Used to have only one NW5.1 running NDS version 8.8.5 (named Server A). Since its data partition is not stable, install a new NW6.5 with SP8 into the same tree for migrating the data (named Server B). Encountered hardware issue and install another new NW6.5 with SP8 into the same tree again (named Server C). Later, hardware problem again and install another NW6.5 with SP8 (named Server D).

Now, only Server A (master replica) and Server D is running. But I noted Server D is a SubRef replica rather than R/W replica. While Server B and Server C is R/W replica, although they no longer exists on the network.

Installation was:
- Server A (up), install Server B
- Server A (up), Server B (down), install Server C
- Server A (up), Server B (down), Server C (down)
- Server A (up), Server B (down), Server C (down), install Server D

Qn1: Why SubRef replica generated automatically but not R/W?

Qn2: how can i make Server D to be an R/W replica while leaving Server A as master replica? I wish to have a R/W replica as the back to the master replica.

I read many material and forum over the net but I still have no idea what to do in order to have a backup replica in my case. And in the following URL, it said not to change the SubRef replica except for disaster recovery. I really get lost right now. Thanks for your help!

If a server has a SubRef replica and you want to promote it to be a real replica on the server, the operation you need to use is the Create Replica operation, not the Change Replica Type operation. This is because a SubRef replica is not a real copy of the partition; rather, it contains just enough information for NDS operations such as tree-walking. Therefore, the only way to change its type is to place a copy of the real replica on that server.

You should never change a SubRef replica type except in a DS disaster recovery scenario, and you should do that only as a very last resort. Refer to the section "Replica Ring Inconsistency" in Chapter 11, "Examples from the Real World" for more information about this process.