On 3/24/06, Per Jessen
All,
probably a little OT here, but I _am_ using SUSE Linux :-)
I'm trying to work out the optimal (or near-optimal at least) RAID configuration of a 24-disk array. The array comes with two redundant RAID controllers each with six SCSI channels. These controllers allow all kinds of RAID0/1/5 configurations, but no RAID6.
I want the array to be able to survive a two disk failure, so RAID6 would be the obvious choice, but ...
So I'm sort of looking at choosing between -
- using plain software RAID6 and ignoring the hardware RAID facilities of the array. - using the hardware controllers to build a combination of RAID0/1/5 that'll give me the two drive failure survivability.
I've had a look around the web, but googling for "two drive failure RAID" almost always leads to someone talking about RAID6 ...
So, opinions/suggestions?
/Per Jessen, Zürich
Per, I did not read all of the responses, but somewhere along the lines you said it was a HP Storage Works box. And some of the responses did not seem to understand raid all that well. I'm certified to design Storage Works raid setups, so if you post details such as the array model number (HSZ80? HSG80?), # of controllers, # of shelves, # of disks per shelf I can give you more feedback. HP rarely recommends raid 5 unless capacity/cost are your driving factors. Normally with Storage Works boxes HP recommends a RAID 10 for reliability and speed. So for you that would be 11 2-drive mirror sets all striped together and 2 hot spares. That can survive 11 failures with one failure per mirror set. As you said somewhere, if you have 2 failures in the same mirror set, your dead. You said somewhere the more drives the more likelihood of a dual drive failure. True, you should always have 2 drives per mirror set so the odds are always the same for a given mirror set. Adding drives simply increases the number of mirror sets you are striping againt and causes a small increase in risk. risk = 11 * odds of a 2-drive mirrorset failure. Storage capacity 11 * one drives capacity. The next step in higher reliability is to move to 3-disk mirror sets. ie. each of the 3 disks have exactly the same info. This is still called RAID 10, but now you will have 7 3-drive mirror sets all striped together with 3 hot spare drives. You can obviously survive a 2 drive failure. risk = 7 * odds of a 3-drive mirrorset failure. Storage capacity 7 * one drives capacity. The next issue you have is a shelf failure. Although rare they do happen. To address this simply ensure that your mirrored drives are always on different shelves. That way even if you have 6 drives on a shelf and the shelf fails you've only lost half of 6 mirror sets. That should be fairly straight-forward to accomplish. Also make sure your hot spares are on different shelves. And for the truly paranoid HP recommends software RAID 1 between 2 different storage works boxes. Not very many people go to this extreme. HTH Greg -- Greg Freemyer The Norcross Group Forensics for the 21st Century