Wednesday, October 12, 2011

Network Attached Storage and backup power

They say you should test your NAS in different ways. One of the aspect of such testing is to see how your NAS responds to a power failure. Most interesting cases are where NAS is combined with UPS unit. Just cutting off the power is not sufficient to check an UPS. In normal operation Network Attached Storage receives the messages from an UPS unit constantly and reacts at once.

However, NAS is unable to accept the notifications on power failure during the start up. Additionally, one should consider that the entire cycle (start up then shut down) may require a couple of minutes.

Let's imagine such a scenario:
  1. power failure occurs,
  2. your NAS unit works on battery for some time and then it shuts down,
  3. then power comes back and the Network Attached Storage begins to start up,
  4. at this moment power is off once again.
Now the most interesting thing - until NAS starts up it cannot find out that the power failure has occurred.
Once Network Attached Storage loads, it finds out that the power is off and sends a command to shutdown. But it may happen that the system will not be able to shut down correctly since UPS is not capable of supplying power long enough due to battery exhaustion (during the first cycle).

You can avoid such an issue by setting the NAS up in such a way that it shuts down as soon as it gets a message about power failure. All this allows saving battery charge to do one more cycle.

Alternative would be to set the Network Attached Storage up to restart manually, but not in automatic mode after the power failure has occurred.

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