I have got two servers with SFP+ capable network cards connected to the Unifi Switch uplink ports. Each of the servers has its own UPS and the switch shares the UPS with one of the servers. The servers are connected using direct attach cables (passive, copper).
I have been experiencing a loss of connectivity after restoration of power between the switch and the server that is on the other UPS (I've tried to swap the switch to another server to verify the behaviour and it indeed reversed) until I physically unplug and re-plug the direct attach cable.
Am I right in thinking that there must be some electrical/ground imbalance due to the systems being on different UPS circuits? I am thinking about getting active direct-attach cables that use optical medium, any other ideas how to resolve this problem?
You might be experiencing a very bad ground loop problem. Unlike UTP copper (and fiber of course), STP and DAC links can be troubled by compensation current.
The problem may be solved by
the groundings on your power sockets may need to be checked and fixed
in case of symmetric UPS power plugs (e.g. Schuko/CEE 7) and especially with both UPS fed from different power phases, try replugging either one or both plugs turned by 180°
Success (or the lack of) may be be verified by measuring AC voltage between each pair of devices (their cases), preferrably with SFP+ DACs unplugged (so you see the ground buzz without compensation).