A journaling file system is a file system that keeps track of the changes that will be made in a journal (usually a circular log in a dedicated area of the file system) before committing them to the main file system. In the event of a system crash or power failure, such file systems are quicker to bring back online and less likely to become corrupted.
Updating file systems to reflect changes to files and directories usually requires many separate write operations. This makes it possible for an interruption (like a power failure or system crash) between writes to leave data structures in an invalid intermediate state.
For example, deleting a file on a Unix file system involves two steps:
- Removing its directory entry.
- Marking space for the file and its inode as free in the free space map.
If a crash occurs between steps 1 and 2, there will be an orphaned inode and hence a storage leak. On the other hand, if only step 2 is performed first before the crash, the not-yet-deleted file will be marked free and possibly be overwritten by something else.
In a non-consistent file system, detecting and recovering from such inconsistencies requires a complete walk of its data structures. This must typically be done before the file system is next mounted for read-write access. If the file system is large and if there is relatively little I/O bandwidth, this can take a long time and result in longer downtimes if it blocks the rest of the system from coming back online.
To prevent this, a journaled file system allocates a special area the journal in which it records the changes it will make, ahead of time. After a crash, recovery simply involves reading the journal from the file system and replaying changes from this journal until the file system is consistent again. The changes are thus said to be atomic (not divisible) in that they either:
- succeed (succeeded originally or are replayed completely during recovery), or
- are not replayed at all (are skipped because they had not yet been completely written to the journal before the crash occurred).
Some file systems allow the journal to grow, shrink and be re-allocated just as a regular file, while others put the journal in a contiguous area or a hidden file that is guaranteed not to move or change size while the file system is mounted. Some file systems may also allow external journals on a separate device, such as a solid-state disk or battery-backed non-volatile RAM. Changes to the journal may themselves be journaled for additional redundancy, or the journal may be distributed across multiple physical volumes to protect against device failure.
The internal format of the journal must guard against crashes while the journal itself is being written to. Many journal implementations (such as the JBD2 layer in ext4) bracket every change logged with a checksum, on the understanding that a crash would leave a partially written change with a missing (or mismatched) checksum that can simply be ignored when replaying the journal at next remount.
A physical journal logs an advance copy of every block that will later be written to the main file system. If there is a crash when the main file system is being written to, the write can simply be replayed to completion when the file system is next mounted. If there is a crash when the write is being logged to the journal, the partial write will have a missing or mismatched checksum and can be ignored at next mount.
Physical journals impose a significant performance penalty because every changed block must be committed twice to storage, but may be acceptable when absolute fault protection is required.
A logical journal stores only changes to file metadata in the journal, and trades fault tolerance for substantially better write performance. A file system with a logical journal still recovers quickly after a crash, but may allow unjournaled file data and journaled metadata to fall out of sync with each other, causing data corruption.
For example, appending to a file may involve three separate writes to:
- The file's inode, to increase its size.
- The free space map, to mark out an allocation of space for append.
- The newly allocated space, to actually write the appended data.
In a metadata-only journal, step 3 would not be logged. If step 3 was not done, but steps 1 and 2 are replayed during recovery, the file will be appended with garbage.
The write cache in most operating systems sorts its writes (using the elevator algorithm or some similar scheme) to maximize throughput. To avoid an out-of-order write hazard with a metadata-only journal, writes for file data must be sorted so that they are committed to storage before their associated metadata. This can be tricky to implement because it requires coordination within the operating system kernel between the file system driver and write cache. An out-of-order write hazard can also exist if the underlying storage:
- cannot write blocks atomically, or
- does not honor requests to flush its write cache
To complicate matters, many mass storage devices have their own write caches, in which they may aggressively reorder writes for better performance. (This is particularly common on magnetic hard drives, which have large seek latencies that can be minimized with elevator sorting.) Some journaling file systems conservatively assume such write-reordering always takes place, and sacrifice performance for correctness by forcing the device to flush its cache at certain points in the journal (called barriers in ext3 and ext4).
Some UFS implementations avoid journaling and instead implement soft updates: they order their writes in such a way that the on-disk file system is never inconsistent, or that the only inconsistency that can be created in the event of a crash is a storage leak. To recover from these leaks, the free space map is reconciled against a full walk of the file system at next mount. This garbage collection is usually done in the background.
Log-structured file systems
In log-structured file systems, the write-twice penalty does not apply because the journal itself is the file system: it occupies the entire storage device and is structured so that it can be traversed as would a normal file system.
Copy-on-write file systems
Full copy-on-write file systems (such as ZFS and Btrfs) avoid in-place changes to file data by writing out the data in newly allocated blocks, followed by updated metadata that would point to the new data and disown the old, followed by metadata pointing to that, and so on up to the superblock, or the root of the file system hierarchy. This has the same correctness-preserving properties as a journal, without the write-twice overhead.
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