MogileFS::ReplicationPolicy::MultipleHosts -- default replication policy

This policy tries to put files onto devices which are on different hosts. If you only have 1 host and 2 devices on that one host, it obviously can't, so it'll grudgingly put it on the same host. But if you request a minimum replica count of 2 and have 3 devices, it'll put 2 copies on different hosts. If you have 4 devices on 2 hosts, and request a minima replica count of 3, you'll get 3 copies on different devices, but two of those devices will be on the same host, and that's considered acceptable, since you have "multiple hosts" covered at least.

MogileFS::Worker::Replicate

MogileFS::ReplicationPolicy