Re: OT: Re: [exim] Reducing Disk IO - Speed Issues

On Thu, Aug 25, 2005 at 03:58:51PM +0100, Nigel Metheringham said:
> On Thu, 2005-08-25 at 16:54 +0200, V. T. Mueller wrote:
> > @Marc: a battery-backed write cache is an optional hardware feature
> > for the better controllers. This is somewhat paradox, because on
> > important high-IO systems - which is where you usually use rather
> > expensive RAID controllers - you rely completely upon UPS triggered
> > shutdown routines in case of a power failure as long as your
> > controller cache is not battery backed. It's quite tricky to restore
> > a database that had just lost a couple of megabytes of transactions...
>
> You still need to have write integrity down to the disks, since even
> with big expensive UPS systems you still find some clown who manages
> trigger the EPO (big red button that turns the power off - normally
> carefully mounted in places where they can be hit accidentally), or
> remove the power cable, or switch off the *wrong* machine.....
>
> Never underestimate the resourcefulness of an idiot in a data centre.

Battery backed write cache has NOTHING AT ALL to do with a UPS. The
battery is on the RAID controller itself.

When the power comes back up and the drives come back up, the RAID
controller flushes the the content of the battery backed cache to the
drives. Integrity is assured. Most higher-end SAN's will have the same
feature.