State Database Replica Conventions

Background

The state database replicas ensure that the data in the state database is always valid. When the state database is updated, each state database replica is also updated. The updates take place one at a time (to protect against corrupting all updates if the system crashes).

If your system loses a state database replica, Enhanced Storage determines which replicas are still valid by using a majority consensus algorithm. This algorithm requires that a majority (half + 1) of the state database replicas be available before any of them are considered non-corrupt. It is because of this majority consensus algorithm that you must create at least three state database replicas when you set up your disk configuration. A consensus can be reached as long as at least two of the three state database replicas are available.

The majority consensus algorithm guarantees the following:


Note - When the number of state database replicas is odd, Enhanced Storage computes the majority by dividing the number in half and then rounding up to the nearest integer. For example, on a system with seven replicas, the majority would be four (seven divided by two is three and one-half, which is then rounded up to four).


Tasks and Issues

If all state database replicas are lost, you could, in theory, lose all data that is stored on your disks. Therefore, create enough state database replicas on separate drives and across controllers to prevent catastrophic failure. Also save your initial Enhanced Storage configuration information, as well as your disk partition information, on a separate system or volume.

Keep these issues in mind: