It has been said that XFS can have problems when near full - Unverified.
xfs_db -c frag -r /dev/device
can be used to get an idea as to the fragmentation percentage.
command can be used to defragment a partition. You can safely run xfs_fsr nightly (or early in the morning) to "defrag" the partition. Doing so nightly will use less resources than doing so "whenever".