Re: Cache battery and Raid Controller Design WAS: Re: Rép. : Question of a Newbie to POWER Systems

True the DASD units will stop spinning. The problem comes with the data in the cache at that moment. It has not been committed to physical disk yet. If the cache was lost at that point, then the data in the cache is lost and the system is corrupted. In a single level storage system, that's potentially very bad news requiring a reload of the system. So the smart thing to do is back up the cache with battery until power is restored, and then commit the cache to disk.

If you had warning, meaning the power goes out, and you have a UPS on the system, you can with the proper knowledge force the cache to disk, clearing it. Now everything is on physical storage and loosing cache is not a big deal. That's basically what happens when the CE replaces the battery. He/She will force a clearing of the cache to disk and then replace the battery. It's an easy process although it has to be done properly.

Jim Oberholtzer
CEO/Chief Technical Architect
Agile Technology Architects, LLC


On 3/30/2011 9:30 AM, sjl wrote:

But if the system loses power, will not also will the disk drives attached
to that RAID controller lose power? At that point, it seems irrelevant to
me to have a battery backup on the RAID controller to keep the data cached
without a way of also powering the disk to allow the data to be written to
storage...

- sjl


Vern wrote:
Bryce

Interesting question - seems to me that the cache battery is a backup -
it preserves anything in the cache in the event of a power outage. The
system could and probably does use the regular power of the system
during normal operations.

The matter of slowing down when the cache battery's life is running out
- I think this happens for a different reason - if the battery can't
keep data in the cache, then you can't rely on the cache. So writes have
to be done synchronously, not through the cache, because the backup is
not trustworthy. This slows it down.

I've not thought of this at all, so my ideas can be far off base!