[ploek]

No, there is no defragmentation for ZFS, unfortunately. A way to get around that is to send the pool's content to another (fresh) ZFS pool, where it would be written sequentially. But for that you would need a set of drives of same (or larger) capacity.

There are ideas on how one would do an actual defrag. They are generally based on a concept called block pointer rewrite, which Matt Ahrens once said could be the 'last feature ever implemented in ZFS', as it would make everything so much more complicated, that it would be hard to add new features afterwards [1].

[1] https://www.youtube.com/watch?v=G2vIdPmsnTI#t=44m53s (Link to the beginning of the explanation, the 'last feature ever implemented' quote is at at around 50:25)