Generally, you should log your largest UFS file systems and the UFS file system whose data changes most often. It is probably not necessary to log small file systems with mostly read-only activity.
If no slice is available for the log device, you can still configure a transactional volume. This strategy might be useful if you plan to log exported file systems when you do not have a spare slice for the log device. When a slice is available, you only need to attach it as a log device.
Consider sharing a log device among file systems if your system does not have many available slices, or if the file systems sharing the log device are primarily read, not written.
When one master device of a shared log device goes into a failed state, the log device is unable to roll its changes forward. This problem causes all master devices sharing the log device to go into the hard error state.