On Thu, 2005-08-25 at 11:10 -0400, Walt Reed wrote:
> 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.
Yes. However vt appeared to be claiming the battery backing was
unnecessary due to the UPS, I was responding that there are faults the
UPS doesn't protect against.
> 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.
Except of course where there is a more interesting failure mode. The
best example I have seen recently is where the neutral got swapped with
one of the phases - OK for 1/3 of the customers, the others had a range
of interesting faults.
Nigel.
--
[ Nigel Metheringham Nigel.Metheringham@??? ]
[ - Comments in this message are my own and not ITO opinion/policy - ]
