ext4
ext4 (fourth extended filesystem) is a journaling file system for Linux, developed as the successor to ext3.
| Developer(s) | Mingming Cao, Andreas Dilger, Alex Zhuravlev (Tomas), Dave Kleikamp, Theodore Ts'o, Eric Sandeen, Sam Naghshineh, others |
|---|---|
| Full name | Fourth extended file system |
| Introduced |
|
| Preceded by | ext3 |
| Partition IDs | 0x83: MBR / EBR. EBD0A0A2-B9E5-4433-87C0-68B6B72699C7: GPT Windows BDP. |
| Structures | |
| Directory contents | Linked list, hashed B-tree |
| File allocation | Extents / Bitmap |
| Bad blocks | Table |
| Limits | |
| Max volume size | 1 EiB |
| Max file size | 16-256 TiB (for 4-64 KiB block size) |
| Max no. of files | 4 billion (specified at filesystem creation time) |
| Max filename length | 255 bytes (fewer for multibyte character encodings such as Unicode) |
| Allowed filename characters | All bytes except NULL ('\0') and '/' and the special file names "." and ".." which are not forbidden but are always used for a respective special purpose. |
| Features | |
| Dates recorded | modification (mtime), data or attribute modification (ctime), access (atime), delete (dtime), create (crtime) |
| Date range | 14 December 1901 - 10 May 2446 |
| Date resolution | Nanosecond |
| Forks | No |
| Attributes | acl, bh, bsddf, commit=nrsec, data=journal, data=ordered, data=writeback, delalloc, extents, journal_dev, mballoc, minixdf, noacl, nobh, nodelalloc, noextents, nomballoc, nombcache, nouser_xattr, oldalloc, orlov, user_xattr |
| File system permissions | Unix permissions, POSIX ACLs |
| Transparent compression | No |
| Transparent encryption | Yes |
| Data deduplication | No |
| Other | |
| Supported operating systems | |
ext4 was initially a series of backward-compatible extensions to ext3, many of them originally developed by Cluster File Systems for the Lustre file system between 2003 and 2006, meant to extend storage limits and add other performance improvements. However, other Linux kernel developers opposed accepting extensions to ext3 for stability reasons, and proposed to fork the source code of ext3, rename it as ext4, and perform all the development there, without affecting existing ext3 users. This proposal was accepted, and on 28 June 2006, Theodore Ts'o, the ext3 maintainer, announced the new plan of development for ext4.
A preliminary development version of ext4 was included in version 2.6.19 of the Linux kernel. On 11 October 2008, the patches that mark ext4 as stable code were merged in the Linux 2.6.28 source code repositories, denoting the end of the development phase and recommending ext4 adoption. Kernel 2.6.28, containing the ext4 filesystem, was finally released on 25 December 2008. On 15 January 2010, Google announced that it would upgrade its storage infrastructure from ext2 to ext4. On 14 December 2010, Google also announced it would use ext4, instead of YAFFS, on Android 2.3.