The Ceph Filesystem service includes the Ceph Metadata Server (MDS) deployed with the Ceph Storage cluster. The purpose of the MDS is to store all the filesystem metadata (directories, file ownership, access modes, etc) in high-availability Ceph Metadata Servers where the metadata resides in memory. The reason for the MDS (a daemon called ceph-mds) is that simple filesystem operations like listing a directory or changing a directory (ls, cd) would tax the Ceph OSD Daemons unnecessarily. So separating the metadata from the data means that the Ceph Filesystem can provide high performance services without taxing the Ceph Storage Cluster.
Ceph FS separates the metadata from the data, storing the metadata in the MDS, and storing the file data in one or more objects in the Ceph Storage Cluster. The Ceph filesystem aims for POSIX compatibility. ceph-mds can run as a single process, or it can be distributed out to multiple physical machines, either for high availability or for scalability.
- High Availability: The extra ceph-mds instances can be standby, ready to take over the duties of any failed ceph-mds that was active. This is easy because all the data, including the journal, is stored on RADOS. The transition is triggered automatically by ceph-mon.
- Scalability: Multiple ceph-mds instances can be active, and they will split the directory tree into subtrees (and shards of a single busy directory), effectively balancing the load amongst all active servers.
Combinations of standby and active etc are possible, for example running 3 active ceph-mds instances for scaling, and one standby instance for high availability.
Above is from Ceph doc, need more details on multiple active MDS.
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