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, monolithic, modular, multitasking, Unix-like operating system kernel. It was originally authored in 1991 by Linus Torvalds for his i386-based PC, and it was soon adopted as the kernel for the GNU ope ...
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. is an American software company that provides open source software products to enterprises. Founded in 1993, Red Hat has its corporate headquarters in Raleigh, North Carolina, with other offices worldwide.
Red Hat has become ass ...
. JBD is filesystem-independent.
ext3,
ext4 and
OCFS2 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
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