Understanding the Majority Consensus Algorithm

Replicated databases have an inherent problem in determining which database has valid and correct data. To solve this problem, Solaris Volume Manager uses a majority consensus algorithm. This algorithm requires that a majority of the database replicas agree with each other before any of them are declared valid. This algorithm requires the presence of at least three initial replicas which you create. A consensus can then be reached as long as at least two of the three replicas are available. If there is only one replica and the system crashes, it is possible that all volume configuration data will be lost.

To protect data, Solaris Volume Manager will not function unless half of all state database replicas are available. The algorithm, therefore, ensures against corrupt data.

The majority consensus algorithm provides the following:

• The system will stay running if at least half of the state database replicas are available.

• The system will panic if fewer than half of the state database replicas are available.

• The system will not reboot into multiuser mode unless a majority (half + 1) of the total number of state database replicas is available.

If insufficient state database replicas are available, you will have to boot into single-user mode and delete enough of the bad or missing replicas to achieve a quorum. See How to Recover From Insufficient State Database Replicas.

When the number of state database replicas is odd, Solaris Volume Manager computes the majority by dividing the number in half, rounding down to the nearest integer, then adding 1 (one). For example, on a system with seven replicas, the majority would be four (seven divided by two is three and one-half, rounded down is three, plus one is four).