UFS logging is the process of storing transactions (changes that make up a complete UFS operation) in a log before the transactions are applied to the UFS file system. Once a transaction is stored, the transaction can be applied to the file system later.
UFS logging provides two advantages. It prevents file systems from becoming inconsistent, therefore eliminating the need to run fsck(1M). And, because fsck can be bypassed, UFS logging reduces the time required to reboot a system if it crashes, or after an unclean halt.
UFS logging is not enabled by default. To enable UFS logging, you must specify the -o logging option with the mount(1M) command when mounting the file system. Also, the fsdb(1M) command has been updated with new debugging commands for UFS logging.
For more information, see System Administration Guide, Volume I.