I hope this helps you out.
NetApp snapshots (and ZFS snapshots, incidentally) do things differently. Instead of copying the old data out of the way before it gets overwritten, the NetApp just writes the new information to a special bit of disk reserved for storing these changes, called the SnapReserve. Then, the pointers that tell the system where to find the data get updated to point to the new data in the SnapReserve.
That’s why the SnapReserve fills up when you change data on a NetApp. And remember that deleting is a change, so deleting a bunch of data fills up the SnapReserve, too.
This method has a bunch of advantages. You’re only recording the deltas, so you get the disk savings of copy-on-write snapshots. But you’re not copying the original block out of the way, so you don’t have the performance slowdown. There’s a small performance impact, but updating pointers is much faster, which is why NetApp performance is just fine with snapshots turned on, so they’re on by default.
It gets better.
Because the snapshot is just pointers, when you want to restore data (using SnapRestore) all you have to do is update the pointers to point to the original data again. This is faster than copying all the data back from the snapshot area over the original data, as in copy-on-write snapshots.
So taking a snapshot completes in seconds, even for really large volumes (like, terabytes) and so do restores. Seconds to snap back to a point in time. How cool is that?
But wait, there’s more.
Snapshots Are Views
It’s much better to think of snapshots as a View of your data as it was at the time the snapshot was taken. It’s a time machine, letting you look into the past.
Because it’s all just pointers, you can actually look at the snapshot as if it was the active filesystem. It’s read-only, because you can’t change the past, but you can actually look at it and read the data.
This is incredibly cool.
Seriously. It’s amazing. You get snapshots with almost no performance overhead, and you can browse through the data to see what it looked like yesterday, or last week, or last month. Online.
