as an example of this, I am currently evaluating sparc system that has
N+N+N redundancy (with voting), this system will cost ~$80K compared to a
single system at <$10K. for some of our firewalls it may well be worth it.
David Lang
On Sun, 4 Jun 2000, James Sutherland wrote:
> Date: Sun, 4 Jun 2000 20:53:37 +0100 (BST)
> From: James Sutherland <jas88@cam.ac.uk>
> To: Bruce Guenter <bruceg@em.ca>
> Cc: linux-kernel@vger.rutgers.edu
> Subject: Re: Hot pluggable CPUs ( was Linux 2.5 / 2.6 TODO (preliminary) )
>
> On Sun, 4 Jun 2000, Bruce Guenter wrote:
>
> > On Sat, Jun 03, 2000 at 08:44:37PM +0100, James Sutherland wrote:
> > > > So, you've essentially got two complete systems (once you add up all the
> > > > components) in a single box.
> > >
> > > No. I have the same components, but organised to make one single machine
> > > with N+N redundancy, rather than a pair of independent machines with no
> > > redundancy at all.
> > >
> > > > What does this buy you above having two completely independant boxes?
> > >
> > > Redundancy. Your approach gives you two machines, each with, say, 99.99%
> > > availability. Mine gives a single machine with, perhaps, 99.9999%. Two
> > > machines without redundancy have much lower availability.
> >
> > I was referring to N machines with network-level redundancy instead of a
> > lower-level redundancy (either shared memory or shared bus interconnect).
>
> I know. The point is, there are applications where that just isn't good
> enough. You need the one machine to work all the time - switching to
> another machine just isn't an option.
>
> For "normal" network tasks, this will usually suffice; HTTP, FTP, SMTP
> etc. failover is adequate. If the machine is doing something important,
> though, it just isn't possible to failover. Think mission critical control
> systems - "hang on reactor, we'll have a spare online in another 30 nanos-
> BOOM. Too late."
>
> More to the point, if you have a pair of identical systems with no
> redundancy, supposing the system HDD fails in one (killing it), then a CPU
> fails in the second (killing it)? A fully redundant machine is fine with
> that: it still has a working system drive, and a working CPU. Your
> machines, OTOH, are now both offline.
>
> > > > I wouldn't be surprised if a single box with all the redundant
> > > > components costs more than the total price of two seperate boxes.
> > > Yes - you are paying through the nose for the extra 9s of availability.
> > > There are markets where the client is more than happy to do so; in mission
> > > critical apps, double the price for an extra 9 is a bargain.
> >
> > Only double the price? That would indeed be a bargain.
>
> Even at greater differentials. However, double the price for N+N
> redundancy gives much better availability than two non-redundant machines.
> As soon as any single component fails in one node, EVERY component of the
> other machine becamse a single point of failure: with my N+N system, two
> of the same component have to fail at once to take me offline. With your
> system, ANY two components failing will take you offline.
>
>
> James.
>
>
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