Journaling block device

Last updated

JBD, or journaling block device, is a generic block device journaling layer in the Linux kernel written by Stephen Tweedie from Red Hat. JBD is filesystem-independent. ext3, ext4 and OCFS2 are known to use JBD. [1] [2]

Contents

JBD exists in two versions, JBD and JBD2. JBD was created with ext3 in 1998. [3] JBD2 was forked from JBD in 2006 with ext4, with the goal of supporting a 64-bit (as opposed to 32-bit-only in JBD) block number. As a result, the maximum volume size in ext4 is increased to 1 EiB compared to 16 TiB in ext3 (assuming 4 KiB blocks). [4] JBD2 is backward-compatible. OCFS2 starting from Linux 2.6.28 uses JBD2. [5] The old JBD was removed with the dedicated ext3 driver in Linux 4.3 (2015). [6]

JBD structures

Atomic handle

An atomic handle is basically a collection of all the low-level changes that occur during a single high-level atomic update to the file system. The atomic handle guarantees that the high-level update either happens or not, because the actual changes to the file system are flushed only after logging the atomic handle in the journal. [2]

Transaction

For the sake of efficiency and performance, JBD groups several atomic handles into a single transaction, which is written to the journal after a fixed amount of time elapses or there is no free space left on the journal to fit it.

The transaction has several states: [2]

Recovery

Based on the transaction states, the JBD is able to determine which transactions need to be replayed (or reapplied) to the file system. [2]

Sources

  1. ↑ "The Linux Journalling API". The Linux Kernel documentation.
  2. 1 2 3 4 Kedar Sovani (June 20, 2006). "Linux: The Journaling Block Device". KernelTrap. Archived from the original on 2007-09-26.
  3. ↑ Stephen C. Tweedie (May 1998). "Journaling the Linux ext2fs Filesystem" (PDF). Proceedings of the 4th Annual LinuxExpo, Durham, NC. Retrieved 2007-06-23.
  4. ↑ Mingming Cao (9 August 2006). "Forking ext4 filesystem and JBD2" (Mailing list). Linux kernel mailing list.
  5. ↑ "Linux 2.6.28". kernelnewbies.org. 2008.
  6. ↑ Corbet, Jonathan. "rm -r fs/ext3". LWN.net.

Related Research Articles

XFS is a high-performance 64-bit journaling file system created by Silicon Graphics, Inc (SGI) in 1993. It was the default file system in SGI's IRIX operating system starting with its version 5.3. XFS was ported to the Linux kernel in 2001; as of June 2014, XFS is supported by most Linux distributions; Red Hat Enterprise Linux uses it as default filesystem.

The ext2 or second extended file system is a file system for the Linux kernel. It was initially designed by French software developer Rémy Card as a replacement for the extended file system (ext). Having been designed according to the same principles as the Berkeley Fast File System from BSD, it was the first commercial-grade filesystem for Linux.

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.

Reiser4 is a computer file system, successor to the ReiserFS file system, developed from scratch by Namesys and sponsored by DARPA as well as Linspire. Reiser4 was named after its former lead developer Hans Reiser. As of 2021, the Reiser4 patch set is still being maintained, but according to Phoronix, it is unlikely to be merged into mainline Linux without corporate backing.

The inode is a data structure in a Unix-style file system that describes a file-system object such as a file or a directory. Each inode stores the attributes and disk block locations of the object's data. File-system object attributes may include metadata, as well as owner and permission data.

In computing, the Global File System 2 or GFS2 is a shared-disk file system for Linux computer clusters. GFS2 allows all members of a cluster to have direct concurrent access to the same shared block storage, in contrast to distributed file systems which distribute data throughout the cluster. GFS2 can also be used as a local file system on a single computer.

The fstab file is a system configuration file commonly found at /etc/fstab on Unix and Unix-like computer systems. In Linux, it is part of the util-linux package. The fstab file typically lists all available disk partitions and other types of file systems and data sources that may not necessarily be disk-based, and indicates how they are to be initialized or otherwise integrated into the larger file system structure.

The extended file system, or ext, was implemented in April 1992 as the first file system created specifically for the Linux kernel. It has metadata structure inspired by traditional Unix filesystem principles, and was designed by Rémy Card to overcome certain limitations of the MINIX file system. It was the first implementation that used the virtual file system (VFS), for which support was added in the Linux kernel in version 0.96c, and it could handle file systems up to 2 gigabytes (GB) in size.

File system Format or program for storing files and directories

In computing, file system or filesystem is a method and data structure that the operating system uses to control how data is stored and retrieved. Without a file system, data placed in a storage medium would be one large body of data with no way to tell where one piece of data stopped and the next began, or where any piece of data was located when it was time to retrieve it. By separating the data into pieces and giving each piece a name, the data is easily isolated and identified. Taking its name from the way a paper-based data management system is named, each group of data is called a "file." The structure and logic rules used to manage the groups of data and their names is called a "file system."

Stephen C. Tweedie is a Scottish software developer who is known for his work on the Linux kernel, in particular his work on filesystems.

Theodore Tso American computer scientist, free software developer

Theodore (Ted) Yue Tak Ts'o (曹子德) is an American software engineer mainly known for his contributions to the Linux kernel, in particular his contributions to file systems. He is the Secondary developer and maintainer of e2fsprogs, the userspace utilities for the ext2, ext3, and ext4 filesystems, and is a maintainer for the ext4 file system.

The Oracle Cluster File System is a shared disk file system developed by Oracle Corporation and released under the GNU General Public License. The first version of OCFS was developed with the main focus to accommodate Oracle's database management system that used cluster computing. Because of that it was not a POSIX-compliant file system. With version 2 the POSIX features were included.

The following tables compare general and technical information for a number of file systems.

e2fsprogs is a set of utilities for maintaining the ext2, ext3 and ext4 file systems. Since those file systems are often the default for Linux distributions, it is commonly considered to be essential software.

The ext4 journaling file system or fourth extended filesystem is a journaling file system for Linux, developed as the successor to ext3.

dm-crypt is a transparent disk encryption subsystem in Linux kernel versions 2.6 and later and in DragonFly BSD. It is part of the device mapper (dm) infrastructure, and uses cryptographic routines from the kernel's Crypto API. Unlike its predecessor cryptoloop, dm-crypt was designed to support advanced modes of operation, such as XTS, LRW and ESSIV, in order to avoid watermarking attacks. In addition to that, dm-crypt addresses some reliability problems of cryptoloop.

LogFS is a Linux log-structured and scalable flash file system, intended for use on large devices of flash memory. It is written by Jörn Engel and in part sponsored by the CE Linux Forum.

Btrfs is a computer storage format that combines a file system based on the copy-on-write (COW) principle with a logical volume manager, developed together. It was initially designed at Oracle Corporation in 2007 for use in Linux, and since November 2013, the file system's on-disk format has been declared stable in the Linux kernel. According to Oracle, Btrfs "is not a true acronym".

A journaling file system is a file system that keeps track of changes not yet committed to the file system's main part by recording the goal of such changes in a data structure known as a "journal", which is usually a circular log. In the event of a system crash or power failure, such file systems can be brought back online more quickly with a lower likelihood of becoming corrupted.

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.