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I’m torn between what I know (or think I do) and what Apple’s documentation states.

The hypothetical setup I’m considering is what is described as RAID 0+1 in Apple’s documentation for the Xserve RAID, which usually means a mirror of two striped sets comprising two identical drives. Like in this diagram: Minimal RAID 0+1 nested setup (Sorry, not allowed to post images yet)

Apple’s documentation along with the RAID Admin setup wizard, states that it can tolerate a single drive failure. Meaning, I assume, if two drives were to fail, there would be downtime, probably spent doing a full colonoscopy on the RAID controller, get new drives, get the backups out, etc, etc.

But isn’t this only in the worst case scenario that two corresponding drives from the two RAID 0 sets, e.g. Disk 0 and Disk 2 in the diagram, are fried? If Disk 0 and Disk 3 were to die simultaneously, shouldn't it be possible for the controller to rebuild the set? Hadn’t noticed this before.

Or does it mean that the diagram is an oversimplification? If you mirror a RAID 0 array with RAID 1, is there no guarantee that the individual drives would be mirrored?

I realize Apple’s enterprise days are long gone and people may not remember much, but if you have any insight on any other systems you’ve worked with, I’d love to hear your experiences.

kumowoon1025
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    Your link is broken. You could lose up to two drives as long as both failures occur in different groups. However, you would never let this happen because you would be monitoring for a single drive failure and replace that drive as soon as possible. You would do that, wouldn't you? – Appleoddity Aug 04 '17 at 05:15
  • I guess I should clarify, that your comment "striped on hardware, mirrored in software" concerns me. Usually RAID 0+1 is done together as a single volume, so to speak. Sounds like you would have two different RAIDs. A RAID of a RAID. Sounds really bad. It is possible that the configuration you speak of could only handle one drive failure. – Appleoddity Aug 04 '17 at 05:17
  • RAID 0+1 is not the same as RAID 10 (aka 1+0). The wikipedia page on [Nested RAID levels](https://en.wikipedia.org/wiki/Nested_RAID_levels) explains the difference between them. RAID 0+1 can only take a single drive failure because it's a mirror of two stripes. You lose a disk in a stripe, the stripe is gone. You lose a disk from each stripe, both stripes are gone. RAID 10 is a stripe of two mirrors. You can lose a single drive from each mirror and still have all of your data accessible. RAID 0+1 is stupid. –  Aug 04 '17 at 05:37
  • @Appleoddity Fixed the link. Sorry about the unclear question, I was going to ask two separate questions, and I was pretty tired when I asked this one. I am curious about the two different RAIDs scenario, but in this hypothetical situation, the whole thing would be handled by a single hardware controller. I’ve edited the question a bit to clarify (at least as much as I could). – kumowoon1025 Aug 04 '17 at 22:34
  • @Appleoddity And yes, of course. I have hot swap carriers I would put in even before drives fail, but multiple drives from the same batch in the same environment failing simultaneously isn’t unheard of. Which is why I also have off-site backups. Let me reiterate also that this is a hypothetical scenario, I am considering a different setup, and I went over the manual; the manufacturer states one drive failure can be tolerated at most, and I am not going to assume I know better, but I was curious about the why behind it. – kumowoon1025 Aug 04 '17 at 22:41
  • @Appleoddity [The RAID Admin tool](http://imgur.com/X7HwdHh) is what confused me, and looking at it again now, it sounds more like it is describing RAID 10??? What do you think? – kumowoon1025 Aug 04 '17 at 22:53
  • @yoonix I had a basic understanding of nested RAID levels, and I’ve edited the question to try to clarify my question. So the controller wouldn’t be able to rebuild the striped array if you lose Disk 0, from the mirrored striped array if you lose Disk 3? It just felt to me like it should be able to. – kumowoon1025 Aug 04 '17 at 22:58
  • Everything you feel it 'should' be able to do it can do.. if you use RAID 10 which uses the exact same amount of space, has the exact same overhead on the raid controllers (there's no parity to calculate). Using RAID 0+1 is literally telling it NOT to mirror individual drives. –  Aug 05 '17 at 02:25
  • @yoonix I understand that, and believe me, I wouldn’t even have looked into RAID 0+1 if RAID 10 was an option. However, for reasons beyond my comprehension, hardware RAID 10 is not available on the Xserve RAID. So you’re saying that Disk 0 and 2 are not identical in 0+1? (referring to the diagram) – kumowoon1025 Aug 05 '17 at 05:42
  • Virtually yes, but the layer of the arrays is built in such a way that the controller is looking at the two devices in the stripe as a single device. Yes, from a data standpoint they could be considered the same. The fact they are RAID 0+1 is an explicit instruction to the raid controller not to treat them as so. If RAID 10 in pure software is an option, I'd consider that. –  Aug 05 '17 at 06:34
  • @yoonix It’s not, frustratingly. Or at least not natively anyway, unless you count two separate software RAID sets nested, which just sounds insane to me, and I don’t even know if that’s supported (haven’t ever tried). So at this point I’m thinking I’ll stick to the striped RAID 5 setup that I have right now, but out of curiosity, are you saying pure software RAID 10 is better than software RAID 0 of hardware RAID 1 sets? – kumowoon1025 Aug 05 '17 at 09:27

1 Answers1

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A RAID 1 over two RAID 0s can have one RAID 0 subarray fail while staying operational, regardless of how many drives in the subarray fail.

Usually, it doesn't make sense to use this layout, a RAID 10 (RAID 0 over RAID 1 subarrays) is more reasonable.

Zac67
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  • I fully agree, but the hardware in question is ancient (the Xserve RAID was released over 10 years ago, and has long since been EOL) and if I wanted to go for RAID 10, the striping would have to be done in software, while the mirroring would be done in hardware, and it just feels icky. RAID 0+1 on the other hand is fully hardware supported. – kumowoon1025 Aug 04 '17 at 23:04
  • I’ve edited the question (and fixed the link) to hopefully make my question a bit clearer. Would you mind looking at it again for me please? – kumowoon1025 Aug 04 '17 at 23:06
  • In theory, a RAID 0+1 could be as resilient as a RAID 1+0. It all depends on how exactly the nested RAID is implemented. If there's insufficient documentation you're on your own or have to test it. In any way, make sure you activate data scrubbing/media patrol to avoid another drive turning out bad when rebuilding. – Zac67 Aug 05 '17 at 07:52