Mac OS X RAID Concatenated Disk Set Update

December 30, 2006 3:41 PM

How long should it take Disk Utility in Mac OS 10.4.8 to update a concatenated RAID when two new drives were added to it?

I have a Mac Mini running 10.4.8. I received two new LaCie 250GB drives for Christmas that I configured into a Concatentated Disk Set using disk utility. I then copied the approximately 400GB of data I had on two other drives to the new RAID set. This worked fine.

Once I was satified that everything was working off the new RAID disk, I added the two older 250GB drives to the concatentated disk set. I confirmed that Disk Utility could erase the two old drives in order to add them. I did this about noon today. It is 6:30pm here and Disk Utility still says "Updating RAID".

Creating the RAID was very fast. I expected the addition of the two additional drives to be reasonably quick as well. Not for any good reason, I guess. I've never set up a RAID before. I am suprised to find it still updating after 6+ hours.

So, hive mind, how long should I expect the update process to take? Should I just wait or is something broken?

posted by rglasmann to computers & internet (4 comments total)

So, after 10 hours of waiting for the "update" to finish, I quit Disk Utility (despite the warning that it might leave my disks in an unusable state).

The RAID disk was fine. All of my data was there. The data was still on the two disks I tried to add to the RAID as well.

I started Disk Utility again and this time I added the drives to the RAID one at a time. This time adding each drive took only seconds. I now have a RAIDed disk set over 900GB in size and my data is intact.

So, not sure what happened the first time, but the problem is solved and the answer to the original question seems to be that it should only take a few seconds to update a concatenated RAID disk set with a new drive.
posted by rglasmann at 7:10 PM on December 30, 2006

Are you aware that if one of those drives fails, your whole RAID will be lost? You might want to reconsider if you really want to do this.
posted by Steve3 at 8:49 PM on December 30, 2006

Indeed, you're multiplying your drive failure rates together. RAID 0 (which is what this is generally called) is great for speedy access to temporary or working data, or giving yourself a bigger working area.

It's an awful idea for permanent storage of important data, as any disk failure risks destroying the entire array. Best way to think about it is, if your machine was in a fire right now, what would you lose, and what would you be pissed about losing? Thats the stuff you need to setup a separate regular backup scheme for.

Harddrives fail all the time. I've seen a dozen go bad over the years, and that's just my own personal kit. Don't wait till it all goes wrong to have a backup of your emails, photos, important docs etc some place safe. Especially with RAID 0...
posted by ArkhanJG at 1:05 AM on December 31, 2006

Striped disk are acceptable *ONLY* for data you really don't care about.

I don't even think this is a striped disk -- I suspect, by the speed, that's it's JBOD -- just a bunch of disks, concatenated. This means you gain nothing, and multiply the chance of losing the volume.

1) Back it up.
2) Rebuild it.

If you really want max speed and reliablity, you want a stripe-and-mirror or mirror-and-strip. (Raid 10 or 01). The first creates a stripe set and mirrors it. The second creates mirror sets and stripes them. One is Faster than the other in some circumstances.

This gets you the speed of stripes and some redundancy. Real RAID controllers will actually speed up reads on mirros, but cheaper ones don't.

However, you're doing software raid. You'll see some gain, but not much.

Pure mirroring (RAID-1) protects data from drive failures, which are one of the most common hardware problems in computers. They don't protect against controller failures or user error, so they are *not* a backup.

For larger disks that are reliable, the answer is RAID-5, distributed parity. You lose one disk worth of storage to the array for parity, but once built, you can lose any drive in the array and it will still work (it will be slower.) You replace the drive, and it rebuilds the data on it. For extra protection, you can add a hot-spare, which will replace the failed drive and start the rebuild anyway.

But for home, that's overkill, and in software RAID, the parity calcs will slow things down quite a bit. So, stick to mirrors, back up your data frequently, unless you want to buy a real hardware RAID controller.
posted by eriko at 7:43 AM on December 31, 2006