Ceph OSDs: A Ceph OSD Daemon (Ceph OSD) stores data, handles data replication, recovery, backfilling, rebalancing, and provides some monitoring information to Ceph Monitors by checking other Ceph OSD Daemons for a heartbeat. A Ceph Storage Cluster requires at least two Ceph OSD Daemons to achieve an active+clean state when the cluster makes two copies of your data (Ceph makes 3 copies by default, but you can adjust it).
Monitors: A Ceph Monitor maintains maps of the cluster state, including the monitor map, the OSD map, the Placement Group (PG) map, and the CRUSH map. Ceph maintains a history (called an “epoch”) of each state change in the Ceph Monitors, Ceph OSD Daemons, and PGs.
MDSs: A Ceph Metadata Server (MDS) stores metadata on behalf of the Ceph Filesystem (i.e., Ceph Block Devices and Ceph Object Storage do not use MDS). Ceph Metadata Servers make it feasible for POSIX file system users to execute basic commands like ls, find, etc. without placing an enormous burden on the Ceph Storage Cluster.
OSD
- OSD는 실제 데이터를 저장하고, 복제, 복구, backfilling, 부하분산 역활을 한다.
ceph는 기본적으로 3개의 복사본을 만들게 되어있지만 수정가능하다.
Monitors
- 모니터는 클러스터의 상태를 체크하고, PG(Placement Group) map, OSD map 관리 및 Ceph state history를 저장하고 관리한다.
MDS
- Ceph Metadata Server는 일반 사용자가 Ceph 데이터를 검색 및 체크(기본 명령어-ls, find,..) 하기 위해 metadata들을 저장하는 서버로 OSD에 부하를 줄이기 위해 사용한다.
(Ceph Block Device와 Object Storage는 MDS를 사용하지 않는다.)
