Journaling filesystem

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 was one of the first UNIX commercial filesystems running under AIX operating systems implementing Journaling. Later in the 2000 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. 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.

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.

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