[G4] EIDE and Ultra ATA RAID

[Philip J Robar]

On Apr 29, 2005, at 3:26 AM, shopdog wrote:

> Phil:
>
> Thank you for the insight. A few things I can't seem to find  
> definite answers on, although you hint at one of them...

First, let me be clear that my postings have been based on things  
I've read and actual use of Sun's Volume Management software. I've  
never set up or tested an OS X RAID 1 (mirror) array. (I've played  
with stripping though.)


> 1) You say the remaining drive should be able to keep running on  
> its own. Do you mean that it should have a full directory, and  
> therefore I should be able to treat it as a single drive (i.e., as  
> if it was never part of an array), and therefore the file should  
> show up and I could get the file, and then continue using the drive  
> as a single drive?

Yes.


> Or do I need to rebuild the array, and only then be able to get to  
> the file?

No.


> Or can I rebuild the array at any point (presumably the array would  
> be rebuilt so that any files I have added since the failure would  
> then be duplicated to the new second drive)?

Yes.


> 2) How does one set up a JBOD, and can OS X Disk Utility do it (My  
> guess is no.)?

Yes, this is new in Tiger. (At least I think it is, I don't have  
Panther running anywhere at the moment.)


> If one drive in a JBOD fails, is it only the data on that drive  
> that is lost? That is my understanding, since files are saved  
> sequentially as the disc space fills up (the system uses one drive  
> first, then the next, etc.), as opposed to saving files at random  
> places across the JBOD. Does a JBOD ever save parts of a file on  
> one of the drives and part of the file on another drive, so that if  
> one drive failed you would lose the file? Or does a JBOD make sure  
> that a file never spans more than one drive? (My guess is it spans  
> drives sometimes.)

There's no guarantee that the data on the surviving disks will be  
recoverable. It varies with the RAID software/hardware, OS, and  
filesystem combination being used. And I don't know the specifics for  
OS X. What you need to keep in mind is that while the files are  
written sequentially the file system spans all of the drives in the  
array. Loosing one of the array's drive is the same as having a huge  
section of a single drive fail - the result is a corrupt file system.  
While you might be able to recover some data with Disk First Aid, or  
Disk Warrior, or an (expensive) data recovery organization, I  
wouldn't count on it. You certainly can't expect to be able to just  
mount the surviving disks and read them as if they were regular  
individual drives.


Phil