redundancy of SCSI controllers possible?

[Carlos Carvalho]

Sorry for the ugly typo on the subject :-(

Torbjorn Lindgren (tl at fairplay.no) wrote on 23 March 1999 13:10:
 >On Mon, 22 Mar 1999, Carlos Carvalho wrote:
 >> All this talk about raid reminded me of a rarely discussed topic.
 >> Suppose you split a raid array in more than one controller, as is
 >> usual for speed reasons, and there's more than one disk on a given
 >> controller. This controller is a single point of failure, since if it
 >> fails the raid is gone.
 >
 >Single point of failure to take the box down, yes. The data is still
 >there, so it's different from a multi-disk failure.

I'm talking about writing, not reading. If you have two disks on each
controller, if one controller fails those disks won't be updated, but
the others will. So the info is only partially written, and I'm not
sure the data is there. Or is the raid code smart enough to detect
this? If so, controller failure isn't a problem at all.

 >You have to decide what you aim is, and how much you are prepared to pay
 >for it. What if the PCI bus fries? What if one CPU fries? Etc etc...

These kinds of failure don't break the synchronization of the raid.

 >> Is it possible to put two controllers on the same bus, and if one of
 >> them fails the other keeps with all disks?
 >
 >IF the aim is to make sure that the box can stay up with single controller
 >failure I would make sure I have enough controllers to never put more than
 >one disk per RAID set on each controller. 

This isn't possible, there aren't enough PCI slots.

 >For large configurations this might be doable without large costs, by
 >using multiple RAID sets (multiple disk on each controller, each one from
 >a different RAID set).

You want even more disks than Doug :-)

 >This should work as long as the failing controller doesn't do something
 >really bad (like blocking the PCI bus until it's removed!)...

Again, this doesn't break the raid.


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