Currently XFS Filesystems can't be shrunk.
To support shrinking XFS filesystems a few things needs to be implemented, based on a list by Dave Chinner :
- A way to check space is available for shrink
- An ioctl or similar interface to prevent new allocations from a given allocation group.
- A variant of the xfs_reno tool to support moving inodes out of filesystem areas that go away.
- A variant of the xfs_fsr tool to support moving data out of the filesystem areas that go away.
- Some way to move out orphan metadata out of the AGs truncated off
- A transaction to shrink the filesystem.
At that point, we'll have a "working" shrink that will allow shrinking to only 50% of the original size because the log (in the middle of the filesystem) will get in the way. To fix that, we'll need to implement transactions to move the log...
- A script from Ruben Porras to check if enough free space is available to support shrinking 
- A patch from Ruben Porras to allow / disallow allocation from an allocation group  plus userspace support for setting / clearing it 
- The xfs_fsr tool in xfsprogs
- The xfs_reno too, see Unfinished_work#The_xfs_reno_tool