HADB’s built-in data replication enables it to tolerate failure of a single node or an entire DRU. By default, HADB won’t survive a double failure , when a mirror node pair or both DRUs fail. In such cases, HADB become unavailable.
In addition to using spare nodes as described in the previous section, you can minimize the likelihood of a double failure by taking the following steps:
Providing independent power supplies: For optimum fault tolerance, the servers that support one DRU must have independent power (through uninterruptible power supplies), processing units, and storage. If a power failure occurs in one DRU, the nodes in the other DRU continue servicing requests until the power returns.
Providing double interconnections: To tolerate single network failures, replicate the lines and switches between DRUs.
These steps are optional, but will increase the overall availability of the HADB instance.