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carterx · Jan 24, 2011, 11:59 AM

I have the two HD's in my MacMini Server RAIDed but I have two identical 2TB HD's with Identical cases that run FW-800 & I was wondering if I can RAID these two drives. Never tried external HD's. I want to use the external HD's as a storage area but to have them backed up at the same time. I know the drives would have to both be on before the computer starts up but wondering if they would work RAIDed?

Thanks,
.

OreoCookie · Jan 24, 2011, 12:13 PM

Possible? Yes. Useful? Probably not.
And I think you're confusing a RAID1 (mirrored RAID) with a backup, it's not. A RAID protects against hardware failure, but it is not a backup.

Thunderbird · Jan 24, 2011, 02:52 PM

You can certainly set up a RAID using external hard drives. It can be very useful, and it's not difficult to set up.

I have used Apple's hard drive utility to set up a RAID 0 using two external USB 2TB hard drives, used exclusively for Time Machine backups. That drive backs up all of my internal and external drives. I also set up two external 2TB Firewire drives in a mirrored raid for my home video collection. As long as the drives that you are using are identical, setting up a software RAID is very simple.

Oneota · Jan 24, 2011, 09:48 PM

Originally Posted by OreoCookie

And I think you're confusing a RAID1 (mirrored RAID) with a backup, it's not. A RAID protects against hardware failure, but it is not a backup.

It can be, though -- if you've got 3 drives, for example, set up a 2 drive mirror, and regularly "break" the mirror by moving one of the drives off-site. Rebuild the mirror by attaching the other one, and you're back in business (actually, your RAID will continue to function just fine in a "broken" state, it'll just be a one-drive RAID). Repeat once a week, or whatever schedule you'd like, and you've got a viable off-site backup strategy, as long as you only need once-in-awhile granularity.

reader50 · Jan 24, 2011, 11:44 PM

Originally Posted by carterx

I have the two HD's in my MacMini Server RAIDed but I have two identical 2TB HD's with Identical cases that run FW-800 & I was wondering if I can RAID these two drives. Never tried external HD's. I want to use the external HD's as a storage area but to have them backed up at the same time. I know the drives would have to both be on before the computer starts up but wondering if they would work RAIDed?

Thanks,
.

They would work, but you would get almost no speed benefit from them over FW800. Most two TB drives top out above 100 MB/s, which is the max FW800 can give. So you'd see a speed benefit only as you get further into the drives, where their individual speeds drop below 100 MB/s.

If you can place them on separate FW800 busses, then you would see a speed benefit with large files. But your Mac Mini will have only one FW800 port.

OreoCookie · Jan 25, 2011, 05:40 AM

Originally Posted by Oneota

It can be, though -- if you've got 3 drives, for example, set up a 2 drive mirror, and regularly "break" the mirror by moving one of the drives off-site. Rebuild the mirror by attaching the other one, and you're back in business (actually, your RAID will continue to function just fine in a "broken" state, it'll just be a one-drive RAID).

This is a very, very bad idea!
First of all, this can easily corrupt your whole RAID silently if the rebuild fails (e. g. if the last rebuilt wasn't allowed to complete). Then all of your data is toast. Secondly, it puts immense strain on the harddrives, especially the one the one(s) you take out. Which makes data corruption all the more likely.

A RAID1 is never intended to be a backup, it provides redundancy against disk failure.

@reader
Regarding throughput, in my experience, I don't come close to 100 MB/s with FireWire 800, I get more like 40 MB/s tops. But you're right, it won't get better when two drives have to share the same bus. For backups or simple mass storage (think music, movies and simple documents), however, I don't think this is a problem.

Oneota · 10:40 AM

Originally Posted by OreoCookie

This is a very, very bad idea!
First of all, this can easily corrupt your whole RAID silently if the rebuild fails (e. g. if the last rebuilt wasn't allowed to complete). Then all of your data is toast. Secondly, it puts immense strain on the harddrives, especially the one the one(s) you take out. Which makes data corruption all the more likely.

A RAID1 is never intended to be a backup, it provides redundancy against disk failure.

I don't do this, personally, but I know there are many sites that do it, and people far smarter than I am have recommended it as a perfectly viable way to do sneaker-net offsite backup on the MacE and the OS X Server lists. It makes more sense to me to just have a second set of disks off-site and use the network to duplicate the data, but if you don't have a good network link to an off-site location, this would do in a pinch, IMO.

(Oh: And if, somehow, the rebuild were to fail, then just put the disk you just pulled out of service back in place, and boom: There's another copy of your data.)

OreoCookie · Jan 25, 2011, 12:08 PM

Originally Posted by Oneota

I don't do this, personally, but I know there are many sites that do it, and people far smarter than I am have recommended it as a perfectly viable way to do sneaker-net offsite backup on the MacE and the OS X Server lists.

They're wrong.
First of all, all that gives you is a clone, a very bad form of backup. (There is a place for cloning, but it augments incremental/differential backups rather than being a substitution for them.) By a large margin, the most common cause for data loss are user errors and software problems. If you only have one copy, then you lose your backup copy after rebuilding the backup. Although this is not a problem that pertains only to RAID1s, it's a very significant drawback.

What is worse is that constantly rebuilding your RAID puts significant additional strain on the hardware and software (if you're using a software RAID). It is very prone to silent errors which are the worst. If rebuilding the backup takes a very long time (> 24 hours), you can be tempted to take out the harddrive before it is completed and then end up with a corrupted RAID array. If, for instance, a rebuilt takes longer than 24 hours and you take home the backup drive daily, then (1) you don't have a backup and (2) you will end up with a corrupted data on your RAID volume.

There was a very funnily written story to that effect by a backup or data recovery professional, although I can't find it anymore. He tells the story where the remaining drive (which always stays in the office) starts dying slowly because of all the strain from work 24 hours. In addition, the system no longer manages to rebuild the RAID volume within the work hours and the people only noticed something like 2~3 weeks later when they were unable to open certain files.

So even if some people you know and you think are smart are recommending this, IMO they don't understand what a RAID is and what it isn't.

carterx · Jan 26, 2011, 11:17 AM

As long as the two Hard Drives are seen I should be able to do it. I would just run RAID 1. I just want the two external HD's to mirror. I'm not worried about speed. I've been running RAID 1 with 2 x 2TB HD's on a Mac Ghost Server at work for a year now & no issues at all.

I just need to give it a try but will post up how it goes.

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