Full definition
On power loss or crash, the filesystem replays the journal on next mount, ensuring consistency without a full fsck (which on large volumes can take hours). Common modes: data journaling (journals data and metadata, slower and safer), ordered (ext4 default, journals only metadata but enforces order), writeback (faster, less safe). In recovery, the journal often holds valuable information: partially written files, pending transactions, recently freed blocks. On ext4, debugfs or JournalParser can extract recent history useful for forensics or recovering a file deleted seconds before the crash.
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