You know those great pictorial articles you see in motoring magazines, the ones with gorgeous photography and prose that serves only as dressing on the full-page, glossy photos? Well, this shall serve to illustrate RAID on the Mac the way those illustrate Ducatis, drift machines, and other vehicles we might not get a chance to try. Similarly, you probably don't want to go down this road with your storage.
From the first image, you should notice a few things. Anyone born before 1981 should recognize that Arnold is the drive we'll be using, and DeVito is the "leftover shit." The reason for this, as careful readers will see, is that LaCie has sold us two different drives as "250GB" drives. Note that the size of these volumes is either 232.9 or 233.8 gigs. At any rate, so I went and created 224gb partitions on these drives, and added them to a raid5 volume group. The original intent was to have four drives and one spare, in a striped set (rather than mirrored or concatenated set). I realize this is suboptimal (two spares being a better choice, with five primaries), but I had the five drives, not seven. Note also that I have a 640gb drive as "scratch," so I can make redundant stuff that I want redundant.
What's really cute here is that it takes a line to say "and 1 other" rather than adding another (meaningless) line of "Arnold." Personally, if I were the programmer, I'd have listed disk5s2, disk4s2, disk3s2, and so on, but my guess here is that Mac users are more likely to associate a "name" with a disk than a device number. This is not the case on Solaris, Linux, or any of the Unixish operating systems I've used (although I've never used HFS on any of them).
Creation of the RAID went swimmingly with the striping – it would add four disks to the stripe. However, when it tried to add a spare, it would fall over with the above message. This looks suspiciously like a command-line error that was simply spat to a dialogue. In any case, it doesn't do the important thing: tell me what the hell happened, how, when, and where.
Finally, deciding that with five drives, I could attain a stripe with parity, and reconstruct a drive from a new drive, I just built a RAID-5 with the five drives:
This may look pretty, but it's actually tame as far as RAID goes. Using a Sun 711-12 and twelve Seagate Cheetah 32gb drives, I had ten disks in a R5 stripe, with two hot spares. I got 220gb or so out of it, but I knew if anything went south, all I'd have to do is get a new 32GB drive off ebay, and run on a spare in the meantime. I really liked DiskSuite, and I wish there were a similar package for the Mac. Even Linux's LVM isn't so bad, once you get used to it. I've never liked the lvcreate/pvcreate dichotomy, but not having any control over that at all makes me very sad.
I've now got about 700gb of stuff on a 1.1tb raid which only has redundancy in its stripe. And, its inability to give me a hot spare means I can't even just buy a couple 250's and hang them off the end of the stripe. Lastly, for bonus credit, can anyone answer this question:
Can I attach the first disk in a FW800 bus to the last disk, forming a ring, with the host somewhere in the middle?
Of course, this makes your bus look a lot like a ring, or a SAN. I don't think FW800 is sophisticated enough to really understand this, even though the cables are tres beefy and shielded. They could probably have run a couple gigabit across them if they'd really wanted to. But nobody wants to dual-path drives on a bus, and nobody really needs more than 800 meg of bandwidth on their storage
Note also I made a raid of the leftover shit – the "DeVitos" – and got something like 32gb out of them, in a 5-disk stripe. It's kinda like an iPod, hanging there off the bus. And, for those still reading, the name of the final 1tb raid (we've had sort of a theme going) is Wilson, named after Greg House's "friend" (you know, he's sort of a backup, but occasionally, he'll get a conscience and do something completely witless that you didn't really expect (but really did, because he's an unreliable pansy)). And you can't have a raid named "wilson" without another one named "hanks" (commence howling wiiiiiiiilsooooooooooooon), although it's strange to have one be like 3% of the size of the other.