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I have a server with 3 hard drives installed, and a total capacity of 6. We're planning to max it out, but our consultant also suggested getting a second RAID controller "for redundancy" to support the new drives. To me, this doesn't make much sense. Even with a second RAID controller running half of the disks, we're still stuck with only half of our disks/programs/data if one of the controllers dies (which isn't much better than running with none). We're putting vmware on the server and he vaguely mentioned some advanced fault tolerance/failover features, but if the disks are inaccessable due to a failed controller, how is it supposed to work?

Counting only reasons for redundancy, not performance, why would I want to have a second RAID controller in my server?

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up vote 8 down vote accepted

In a 'single box high availability' design then yes, you'd want a second controller, ideally on a second bus too. But this kind of approach has given way to a cheaper design based around clustering where one box failure doesn't stop service. So it depends on if you plan to use a clustered environment or rely on a single box. Even if your answer is the latter having dual controllers may be seen as adding extra complexity and maybe being overkill.

edit - based on your comment about using ESXi on your other question I'd have to say that its clustering is fabulous, we have many 32-way clusters that work brilliantly.

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AFAIK, we're not going to use clustering. How would a second controller in a single box benefit me? Is there such a thing as controller failover? – Bigbio2002 Aug 22 '11 at 20:07
Not in an ESX/ESXi world no - a single one would be fine, make sure you get a controller that will make one big R10 array of all 6 disks but allow you to create these 2TB (or less) logical disks ok. HP's Pxxx-series let you do that btw. – Chopper3 Aug 22 '11 at 20:20

A second RAID controller which is actively used is not for redundancy. Only if it is a cold-stand-by controller where you switch all your disks to when the first one dies. Then you have redundancy (for the controller). But beware of doing so, as posted here.

So the RAID is for redundancy of disks leading to a single point of failure at the controller. Having a second (unused) controller may solve this as you could switch all the disk to the new one. If this works depends on other factors...

I'm no native speaker, but for me "fault-tolerance" is something different than "redundancy". Can some English speaker help me out here?

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Redundancy is a way of achieving fault-tolerance :). I was looking for something along the lines of a cold-standby or a failover controller. Is this a feature that's supported, or would I have to manually swap out the cards? – Bigbio2002 Aug 22 '11 at 19:00
I have never seen a controller where the switching of disks is done automatically. This is either because I didn't looked for it or because I can't imagine how you should circuit the cables between one disk and two controllers. – mailq Aug 22 '11 at 23:57
Dual-ported drives are quite common in enterprise environments (think SAN shelves) - but the prices go up by a factor of 2 or 3, obviously. – adaptr Feb 16 '12 at 8:10

You'd need dual-ported SAS drives to provide actual failover on multiple controllers. While these do exist, it is decidedly uncheap - not in the price range of a single server that only has internal storage.

These are technologies often employed in SAN systems, where controller death is a real issue.

For a single server with no other failover capabilities, a second controller will not gain anything - it will just cost more money and provide the consultant with more profit.

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