Journaling block device

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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.
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