How Does DiskSuite Use State Database Replicas?

The state database replicas ensure that the data in the metadevice state database is always valid. When the metadevice 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, DiskSuite must figure out which state database replicas still contain non-corrupted data. DiskSuite determines this information by 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.

To protect data, DiskSuite will not function if a majority (half + 1) of all state database replicas is not available. The algorithm, therefore, ensures against corrupt data.

The majority consensus algorithm guarantees the following:

• The system will stay running with exactly half or more state database replicas.

• The system will panic if more than half the state database replicas are not available.

• The system will not reboot without one more than half the total state database replicas.

When the number of state database replicas is odd, DiskSuite 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).

During booting, DiskSuite ignores corrupted state database replicas. In some cases DiskSuite 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.

If all state database replicas are lost, you could, in theory, lose all data that is stored on your disks. For this reason, it is good practice to create enough state database replicas on separate drives and across controllers to prevent catastrophic failure. It is also wise to save your initial DiskSuite configuration information, as well as your disk partition information.

Refer to Solstice DiskSuite 4.2.1 User's Guide for information on adding additional state database replicas to the system, and on recovering when state database replicas are lost.