Why Do You Recommend One Drive Per OSD¶
Ceph OSD performance is one of the most common requests for assistance, and running an OS, a journal and an OSD on the same disk is a frequently the impediment to high performance. Total throughput and simultaneous reads and writes are a major bottleneck. If you journal data, run an OS, or run multiple OSDs on the same drive, you will very likely see performance degrade significantly–especially under high loads.
Running multiple OSDs on a single drive is fine for evaluation purposes. We even encourage that in our 5-minute quick start. However, just because it works does NOT mean that it will provide acceptable performance in an operational cluster.