Each DRU contains a complete copy of the data in HADB and can continue servicing requests if the other DRU becomes unavailable. However, if a node in one DRU and its mirror in another DRU fail at the same time, some portion of data is lost. For this reason, it is important that the system is not set up so that both DRUs can be affected by a single failure such as a power failure or disk failure.
Follow these guidelines when setting up the HADB nodes and machines:
Set up each DRU with a number of spare nodes equal to the number of nodes running on each machine. This is because if each machine in the configuration runs n data nodes, the failure of a single machine brings down n nodes.
Run the same number of HADB nodes on all machines to balance load as evenly as possible.
Do not run nodes from different DRUs on the same machine. If you must run nodes from different DRUs on the same machine, ensure that the machine can handle any single point of failure (for failures related to disk, memory, CPU, power, operating system crashes, and so on).