ext3: Файловая система с журналированием для Linux

Journaling file system for Linux
the gene

ext3, or third extended filesystem, is a journaled file system that is commonly used by the Linux kernel. It used to be 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 a 1998 paper, and later in a February 1999 kernel mailing list posting. The filesystem was merged with the mainline Linux kernel in November 2001 from 2.4.15 onward. 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.

Because ext3 aims to be backward compatible with the earlier ext2, many of the on disk structures are similar to those of ext2. Consequently, ext3 lacks recent features, such as extents, dynamic allocation of inodes, and block sub allocation. A directory can have at most 31998 subdirectories, because an inode can have at most 32,000 links (each direct subdirectory increases their parent folder inode link counter in the " " reference). On ext3, like for most current Linux filesystems, the system tool "fsck" should not be used while the filesystem is mounted for writing. There are userspace defragmentation tools, like Shake and defrag. Shake works by allocating space for the whole file as one operation, which will generally cause the allocator to find contiguous disk space. If there are files which are used at the same time, Shake will try to write them next to one another. Defrag works by copying each file over itself. However, this strategy works only if the file system has enough free space. A true defragmentation tool does not exist for ext3. However, as the Linux System Administrator Guide states, "Modern Linux filesystem(s) keep fragmentation at a minimum by keeping all blocks in a file close together, even if they can't be stored in consecutive sectors. Some filesystems, like ext3, effectively allocate the free block that is nearest to other blocks in a file. Therefore it is not necessary to worry about fragmentation in a Linux system." While ext3 is resistant to file fragmentation, ext3 can get fragmented over time or for specific usage patterns, like slowly writing large files. Consequently, ext4 (the successor to ext3) has an online filesystem defragmentation utility e4defrag and currently supports extents (contiguous file regions).

ext3 does not support the recovery of deleted files. The ext3 driver actively deletes files by wiping file inodes for crash safety reasons. There are still several techniques and some free and proprietary software for recovery of deleted or lost files using file system journal analysis; however, they do not guarantee any specific file recovery.

e3compr is an unofficial patch for ext3 that does transparent compression. It is a direct port of e2compr and still needs further development. It compiles and boots well with upstream kernels, but journaling is not implemented yet.

Unlike a number of modern file systems, ext3 does not have native support for snapshots, the ability to quickly capture the state of the filesystem at arbitrary times. Instead, it relies on less space efficient, volume level snapshots provided by the Linux LVM. The Next3 file system is a modified version of ext3 which offers snapshots support, yet retains compatibility with the ext3 on disk format.

ext3 does not do checksumming when writing to the journal. On a storage device with extra cache, if barrier=1 is not enabled as a mount option (in /etc/fstab), and if the hardware is doing out of order write caching, one runs the risk of severe filesystem corruption during a crash. This is because storage devices with write caches report to the system that the data has been completely written, even if it was written to the (volatile) cache. If hard disk writes are done out of order (due to modern hard disks caching writes in order to amortize write speeds), it is likely that one will write a commit block of a transaction before the other relevant blocks are written. If a power failure or unrecoverable crash should occur before the other blocks get written, the system will have to be rebooted. Upon reboot, the file system will replay the log as normal, and replay the "winners" (transactions with a commit block, including the invalid transaction above, which happened to be tagged with a valid commit block). The unfinished disk write above will thus proceed, but using corrupt journal data. The file system will thus mistakenly overwrite normal data with corrupt data while replaying the journal. If checksums had been used, where the blocks of the "fake winner" transaction were tagged with a mutual checksum, the file system could have known better and not replayed the corrupt data onto the disk. Journal checksumming has been added to ext4. Filesystems going through the device mapper interface (including software RAID and LVM implementations) may not support barriers, and will issue a warning if that mount option is used. There are also some disks that do not properly implement the write cache flushing extension necessary for barriers to work, which causes a similar warning. In these situations, where barriers are not supported or practical, reliable write ordering is possible by turning off the disk's write cache and using the mount option. There is also a potential issue with the barrier implementation related to error handling during events, such as a drive failure. It is also known that sometimes some virtualization technologies do not properly forward fsync or flush commands to the underlying devices (files, volumes, disk) from a guest operating system. Similarly, some hard disks or controllers implement cache flushing incorrectly or not at all, but still advertise that it is supported, and do not return any error when it is used. There are so many ways to handle fsync and write cache handling incorrectly, it is safer to assume that cache flushing does not work unless it is explicitly tested, regardless of how reliable individual components are believed to be.

Ext3 stores dates as Unix time using four bytes in the file header. 32 bits does not give enough scope to continue processing files beyond January 18, 2038 the Year 2038 problem.

On June 28, 2006, Theodore Ts'o, the principal developer of ext3, announced an enhanced version, called ext4. On October 11, 2008, the patches that mark ext4 as stable code were merged in the Linux 2.6.28 source code repositories, marking the end of the development phase and recommending its adoption. In 2008, Ts'o stated that although ext4 has improved features such as being much faster than ext3, it is not a major advance, it uses old technology, and is a stop gap; Ts'o believes that Btrfs is the better direction, because "it offers improvements in scalability, reliability, and ease of management". Btrfs also has "a number of the same design ideas that reiser3/4 had".