I posted a (specific) question last night which was really indicative of my lack of (more general) knowledge about how the Drobo manages space. I was thinking about this more and I think I’ve come up with a theory – I’d love it if someone could confirm or deny this. (Then, my other question would be answered, too)
My theory: Every Drobo volume can have two kinds of partitions in it – for lack of better names I’ll call them “sparse-friendly” partitions and “sparse-unfriendly” ones. The Drobo understands the filesystem format of sparse-friendly partitions, and is able to store them on its physical storage in a sparse manner. So you can have a sparse-friendly partition that is much larger than the free space on your Drobo, and that works just fine.
Sparse-unfriendly partitions, by contrast, are “wired” – physical space is allocated for them immediately and stays allocated to them forever. The Drobo doesn’t necessarily understand the filesystem format.
If you remove a disk (and don’t replace it, but just wait until everything is green again) so that the amount of total space is permanently lowered, then you’d better have enough space to cover all your sparse-unfriendly partitions plus all the used space in your sparse-friendly partitions. (Or else what? I’m guessing it just means the Drobo will remain in degraded mode until you add another disk)
So the instructions floating around the net about how to create a Time Machine partition (which say there’s something magical about the “last” partition in a volume) are incorrect. It’s not the fact that it’s the last; it’s the fact that it’s sparse-friendly, whereas the first partition may or may not be.
If all of this is correct, then my one question is: What exactly determines whether a partition is sparse-friendly or sparse-unfriendly? Is it the filesystem format (e.g. is an HFS+ partition created by Disk Utility just as good as an HFS+ partition created by Drobo Dashboard)? Is it who created it (e.g. an HFS+ partition created by Drobo Dashboard will always be sparse-friendly, and an HFS+ partition created by Disk Utility will never be)?
Is there a danger that you might be able to somehow create a partition that the Drobo thinks is sparse-friendly, but really is in a format that the Drobo doesn’t quite understand (e.g. case-sensitive HFS+), and so it might incorrectly think that a certain block is free (and doesn’t need to be backed by actual storage) when it’s actually not free? Or is the Drobo pretty good about determining what’s sparse-friendly and what it should just allocate space for up-front and then leave alone?
Is there anything inherently unsafe about using sparse-unfriendly partitions? I’m trying to figure out why people are so adamant that “using Disk Utility is a bad thing”.
Or is my theory totally incorrect?