6.28 will often clear the contents of the file before the crash, but never write the new version, thus losing the contents of the file entirely. Many people[who?] find such a behavior unacceptable. A significant issue is that fixing the bug by using fsync more often could lead to severe performance penalties on ext3 filesystems mounted with the data=ordered flag (the default on most Linux distributions). Given that both file-systems will be in use for some time, this complicates matters enormously for end-user application developers. In response, Theodore Ts'o has written some patches for ext4 that cause it to limit its delayed allocation in these common cases. For a small cost in performance, this will significantly increase the chance that either version of the file will survive the crash. The new patches are expected to become part of the mainline kernel 2.6.30. Various distributions may choose to backport them to 2.6.28 or 2.6.29, for instance Ubuntu made them part of the 2.6.28 kernel in version 9.04—Jaunty Jackalope.

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