Hi Scott:
Here's the deal with space reservations or more precisely, the problem they try to fix and more importantly other ways of addressing the same problem. Bottom line, space reservations are never needed unless you choose to use them as a way to solve this problem.
The problem, which is not unique to NetApp SAN, but any SAN product that allows snapshot technology is that when you take a snapshot, you have to save blocks as they are deleted or changed. You have to store those blocks somewhere and that space can run out. Let's say you delete a bunch of blocks inside of a LUN. The host believes those blocks are free, but in reality, those blocks must be kept somewhere as long as a snapshot points to them. So if you run out of physical space in the storage when the host thinks there is space in the LUN, that's a problem.
The way most SAN vendors deal with this issue is through a choice or combiination of 2 features. One is auto-grow. As the space to store the blocks get low (in the case of NetApp, it's thecontaining FlexVol, others may use a separate area, but the concept is the same), you automatically grow that space. Of course you set policies around that growth (thesholds, grow by how much, and to what ulimate size). This simply adds more blocks to the pool. One of the neat things that NetApp brings to the table here that many SAN vendors do not is that if you fix the situation (a snap falls off or you choose to delete it) and blocks become free, NetApp gives you the ability to shrink the volume back down to where it was while keeping all the other snapshots around. Many vendors cannot do this without deleting all snapshots.
The second method is snapshot autodelete. So as space fills up, you delete older snapshots until you have sufficient space again. NetApp also has this option. Most storage systems, NetApp included allow you to use these methods together, (say grow first, then start deleteing). But the important thing to know is you can use either or both of these methods with NO space reservations. You'll want to keep an eye on your volume usage just in case, but the storage system will handle the vast majority of the cases just fine.
Now, NetApp offers you a third option. Many years ago, this was the only option but that hasn't been true since 7.1 was released and introduced autogrow and auto delete. This 3rd option is using space reservations. By reserving space up front in the volume that is only used once the rest of the volume is filled up, you can always have free blocks because once you start using the reserved space, new snapshots are failed so all blocks that are used in the reserved space will never be in a snapshot thus they can be freed if the host frees the corresponding block in the LUN (this assumes 100% reserved, you can set this value lower if you know your environment and choose to do so). Space reservations don't really impact performance, they just give you a cushion against this condition.
So, many customers don't use space reservations at all. And if you don't take snapshots in that volume, you don't need to do any of these things.
Anyway, I hope this helps.
