UFS Logging

UFS logging bundles the multiple metadata changes that make up a complete UFS operation into a transaction. Sets of transactions are recorded in an on-disk log, and then applied to the actual UFS file system's metadata.

At reboot, the system discards incomplete transactions, but applies the transactions for completed operations. The file system remains consistent because only completed transactions are ever applied. This consistency remains even when a system crashes, which normally interrupts system calls and introduces inconsistencies into a UFS file system.

UFS logging provides two advantages:

• If the file system is already consistent due to the transaction log, you might not have to run the fsck command after a system crash or an unclean shutdown. For more information on unclean shutdowns, see What the fsck Command Checks and Tries to Repair.

• Starting in the Solaris 9 12/02 release, the performance of UFS logging improves or exceeds the level of performance of non-logging file systems. This improvement can occur because a file system with logging enabled converts multiple updates to the same data into single updates, and so reduces the number of overhead disk operations required.

The log is allocated from free blocks on the file system, and it is sized at approximately 1 Mbyte per 1 Gbyte of file system, up to a maximum of 64 Mbytes. The log is continually flushed as it fills up. The log is also flushed when the file system is unmounted or as a result of any lockfs command.

UFS logging is enabled by default for all UFS file systems.

If you need to disable UFS logging, add the nologging option to the file system's entry in the /etc/vfstab file or when you mount the file system manually.

If you need to enable UFS logging, specify the -o logging option with the mount command in the /etc/vfstab file or when you mount the file system manually. Logging can be enabled on any UFS file system, including the root (/) file system. Also, the fsdb command now has new debugging commands to support UFS logging.

In some operating systems, a file system with logging enabled is known as a journaling file system.