Raid Utility. Write cache disable due to insufficient battery charge

Question

Is it the baterry? It gaves me the message every day at star.

Answer 1

The write cache is a performance-enhancing technique that improves write performance.
Ordinarily (without the cache), the host system sends data to the RAID to be stored. The RAID controller works out where the data should be written (based on the RAID parameters of the array), spins the disks, writes the data, verifies the write and then signals back to the host that the write was performed succesfully.
Sounds like a lot - and it is - but it all happens in a few milliseconds so you don't really notice too much - especially at low load.
At higher loads, though, it may take longer to queue the incoming requests - work out where to write the data, spin the disks, wait for other data to be written, etc.
That's where the cache comes in. With the write cache enabled, the host sends data to be written to the array. The controller stores this data in cache and immediately sends a confirmation back to the host. This lets the host get on with other tasks (possibly including sending more data to write), without waiting for the (slower) disks.
Writing to cache, since it's really just RAM, can be orders of magnitude faster than writing to disk, so the host gets a confirmation quicker, and it's not stuck waiting for other tasks to write to the disks, too.
There is a risk, though - if the power goes out, for example, there's a risk that any data in the (volatile) cache doesn't get written to disk in time and data loss/corruption occurs. That's where the battery comes in. It ensures the RAID can maintain that cache data (for up to 72 hours), so that it can be written to disk when the power is restored.
If the XServe RAID can't rely on the battery it disables the write cache to avoid the potential data loss.
As for whether you'll notice, that depends on your use pattern. Small writes won't have much of an impact, and it doesn't significantly affect read performance, so it's up to you.
Personally, I'd look to fix the issue, but that's me.