RPC
~~~

The most common activity in the Lustre protocol is for a client to
initiate a Remote Procedure Call (RPC) directed at a specific
target. There are two additional activities supported by the Lustre
protocol. A server may initiate an RPC to the target on another
server, such as an MDS RPC to the MGS for configuration data, or an
RPC to an OST to update the MDS's state with available space
data. Finally, in a "call-back" RPC the service side (where the target
resides) initiates the RPC, which may be directed at a client or
another server.