What's the best way to RAID my Mac Pro?
May 11, 2010 11:05 AM   Subscribe

I'm looking for answers and advice about configuring an internal RAID array in a Mac Pro.

I don't have a lot of real-world experience with RAID, but my work computer was spec'd out with four 640GB hard drives and a hardware RAID controller.

It's total overkill for my work (web development), but I suppose that is a good problem to have.

I'm not sure which configuration is best for me, but I do know the current setup (1 boot JBOD drive, and 3-disk RAID 5, which I'm not responsible for) is not ideal.

I could go for performance and set all 4 drives as a RAID 0+1.
The downside to this is I'd have to find another backup solution. Also, since I'm in web, I don't usually work with huge files... will I even gain that much?

I'm leaning toward going for redundancy and backup by splitting the drives into two mirrored sets of two: one for use and one for Time Machine. Then I'll have instant failover if one drive in either set fails and also will have a local backup.

My question about this approach is: could I get a fifth drive to occasionally cold swap with one of the boot drives so I could have one semi-updated drive kept offsite? Will the RAID be able to handle that sort of regular intentional degradation and efficiently diff the out-of-sync drive?

Also, just a general question: does the Apple RAID controller do anything to the drives that would make them unusable in a different system without a RAID?
posted by Mr. Anthropomorphism to Computers & Internet (8 answers total) 2 users marked this as a favorite
Best answer: do know the current setup (1 boot JBOD drive, and 3-disk RAID 5, which I'm not responsible for) is not ideal.

Why do you think so?

Generally the reason you use a hardware RAID card, instead of software RAID, is so that you can use a parity-based RAID level like RAID 5. The usual advice to use 0+1 instead of 5 is because most people don't already own a hardware accelerator, and in the past few years it's been much cheaper to purchase more drives and offset the capacity hit that 0+1 requires (and use software RAID) than buy a real RAID card. But you already own the card ... IMO that changes the equation.

I realize that "RAID 5 versus RAID 10" is sort of a holy-war topic but if you already have the system set up with RAID 5, absent some compelling reason to change it, I wouldn't.

As for your Time Machine backup idea, while I guess you could do this, given the price of external hard drives I'm not sure I would. I think it makes more sense to buy one (or preferably two!) external hard drives and use them as TM volumes, periodically swapping the two on and off-site.

While having a TM volume inside your computer would protect you against accidentally deleting a file, it wouldn't protect you against theft or destruction of the machine, which you should consider as a fairly common scenario. Also, because it would be using drives of the exact same type as your main disks, exposed to the exact same environment, I'd be pretty nervous about the possibilities of simultaneous failure.

My personal recommendation for backups is a top-loading SATA drive enclosure, one that you can just put a bare SATA drive into, like loading a cartridge into a Sega Genesis. When the drive gets full you can eject it, put it back in its original shipping package for storage, and drop in a new one. By far the lowest cost-per-MB of any backup option I've run across, and you can get them in eSATA, USB, and FireWire versions.
posted by Kadin2048 at 12:40 PM on May 11, 2010

Why is the current configuration not ideal? OS boot + Raid 5 data is a fairly common set up when you had a RAID card.
posted by jmd82 at 12:45 PM on May 11, 2010

Best answer: Oh, and I never really answered your question about the fifth drive ... I would not do that. If you want an off-site backup you should do it explicitly, and not use the fail/rebuild functionality of the RAID in mirrored mode to accomplish it. First, because there would be performance implications -- you'd lose the advantage of having the RAID system while the array was rebuilding -- and second because it's just not safe or anywhere close to best practice. When the array is rebuilding you don't have any other fault-tolerance, so a failure in one of the remaining drive or drives (which isn't that unlikely; you're creating a lot of I/O as a result of the rebuild) would mean you'd have to recover from the drive you'd removed ... which might or might not get recognized by the RAID card as part of the array ... ugh, it just doesn't seem like a good idea to me. I wouldn't want to go there unless I really had to go there.

In general you seem to be trying to do down a path that mixes backup in with fault-tolerance, and that is just not what RAID does well. You didn't ask for the RAID and aren't convinced you need it -- fair enough. But I think you're better off just ignoring it and letting it do its thing (and do backup in some normal way, with external drives or a NAS or whatever) than trying to shoehorn it into a backup role that it's not well-designed for.
posted by Kadin2048 at 12:56 PM on May 11, 2010

Response by poster: I don't think the 3-disk RAID 5 is ideal because I don't have instant failover. If I lose my boot drive I have to get a replacement, install OS X, and then restore from Time Machine.

Thanks for the input. Barring any more, I'll probably put all 4 drives in a RAID 01/10 or 5 and look elsewhere for backup.
posted by Mr. Anthropomorphism at 1:09 PM on May 11, 2010

That's one way to go; I think the reason Apple doesn't set them up that way out of the box is because Boot Camp doesn't work with the RAID card.

If you're OK with that limitation then no reason not to put all four disks into the array. (I'd go with 5, but YMMV.)
posted by Kadin2048 at 1:40 PM on May 11, 2010

RAID-5, I mean, not five disks. Sorry.
posted by Kadin2048 at 1:42 PM on May 11, 2010

Just don't use raid 5 for your boot disk.

If you want to go for complete overkill then add a pair of 40ish gig drives to use in raid 0 for your boot volume and those other 4 in raid 5 for your data.
posted by phearlez at 1:50 PM on May 11, 2010

That link is talking about DB servers that do a ton of log writing; a typical OS X install, being used as a desktop or workstation, doesn't really write that much to its logs. There's some on boot and here and there while in operation but I doubt it's that significant. It's not like an application server that's constantly spewing MBs of logs.

Also I suspect the Apple RAID card, just judging by the size of the heatsinks, is bringing a fair amount of processor power to bear on the parity calculations. I'd bet that although it may be a bit slower while writing than RAID 10 of the same drives, it's going to be faster than a single disk (which is apparently Apple's default). Plus, a lot of the boot process is reading from disk into memory, and that's when you really feel the speed issue; it's not that write-intensive a process.

The other consideration is that two older 40GB disks are going to have a much lower native transfer speed than a modern disk operating at the same rotational speed, because the areal density is a lot lower. It's entirely possible that two old 40GB disks in RAID 0 would be slower, both read and write, than 4 high-density 500GB disks in RAID 5 behind a good card.
posted by Kadin2048 at 3:41 PM on May 11, 2010

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