Single-node Ceph cluster

Sometimes I need a Ceph cluster for testing and I need it to be as simple as possible. Fortunately, Ceph can be convinced to run, self-contained, on a single machine. Of course, such a cluster should only be used for testing purposes, since it provides no failure resistance.

Single-node cluster requires some special configuration, but not too complicated. First, there can only be one MON in such a cluster. Second, the default "CRUSH chooseleaf" must be set to 0. Third, "size" and "min_size" pool parameters must be set appropriately, depending on the number of OSDs in the cluster.

Example ceph.conf for single-node cluster with two OSDs:

[global]
fsid = 44af0c2b-0423-4823-86f5-b21b5ed782b0
mon_initial_members = vanguard2
mon_host = 10.100.12.15
auth_cluster_required = cephx
auth_service_required = cephx
auth_client_required = cephx
osd_crush_chooseleaf_type = 0
osd_pool_default_size = 1
osd_pool_default_min_size = 1

For a single OSD, change the last line to osd_pool_default_min_size = 0. Ceph wants to provide failure resistance and will normally go into HEALTH_WARN if the cluster is not configured to sustain the failure of at least one OSD without data loss.