It's good to have multiple options if you want to host databases in the cluster.
For example you could use NFS for 90% of the storage needs for logging and sharing files between pods. Then use local storage, FCOE, or iSCSI-backed PVs for databases.
If you are doing bare hardware and your requirements for latency are not too stringent then not hosting databases in the cluster is also a good approach. Just used dedicated systems.
If you can get state out of the cluster then that makes things easier.
All of this depends on a huge number of other factors, of course.
For example you could use NFS for 90% of the storage needs for logging and sharing files between pods. Then use local storage, FCOE, or iSCSI-backed PVs for databases.
If you are doing bare hardware and your requirements for latency are not too stringent then not hosting databases in the cluster is also a good approach. Just used dedicated systems.
If you can get state out of the cluster then that makes things easier.
All of this depends on a huge number of other factors, of course.