Acronym for Journalling File System.
The idea behind a journalling file system is that you write out what changes you are about to make to the file system in a special area (called a "Journal"), then every so often you go and "commit" all the changes (actually go and do them). This actually tends to improve performance in the case where you are frequently updating the same data, for example for databases.
If a crash occurs due to a power outage or other event, then when the machine starts up it looks at the journal, and verifies that the journal is correct, if it is correct it actions all the journal entries, and if it is invalid, then it ignores them.
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