JBD, or journaling block device, is a generic block device journaling layer in the

Linux kernel The Linux kernel is a Free and open-source software, free and open source Unix-like kernel (operating system), kernel that is used in many computer systems worldwide. The kernel was created by Linus Torvalds in 1991 and was soon adopted as the k ...

written by

Stephen Tweedie Stephen C. Tweedie is a Scottish software developer who is known for his work on the Linux kernel, in particular his work on filesystems. After becoming involved with the development of the ext2 filesystem working on performance issues, he led ...

from

Red Hat Red Hat, Inc. (formerly Red Hat Software, Inc.) is an American software company that provides open source software products to enterprises and is a subsidiary of IBM. Founded in 1993, Red Hat has its corporate headquarters in Raleigh, North ...

. JBD is filesystem-independent.

ext3 ext3, or third extended filesystem, is a journaling file system, journaled file system that is commonly used with the Linux kernel. It used to be the default file system for many popular Linux distributions but generally has been supplanted by ...

ext4 ext4 (fourth extended filesystem) is a journaling file system for Linux, developed as the successor to ext3. ext4 was initially a series of backward-compatible extensions to ext3, many of them originally developed by Cluster File Systems for ...

and

OCFS2 The Oracle Cluster File System (OCFS, in its second version OCFS2) 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 accom ...

are known to use JBD. JBD exists in two versions, JBD and JBD2. JBD was created with ext3 in 1998. 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 The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable unit ...

compared to 16 TiB in ext3 (assuming 4 KiB blocks). JBD2 is backward-compatible. OCFS2 starting from Linux 2.6.28 uses JBD2. The old JBD was removed with the dedicated ext3 driver in Linux 4.3 (2015).

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.

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: * Running - it means that the transaction is still live and can accept more handles * Locked - not accepting new handles, but the existing ones are still unfinished * Flush - the transaction is complete and is being written to the journal * Commit - the transaction is written to the journal and now the changes are being applied to the file system * Finished - the transaction has been fully written to the journal and the block device. It can be deleted from the journal.

Recovery

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

Sources

Linux kernel features Linux file system-related software Disk file systems {{linux-stub