UFS logging is the process of writing file system "metadata" updates to a log before applying the updates to a UFS file system.
UFS logging records UFS transactions in a log. Once a transaction is recorded in the log, the transaction information can be applied to the file system later.
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. Because the file system is never inconsistent, it does not need checking by fsck(1M).
A system crash can interrupt current system calls and introduce inconsistencies into a UFS. If you mount a UFS without running fsck(1M), these inconsistencies can cause panics or corrupt data.
Checking large file systems takes a long time, because it requires reading and verifying the file system data. With UFS logging, UFS file systems do not have to be checked at boot time because the changes from unfinished system calls are discarded.