ZFS Design

There's been some work done recently that might have some bearing on this. The fragmentation itself won't be reduced (really it's intrinsic to the design of a log-structured filesystem), but peformance when free space is running low should be much improved. See http://permalink.gmane.org/gmane.os.illumos.zfs/2565 - there's been some more discussion on the topic, but you'll have to dig around the list to find it.

>Not allowing to shrink a filesystem

This, along with actual defragmentation, would require block pointer rewrite functionality. This is a blocker for a lot of features, and has been talked about for years without any visible progress. Someone (might have been Matt Ahrens) mentioned a while back that they had been working on BP rewrite at Oracle, shortly before all the ZFS developers of note left the company. Reading between the lines, I suspect this might be taken as an indication that those most qualified to do that work might be concerned about legal repercussions if they did so (but this is purely speculation on my part).

Regardless, there's recently been some noise at Nexenta about implementing BP rewrite (see http://blog.gmane.org/gmane.os.illumos.zfs/month=20130701 for example), so you never know.