OS X RAID question - striping and mirroring

[nagromme]

Can you partition a pair of drives so that part of your storage is striped, and part mirrored?

For example, would OS X's RAID tools allow the following?

* Two 500 GB drives.

* 250 from each are STRIPED to make a fast 500 GB volume.

* The remaining 250 from each are MIRRORED for use as a 250 GB backup destination.

* Total storage 750 GB: 500 fast, 250 redundant.

Or can something similar be done? I'd love to get the speed of striping, but if one drive fails all is lost. So setting aside some backup space for work-in-progress would be great.

TIA!

 

[glassbathroom]

An interesting question. I hope someone can help us with an answer.

 

[nagromme]

I know you can always just keeping adding drives for backup purposes--but if you don't NEED a whole striped drive, it's nice to save cost--and space for future expansion, by using "only" 2 drives.

Another possible scenario:

250 striped on each (500 fast GB)

200 mirrored on each (200 redundant backup GB)

50 non-RAID on each... one for Boot Camp, one for Linux.

Related Q: when you set up multiple partitions like this, can you tell which part of the drive is fastest? I'd want to use the fastest area for the striped array... and the slowest for Boot Camp which would see little use.

 

[scan]

Can you partition a pair of drives so that part of your storage is striped, and part mirrored?

For example, would OS X's RAID tools allow the following?

* Two 500 GB drives.

* 250 from each are STRIPED to make a fast 500 GB volume.

* The remaining 250 from each are MIRRORED for use as a 250 GB backup destination.

* Total storage 750 GB: 500 fast, 250 redundant.

Or can something similar be done? I'd love to get the speed of striping, but if one drive fails all is lost. So setting aside some backup space for work-in-progress would be great.

TIA!

okay I think I understand you correctly and in short: No, you can't.

Now, I have never using OSX raid but from a raid theoretical point of view you can't just partition some space from the drives to mirror because if 1 drive fails, the partition not only fails the whole drive fails.

As a rule of thumb about RAID setups:

If you want RAID0+1, which is what you're looking for, you need 4 (identical) drives. I'm pretty sure they have to be identical (ie same manufacturer) but they dont' have to be the same size.

I know you can always just keeping adding drives for backup purposes--but if you don't NEED a whole striped drive, it's nice to save cost--and space for future expansion, by using "only" 2 drives.

Another possible scenario:

250 striped on each (500 fast GB)

200 mirrored on each (200 redundant backup GB)

50 non-RAID on each... one for Boot Camp, one for Linux.

Related Q: when you set up multiple partitions like this, can you tell which part of the drive is fastest? I'd want to use the fastest area for the striped array... and the slowest for Boot Camp which would see little use.

Again, this is not possible. Mirroring needs separate drives, period.

 

[nagromme]

You could be right--but maybe we're not understanding each other. I don't see any logical impossibility in my plan: the mirroring DOES have two separate drives--it just doesn't get the whole of each drive.

I'm not trying to mirror from one drive to itself--that wouldn't make sense. I'm mirroring part of one drive onto part of the other.

I realize that if one of the drives failed:

* I would lose the entire striped volume.

* I would lose one of the mirrored halves.

But I would think that the remaining mirrored copy, on the good hard drive, would still be there. (Along with my larger/less-frequent external backups of course.)

That's not to say it's common practice or that OS X can do it--that's what I'm wondering about--but I don't see how what you're saying makes my plan theoretically impossible.

Anyway, thanks for any food for thought!