Description
I had already added some comments to the code review at xapi-project/xen-api#2232, which still apply to the current design. I'll paraphrase:
When adding the is_fcoe_supported call into the metrics/monitor loop, the command would be run every 5 seconds, while I'd expect the FCoE capabilities of a NIC to not change that often, or even never. I assume that the field means that the field refers to the inherent capabilities of the NIC, rather than whether FCoE is currently "on" (it is not a status field).
Therefore, it would be sufficient to handle fcoe_supported in the same way as we handle the MAC address of a PIF: we set the field when the PIF is created, and update it each time xapi starts (in case the hardware was changed/replaced when xapi wasn't running). Code for the former can go around here: https://github.com/xapi-project/xen-api/blob/master/ocaml/xapi/xapi_pif.ml#L327 for PIF.introduce/scan; the latter here: https://github.com/xapi-project/xen-api/blob/master/ocaml/xapi/xapi_pif.ml#L69 which is called by the dbsync startup code. There is no need to do anything on PIF.plug (Nm.bring_pif_up).
The above also implies that the field should be on the PIF class directly, rather than the PIF_metrics class. Perhaps this deserves a more general PIF.capabilities field where we can put various (offload) capabilities of the NIC.