Is It Possible To Partition A RAID 1 MIRROR?

If the two drives are already partitioned (but not yet RAIDed), then you can RAID a partition on one drive with a partition on the other drive. For example, I have two 300 GB drives each of which are partitioned into three volumes: 172 GB, 50 GB, and 50 GB. I have a striped RAID created with the two 172 GB slices. I have a second mirrored RAID that uses two of the 50 GB slices. A third, striped, RAID uses the remaining 50 GB slices on each drive.

Note that if you have installed OS X on one of your volumes or drives, then you cannot create a RAID with it. You must first create a blank RAID, then install OS X.

Yes, RAIDing will erase all data. However, you still cannot create a RAID using a startup volume. You need to boot the computer from the Tiger DVD. Partition the drives as desired using Disk Utility (Utilities menu of the Tiger DVD.) Create your desired RAID(s), then install OS X on the desired volume.

Let's say you partition each drive into two volumes each that are 125 GBs (basically dividing a 250 GB drive into two equal volumes.) Create a mirrored RAID using the first volume of Disk 1 and the first volume of Disk 2. You now have a 125 GB startup volume with a mirror. Next, create a striped RAID using the second volume of Disk 1 and the second volume of Disk 2. You now have a 250 GB striped RAID for fast data storage, backup, etc.

You can now install OS X on the first RAID (the mirrored one.)

Good luck with it.



Why reward points?(Quoted from Discussions Terms of Use.)
The reward system helps to increase community participation. When a community member gives you (or another member) a reward for providing helpful advice or a solution to their question, your accumulated points will increase your status level within the community.
Members may reward you with 5 points if they deem that your reply is helpful and 10 points if you post a solution to their issue. Likewise, when you mark a reply as Helpful or Solved in your own created topic, you will be awarding the respondent with the same point values.

Kappy,

I'm very curious about your method and choice (combinations) of RAIDing as you've described. Would you mind discussing it?

I've never created a raid. I've been holding off until I understand more (much more) about it, so I can approach it in the best, most intelligent way possible, the first time.

Hi Kappy;

While I agreed that you can do what you suggest, I don't think you will see very good performance. In the testing I did it appears that the head movement to get between the partition negates the performance improvement that RAID is supposed to give you. About the only way I found to actually get any performance increase is to avoid partitioning when creating RAID arrays.

Allan

Allan,

It does work provided you pair partitions on separate drives. I agree that RAIDing two partitions on the same drive makes no sense. However, RAIDing slices from separate drives does work well. I am using just such striped RAIDs on my Mac Pro with the expected results based on benchmarks.

I assure you I would not suggest such an arrangement had I not tried it myself to verify it was feasible.

I'm not really sure what to discuss. The arrangement was described in a previous post. So you'll need to tell me more about what questions you have.

Practically I don't really see any reason for using RAIDs for a home system. Using a RAID as a startup volume is risky at best. If you use a mirrored RAID every transaction created is mirrored on the other drive. This includes any data or directory corruption. If the main drive gets corrupted, then so will the mirror. That obviates any safety that you would have with a simple backup. A mirrored RAID is useful only when the main drive fails.

A striped RAID provides an increase in speed, but the main purpose of striped RAIDs is fast secondary storage for data redundancy, hence the term Redundant Arrays of Inexpensive Disks (RAID). Such RAIDs are not intended as substitutes for a startup volume and they provide no redundancy in this usage unless you maintain a separate backup on a single drive.

The only reason I have the RAIDs setup on my system is so I can answer questions like the ones in this topic. Unless your running a large server that requires data redundancy and fast secondary storage, I can see no real advantage for the home system except for cocktail party discussions.

As an aside you do not need to defrag. OS X defrags all files less than 20 MBs on the fly. Only if you keep a large number of very large files might you need to defragment the drive. Otherwise it's unnecessary.

Be sure you are using the current version of Retrospect as well as the most recent driver plug-in. Retrospect should not cause any drive corruption. If you are having drive corruption problems you should first repair the drives before combining them into a RAID. It's best to first completely erase the drives before RAIDing them.

Excellent answer. Thanks for putting my "to RAID or not to RAID" question to rest.