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

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On 03/30/2011 08:32 AM, Bryce Martin wrote:

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....

The cache battery *is* a backup, not a primary source.

The reason that performance goes to pot when the cache battery dies is
because the system now knows that there's no backup to that memory. If
system power dies then the data contained in the cache is lost. So it
disables write cacheing to get data to permanent storage as soon as
possible.

The performance hit is due to that behavior. It's a design choice made
to protect the data.

Barry

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