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

[Jim Oberholtzer <midrangel@xxxxxxxxxx>]

Bryce:

The battery is the back up power source, however IBM cannot know when a 
power event might happen requiring the raid card to maintain the cache 
while the primary power is out.  Since loosing the cache could be way 
worse than slowing you way down, they chose to force the issue when the 
battery is weak.  Even rechargeable batteries need replacing and that's 
what we are discussing.  When does the rechargeable battery need to be 
replaced?  Keep in mind the whole goal of the battery in the first place 
is to ensure there is no data loss when the power decides to go out, 
even momentarily.

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


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