There are fundamental differences in approach between Ceph and Gluster. Ceph is at base an object-store system, called RADOS, with a set of gateway APIs that present the data in block, file, and object modes. The topology of a Ceph cluster is designed around replication and information distribution, which are intrinsic and provide data integrity.
Red Hat describes Gluster as a scale-out NAS and object store. It uses a hashing algorithm to place data within the storage pool, much as Ceph does. This is the key to scaling in both cases. The hashing algorithm is exported to all the servers, allowing them to figure out where a particular data item should be kept. As a result, data can be replicated easily, and the lack of central metadata files means that there is no bottleneck in accessing, as might occur with Hadoop.