journal
also: journaling, ext4 journal
A log of filesystem changes written sequentially before they are committed to disk, allowing recovery from crashes without data corruption. The most common implementation is ext4's journaling feature.
A journal is a dedicated area of a filesystem that records metadata changes (and optionally file data changes) before they are permanently written to disk. When the system crashes, the journal allows the filesystem to quickly recover by replaying or undoing incomplete transactions, rather than requiring a time-consuming full filesystem check.
Most modern Linux filesystems like ext4, XFS, and Btrfs include journaling. For example, when you create a file, the journal first records "create file X in directory Y," then writes the actual changes to the filesystem. If power fails mid-operation, the journal indicates what was incomplete.
There are different journaling modes: journal mode logs data and metadata (safest, slower), ordered mode journals only metadata but ensures data writes complete first (default for ext4), and writeback mode journals only metadata (fastest, less safe).