A journaled file system records changes in a log so it can recover filesystem structures to a consistent state after a crash. This mainly protects the file system’s bookkeeping—such as metadata used to locate files and track storage—not necessarily the latest contents of your files.
What does “journaled file system” mean?
A file system organizes file contents and the metadata that describes them, including directory entries, file records, and which storage blocks are in use. A journaled file system records changes as transactions in a journal, or log. Once a transaction is committed, the file system can use its journal during recovery if a crash interrupts updates to its main structures.
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For ext4, recovery replays valid committed transactions. An interrupted transaction without a valid commit record is not treated as a completed transaction. The exact structures logged and the recovery process vary by file system; journaling is a general design approach, not one identical implementation.
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What does journaling protect?
Journaling is principally about keeping filesystem structures consistent after an interrupted update. That can help the system recognize files and track storage correctly when it restarts. It does not automatically mean that file contents are journaled or that the newest data written by an application will survive a crash.
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Ext4’s default ordered mode journals metadata but does not guarantee that file data is consistent after a crash. Journaling also does not prevent hardware failure, replace a backup, or guarantee that every application write acknowledged before a power loss has reached durable storage.
How ext4’s journaling modes differ
Ext4 documents three data modes. They differ in what goes into the journal, how file data is ordered relative to metadata updates, and the performance tradeoff. These are ext4-specific options, not a menu shared by every journaled file system.
| ext4 mode | What is journaled and ordered | Documented performance tradeoff |
|---|---|---|
| writeback | Metadata is journaled; file data is not. Data writes are not ordered before the related metadata commit, so a crash can cause incorrect data to appear in files written shortly before it. | Typically ext4’s best performance. |
| ordered | Metadata is journaled. File data blocks associated with metadata changes are written before the metadata is committed, but the file data itself is not journaled. | Generally somewhat slower than writeback, but significantly faster than full journal mode. |
| journal | Both file data and metadata are written through the journal; data is then written to its final location. | Slower than the other modes, except when data needs to be read from disk; it provides the broadest journaling protection of these three modes. |
These descriptions are from the Linux kernel’s ext4 documentation. The kernel notes that XFS and JFS default behavior is similar to ext4’s metadata-journaling level, but that does not make ext4’s three modes universal options.
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What journaling does not guarantee
- It is not a backup. A journal supports recovery of filesystem structures; it is not a separate copy of your files.
- It does not necessarily preserve recent file contents. Whether data is journaled or merely ordered depends on the file system and its settings.
- It is not a guarantee against every kind of data loss. The documented purpose is crash recovery and filesystem consistency, not protection from all hardware or software failures.
- It does not replace durable-write practices. Applications that need reliable persistence still need to use appropriate application- and system-level mechanisms.
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