The external HSM coordinator allows to realize environments in which HSM requests are generated by means other than lfs; these could be policy engines, job schedulers, web GUIs, etc. Such entities may not operate on Lustre FIDs, but on pathnames, instead. The current HSM copytool API accepts FIDs as parameters, so it might not be an ideal match for such deployments. Furthermore, it requires that copytools register with the internal HSM coordinator, so that the coordinator can send HSM requests to the copytools; however, the scheduling of HSM requests can be performed outside of Lustre when using an external HSM coordinator, so copytool registration is not needed. The current API also accepts opaque copytool data parameters, which could be considered a more complicated implementation than what is necessary.

Provide a new HSM copytool API that is more adequate for use in such environments and is simpler to use.