Shrinking Support

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 [1]:

• 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...

Available pieces

• A script from Ruben Porras to check if enough free space is available to support shrinking [[2]]

• A patch from Ruben Porras to allow / disallow allocation from an allocation group [3] plus userspace support for setting / clearing it [4]

• The xfs_fsr tool in xfsprogs

• The xfs_reno too, see Unfinished_work#The_xfs_reno_tool

• An untested patch from Dave Chinner for a xfs_swap_inodes ioctl that allows to not just defragment extents but moving the whole inode [5] and a patch to xfs_reno to use it from Ruben Porras [6]