State database replicas contain configuration and status information for your Enhanced Storage configuration. Multiple copies (replicas) are maintained to provide redundancy and to prevent the database from being corrupted during a system crash (at most, only one copy if the database will be corrupted).
How does Enhanced Storage use replicas?
The system will stay running with exactly half or more replicas. The system will panic when fewer than half the replicas are available to prevent data corruption.
The system will not reboot without one more than half the total replicas. In this case, you must reboot single-user and delete the bad replicas (using the metadb command).
As an example, assume you have four replicas. The system will stay running as long as two replicas (half the total number) are available. However, in order for the system to reboot, three replicas (half the total plus 1) must be available.
During booting, Enhanced Storage ignores corrupted state database replicas. In some cases, Enhanced Storage tries to rewrite state database replicas that are bad. Otherwise they are ignored until you repair them. If a state database replica becomes bad because its underlying slice encountered an error, you will need to repair or replace the slice and then enable the replica.