ZFS does not have a huge problem with fragmentation. However, over time and especially when the ZFS filesystem gets over 70% full, fragmentation occurs.

The FRAG column of a zpool list command shows how fragmented the free space is.

The easiest and most sure way to defragment a ZFS pool is to use zfs send and zfs recv to copy the data out of the fragmented pool and into a new, unfragmented pool.

https://pthree.org/2012/12/20/zfs-administration-part-xiii-sending-and-receiving-filesystems/

http://docs.oracle.com/cd/E19253-01/819-5461/gbchx/index.html

http://docs.oracle.com/cd/E19253-01/819-5461/gbinw/