Xserve RAID: About Using Uninterruptible Power Supplies (UPS)

Uninterruptible power supplies (UPS) are designed to prevent blackouts, brownouts, sags, and surges from affecting your server, data, or other electronic equipment.

You should use a UPS in every configuration of Xserve RAID. In case of power interruption or power supply failure, each power supply can maintain the functionality of Xserve RAID. In many cases, a UPS will be used on one power supply and the other will be connected to an isolated ground outlet, which is not on the same power leg as the UPS. Ideally, the two power supplies should be connected to two separate power sources, and both power sources should have a UPS system attached.

UPS monitoring is performed by Xserve RAID through a serial port connection on the RAID controller. Each controller has its own port so each Xserve RAID can monitor two different UPS sources. When Xserve RAID detects that input power has been switched to UPS power, the system automatically changes the cache mode from high-throughput write-back to safer write-through. Overall system throughput is reduced, but transactions are protected in the event that the UPS fails. If you use an APC UPS, Xserve RAID notifies you when the UPS signals that it is critical (that is, about 20 percent power remains).

Xserve RAID works with optional cache backup batteries, which can maintain the current transaction data in the cache for over 72 hours. In this case, even if the UPS has less than 20 percent power remaining, you can still choose to not shut down the Xserve RAID, because the last transactions will be maintained in the cache in case of UPS failure.

Once power is restored, Xserve RAID will write the transactions and begin charging the batteries. If battery power is low, Xserve RAID will stay in write-through cache mode until the battery charge is over 50 percent before it will switch back into write-back mode.

Warning: Without a UPS connected to at least one of the power supplies, the cache backup battery modules will back up the controller cache, but you could experience data loss if you do not set the drive write cache to off.