Is the battery redundant if the server is on a properly-sized UPS? There is an option to force write-back mode and I want to understand the risk. It is my understanding that it helps against power failures while data in the cache has not been written out to physical disk. But if the server is on a UPS, then there is no risk of power loss, especially if it is a smart UPS and the server is configured to shut down gracefully when the battery reaches a low state. Am I missing something?
What purpose does the battery serve on a battery-backed caching RAID controller if the server is on an uninterruptible power supply
backupbatterycacheraidups
Best Answer
Your understanding is essentially correct.
Allowing the controller to cache write data in its local RAM improves write speed, however it exposes you to a risk of having the OS think data has been written to disk (because the controller has it) and having power fail before the controller completes that write. The battery back-up allows the data in the cache to survive until the machine is powered on again, mitigating that risk.
As far as benefit if your system is already on a UPS, what happens if you exceed your UPS runtime (possible even with a smart UPS), or if the server's power supplies blow up? Or if someone accidentally yanks the power cords out?
Many thing can cause a power failure. A UPS only protects against a few of them.