Techniques
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.
Read more about this topic: Journaling File System
Famous quotes containing the word techniques:
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—Neil Kurshan (20th century)
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—John Steinbeck (1902–1968)