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

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!

Vern

On 3/30/2011 7:32 AM, Bryce Martin wrote:

> Jim Said....
> "Write cache has a massive impact on I/O performance. The more write
> cache the better. Ask anyone who has lost the batteries on the raid
> card what happens when write cache goes away. Ugly things happen."
>
> This got me thinking about RAID Card design....
>
> Why the heck isn't the cache batter a backup, not a primary power source?
> If the cache batter goes shouldn't the controller still have enough
> electrical input to keep its cache alive? This just seems like poor
> design, and a major oversight. Maybe I don't understand hardware design
> (that is probably the case), but I'm failing to see what the purpose is of
> the battery vs straight electrical input from the system? I would think
> you'd want a cache battery in the case of a hardware failure so you don't
> lose data, but I would think that it should be a backup source....
>
> Thanks
> Bryce Martin
> Programmer/Analyst I
> 570-546-4777