- Jun 26, 2017
from my point of view it's not true - you can build an 3-Node ceph cluster without issues.
One node can fail, without data loss.
But the downtime of the failed node should not be to long. Because ceph can't remap the data to other osds to reach the replica-count of three again.
But this depends on the amount of data. Often, in much bigger ceph-setups, it's makes not realy sense to map all data to other nodes, because you are faster to bring the failed node back (spare server...). E.g. if one node have 10 4-TB OSDs you need a long time the rebalance the data across the other nodes.
And you need the free space on the other nodes of couse!
But ceph win with more nodes (more speed, less trouble during rebalance).
So what happens when one of the three machines with ceph goes down? Can't it function with just two copies, or does it try to create a third copy on the existing free space?