The SPS itself is a UPS actually. The Storage Processor Enclosure and Vault DAE both plug in to the SPS for power, and the SPS plugs into the datacenter power/mains. If power is lost to the array, all DAEs EXCEPT the Vault DAE will lose power and all LUNs will go offline. However, the SPS powers the Processor Enclosure and Vault DAE long enough to save the cache to the vault and power off cleanly.  When power is restored to the system, the processor will read the cache data from the vault upon boot and commit that data to disk. Attaching a UPS to the SPS will just prolong the time until the SPS notices the power outage and starts the vault process. It’s unnecessary.  Richard J Anderson
The UPS protection that normally protects the PDUs in our datacenter has been taken offline for emergency maintenance. Our CX4-240, servers, switches, etc., are all now subject to outages in mains power for the next several weeks. This array services a few VMs as well as containing all of the secondary  MirrorView/A mirrors, which receive replicated data from another CX4-240 in  another datacenter; I'm not sure whether our use of MV factors into  this.  We thought it might be wise -- and this is just a guess -- to install our own small UPSs, just to avoid mains power suddenly dropping to the SPSs, but we don't have enough to fully protect the whole array (all of the DAEs), just enough to plug each SPS into the UPS.  If we were to do this, and mains power were to drop, all of the servers, FC switches, and DAEs would turn off, while the SPSs would continue running. We'd have time to flip the switches on the SPSs and have the SPs dump their cache contents to the vault drives. We're not sure if this "buys" us anything, compared to simply having the mains power simultaneously drop for everything. We're also thinking this could create an unexpected (and destructive?) failure mode: the DAEs turn off, but the SPSs retain mains power.  Would this failure mode actually be worse than just having power drop to everything, including the SPSs? Are we better off having either all or none of our equipment on UPSs, as opposed to having only the SPSs protected by UPSs?  What's the worst that could happen if we leave everything as-is, and mains power drops to everything simultaneously...for either just a moment or an extended outage?
Is it better to UPS-protect only the SPSs, or nothing at all?