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

[Vern Hamberg <vhamberg@xxxxxxxxxxx>]

At last! Good to be right once a year, anyhow!

:-)

On 3/30/2011 8:06 AM, Jim Oberholtzer wrote:
> Vern,
>
> Your exactly correct.
>
> Jim Oberholtzer
> CEO/Chief Technical Architect
> Agile Technology Architects, LLC
>
>
> On 3/30/2011 7:49 AM, Vern Hamberg 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!
> >
> > 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