We have a PowerEdge 2420 24U rack for our VM environment of two Dell R710 servers, Dell PowerConnect 5424 switches, and Dell/EMC AX-4 SAN. Everything is connected to a single 16 amp PDU which in turn is connected to a single 3kVA UPS. As the servers and SAN have dual power supplies, we'd like to split the PSUs across two (smaller) UPSs.
Is there any reason why we can't just connect any device straight to the UPSs? I'm not clear what the PDU benefit is given a UPS's features?
Of course, with the above in place, we would endeavour to have each UPS on a separate AC circuit.
It's very much up to you as to how you design your power distribution. A PDU can give you benefits depending on which type of PDU you have. A "basic" PDU doesn't add a lot of value other than splitting one large electrical feed into other smaller ones (and providing you with the right connections such as C13 outlets). Typically a UPS has a number of outlets on the rear so if the UPS has enough connections of the correct type then you don't need a PDU. However PDUs can add value where you use "Monitored", "Switched" or "Managed" technologies which allow remote monitoring of power draw (sometimes per circuit), remote switching over an IP network and remote alarming for overload conditions.
Also note that the Dell UPS has "load segments" which allow you to individually control the on/off profile of banks of outlets. If you connect all your servers to a PDU on one load segment you remove the ability to use this feature. Typically you would use load segments for bringing down less essential equipment first, and keeping more essential equipment up and running for longer. You can also ensure that servers shut down before storage arrays to avoid data corruption. Conversely you can also start up equipment preferrentially, e.g. storage & switches before servers.
Another point to note is that if you use UPSs in redundant configurations, be sure the s/w can handle this easily. The Dell UPS s/w handles this situation well with no additional license fee required. You don't want one server running out of batteries and sending a shutdown command to the server, when the UPS on the B side still has battery power remaining.
Connecting a UPS to both A & B feeds is certainly the best way to create redundancy in your system at a reasonable cost point.