Administering filesystems

Understanding intent logging

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.

NOTE: Intent logging does not increase the reliability of filesystem. Only transactions concerning file meta data (the structures concerned with storing data) are logged. The purpose is to minimize system downtime. For optimum benefit, intent logging should be enabled.

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.