Messages in this thread | | | Date | Mon, 26 Jul 1999 23:23:26 -0500 | From | Walter Lundby <> | Subject | Re: High-availability question |
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Steve Underwood wrote:
> "Stephen C. Tweedie" wrote: > > > > The data corruption caused by a not correctly plugged in SCSI cable > > > resulted in both mirror sets being corrupted and the corrupted data > > > also being mirrored over to the other system. > > > > Sure --- garbage in, garbage out. You still aren't proof against an > > application failure or against writing the wrong data. That doesn't > > mean that having a redundant storage subsystem is useless: it just means > > that such redundancy is only part of the problem. > > > > btw, Tandem would have caught that scsi error: the existence of such > > fault tolerant systems shows that it _is_ possible to guard against > > random machine failures. You are still in trouble if the application is > > buggy, of course. > > You don't need a Tandem to catch this. You just need to turn on the SCSI > parity. For some reason the majority of systems seem to have it turned off. > On some older systems parity didn't work properly, but I don't think that is > an issue today. Still, most systems I look at have parity turned off. > > Stratuses die. Tandems die. They die less often, but the consequences are > worse. In my experience a Stratus will have more than one reboot a year, due > to various troubles. A just reboot takes over an hour, so one reboot per year > brings availability down to 99.99%. Tandems have a pretty slow restart too. > The place were Tandems seem to score well is data integrity - banks don't > loose many transactions through them, even when they do down. > > Steve > >
Don't brag about the Tandems too much. With the Helix series they disable the CRC check in the drive and do it in the io-controller and then modify non-stop unix to handle it. This makes for very slow disk-io and very expensive drives. We have had a very hard time getting Tandems to 5 9s which cost dearly in payments to customers. Oh well...
A fault tolerant Corba TAU orb is one of the solutions considered for future replacement of standard fault tolerant systems such as Tandem.
If anybody has a real good and speedy 5 9's solution I am all ears...
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