Journaling

Please describe journaling in filesystems such as XFS.

Journaling in modern computing is a crucial technique that ensures data consistency within a file system even in the face of power outages or system crashes. One implementation of this concept can be found in the XFS file system. XFS employs journaling for its metadata, where changes to the file system are initially logged in a sequential journal before being applied to the actual disk blocks. This journal acts as a circular buffer of disk blocks and isn't typically accessed during normal file system operations.

The XFS journal can be stored within the file system's data section as an internal log or on a separate device to reduce conflicts with disk activity. Entries within the journal primarily describe the modified sections of disk blocks resulting from various file system operations. These journal updates are carried out asynchronously to prevent performance degradation.

In the event of a system crash, the file system can recover by reapplying operations recorded in the journal that were executed just before the crash. This process ensures the consistency of data within XFS file systems. Automatic recovery takes place when the file system is mounted after a crash. Interestingly, the recovery speed isn't tied to the file system's size but rather depends on the quantity of file system operations that need to be reapplied. This approach helps maintain data integrity even when unexpected interruptions occur.