Shrinking Support

Currently XFS Filesystems can't be shrunk.

To suppor shrinking XFS filesystems a few thinks needs to be implemented, based on a list by Dave Chinner [1]:

• 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.

• An ioctl or similar interface to prevent new allocations from a given allocation group

• Some way to move out orphan metadata out of the AGs truncated off

• A transaction to shrink the filesystem.

At that point, we'll got a "working" shrink that will allow shrinking to only 50% of the original size because the log will get in the way. To fix that, we'll need to implement transactions to move the log...