OSD device strategy
With either back end, FileStore or BlueStore, it is strongly advised in production to deploy only one OSD per physical drive, be it HDD or SSD. Ceph's tools and code are designed with this strategy in mind; ignoring it may cost you exaggerated seek overhead, device wear, or operational issues including confounded CRUSH replication rules.
Similarly it is advised to deploy OSDs directly onto underlying physical devices, as opposed to hardware, embedded, or software RAID volumes. There was once interested in underlying RAID as a means to conserve RAM by limiting OSD count, or for specialized failure domains. However with modern hardware and the substantial refinements and improvements achieved by successive Ceph releases, ...
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