Ceph MDS stands for Metadata Server and is required only for a Ceph filesystem (CephFS) and other storage method blocks; object-based storage does not require MDS services. Ceph MDS operates as a daemon, which allows a client to mount a POSIX filesystem of any size. MDS does not serve any data directly to a client; data serving is done by OSD. MDS provides a shared coherent filesystem with a smart caching layer; hence, drastically reducing reads and writes. MDS extends its benefits towards dynamic subtree partitioning and single MDS for a piece of metadata. It is dynamic in nature; daemons can join and leave, and takeover of failed nodes is quick.

MDS is the only component of Ceph that is not production ready; current metadata servers are not currently scale only, and one MDS is supported as of now. A lot of Q&A work is going on to make it production ready; we can expect some news very soon.

MDS does not store local data, which is quite useful in some scenarios. If an MDS daemon dies, we can start it up again on any system that has cluster access. A metadata server's daemons are configured as active and passive. The primary MDS node becomes active, and the rest will go into standby. In the event of a primary MDS failure, the second node takes charge and is promoted to active. For even faster recovery, you can specify that a standby node should follow one of your active nodes, which will keep the same data in memory to prepopulate the cache.