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In a journaling file system, the information about the changes is recorded in a separate log (the journal) before the indexes to the files are updated.If a power or other system failure corrupts the indexes as they are being rewritten, the operating system can use the log to repair them when the computer is restarted.

In a journaling file system, the information about the changes is recorded in a separate log (the journal) before the indexes to the files are updated. If a power or other system failure corrupts the indexes as they are being rewritten, the operating system can use the log to repair them when the computer is restarted. From another words, 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 goal 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.[1][2]

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.[3]