RAID 5: Two (2) drives on One (1) controller, What happens if a controller fails?

[bobdazzla]

Dearest Raid Gentlemen:

Putting together my first raid system, will be doing it through software in linux.

I have a handful of promise ide controllers that have 2 IDE channels per card. I know that running two drives on a cable (in a master/slave config) is a definate no go, because if 1 drive failed it would probably take out the other drive on the cable (leading to corrpution).

With that understood, my question then: is it safe to run 2 drives, 1 per ide channel, on 1 controller?

I'm guessing I'd be safe if 1 disk died (it would take out that 1 ide channel, but the other ide channel on the card would keep running), but what would happen if the controller died? Both drives would be taken offline, leading the array to break.

After replacing the failed controller, would there be so much corrpution on each of the two drives that the array would be useless? Or after replacing the failed controller, would the array be fine? Or does it really depend on other circumstances (were the drives in the middle of a write, did the controller send corrupted signals to the drives right before it died, etc.)?

I ask because I have 7 drives total, and not enough open PCI slots to provide each and every drive with it's own individual IDE controller. If the answer is "no, it is not safe to have 2 drives on 1 controller in a raid setup", then i guess i'll have to make due with 6 drives, not 7.

Thanks in advance

 

[RadiclDreamer]

You can usually replace the controller with the same make/model and still be good

 

[bobdazzla]

I think what I'm really asking is whether or not a failed controller leads to corruption on the drives that were attached to it.

If it doesn't, cool, I can run 2 cards on a controller and if it fails, just swap it out. But if a failed controller DOES cause corruption on all drives attached, this corruption might prevent my raid 5 from booting, since the damage was on more than 1 drive (and raid 5 can't handle a failure on more than 1 drive).

ps - ignore that last sentence of my original post with the "controller that offers more than 2 ide channels"... obviously that wouldn't help, wasn't thinking.

 

[EatSpam]

If the controller were to fail, you might lose both drives attached to the controller and potentially the entire RAID 5. Definately a risk. However, controllers are solid state devices and generally don't fail, so its not really a bad risk.

 

[jose]

You need 3 drives to do raid 5 ..

http://www.acnc.com/04_01_05.html

BTW, If your going to do raid 5 get a LSI controller.
Most of the onboard raid controllers are software based anyway.

Regards,
Jose

 

[bobdazzla]

Thank you for the input spam, I think you are right, and that a controller failure is very unlikely. Even if it did happen and DID lead to corruption that would cause the array to fail, I can't imagine that it would cause a super-catastrophic corruption of the filesystem or anything, so if I absolutely had to I could run a "forced recovery" using mdadm (the raid management tool I will be using, more information on it here: http://en.tldp.org/HOWTO/Software-RAID-HOWTO-8.html#ss8.1).

As I understand it, promise cards aren't that flaky, so I'm gonna try 2-drives-on-1-controller and hope for the best. Thanks again.

 

[EatSpam]

Linux software RAID is an excellent choice. I use it on my 16 drive storage server. Very very stable and quick, too. Fastest RAID 5 writes shy of a good hardware controller - and faster than an LSI, jose.

BTW, you cannot boot from Linux software RAID 5. Software RAID 1 works fine. For RAID 5 booting, you'd need a hardware or fakeraid controller ....