Journaling file system

A journaling file system is a file system that keeps track of changes not yet committed to the file system's main part by recording the intentions of such changes in a data structure known as a 'journal', which is usually a circular log. In the event of a system crash or power failure, such file systems can be brought back online more quickly with a lower likelihood of becoming corrupted. A journaling file system is a file system that keeps track of changes not yet committed to the file system's main part by recording the intentions of such changes in a data structure known as a 'journal', which is usually a circular log. In the event of a system crash or power failure, such file systems can be brought back online more quickly with a lower likelihood of becoming corrupted. Depending on the actual implementation, a journaling file system may only keep track of stored metadata, resulting in improved performance at the expense of increased possibility for data corruption. Alternatively, a journaling file system may track both stored data and related metadata, while some implementations allow selectable behavior in this regard. In 1990 IBM JFS, introduced with AIX 3.1, was one of the first UNIX commercial filesystems that implemented journaling. Later in 1993 Microsoft NTFS filesystem and in 2001 ext3 implemented journaling. 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 three steps: If a crash occurs after step 1 and before step 2, there will be an orphaned inode and hence a storage leak; if a crash occurs between steps 2 and 3, then the blocks previously used by the file cannot be used for new files, effectively decreasing the storage capacity of the file system. Re-arranging the steps does not help, either. If step 3 preceded step 1, a crash between them could allow the file's blocks to be reused for a new file, meaning the partially deleted file would contain part of the contents of another file, and modifications to either file would show up in both. On the other hand, if step 2 preceded step 1, a crash between them would cause the file to be inaccessible, despite appearing to exist. Detecting and recovering from such inconsistencies normally requires a complete walk of its data structures, for example by a tool such as fsck (the file system checker). 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 drive 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.