Intent logging minimizes system downtime after abnormal shutdowns by logging filesystem transactions. When the system is halted unexpectedly, this log can be replayed and outstanding transactions completed. The check and repair time for filesystems can be reduced to a few seconds, regardless of the filesystem size.
The ability to locate and check only affected areas of the disk for
inconsistencies is central to the logging mechanism. The structure of the
mechanism is described in detail in
``Fast filesystem recovery for vxfs''.
If the system crashes before the log is written, it is as if the change (any modifications to the filesystem) never occurred. If the system crashes after the log is written, but before the transaction complete, fsck either completes the change or undoes it. If the system crashes after the transaction is completed, then the modification has been completed, and there is nothing for fsck to do.