It's even smarter thn that. It makes a copy of the super block. The new
super block points at the current root directory. When the first file
gets modified, the tree of directories leading up to that file are
duplicated and the new superblock points to them. So if all you ever do
is change 1 file in your file system, the cost is a handful of inodes,
not the cost of all the inodes in the old system.
Think of it as copy on write for meta data as well as data. It's very
cool. I'm enough of a file system expert to view the NetApp work as
some of the best file sytem work in the last 20 years. There may well
be other work that I don't know about that is better, but it is unlikely,
I tend to stay somewhat current on that stuff, it's interesting to me.
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