[G4] EIDE and Ultra ATA RAID

[shopdog]

Phil:

Excellent response.  Very informative.  I think when I pick up Tiger 
(soon), I will try a RAID 1 (mirror) and test it out to see what 
happens if one drive fails.  I'm tired of trying to maintain an exact 
backup.  I mean, I will still do an occasional offsite backup, but I 
would also like one that is in-house.

Sounds as though I will stay away from striping and JBOD, unless it's 
just for non-critical things, or I want to use JBOD to link some small 
drive together.  Of course I would still back them up.  My biggest 
concern is data loss.

Any suggestions on good backup software?  I'd like for it to be able to 
backup to DVD (span several, if possible).

shopdog

On Apr 29, 2005, at 7:57 AM, Philip J Robar wrote:

>
> 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
>
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