Re: [RFC V3 0/3] xfs: Add support to shrink multiple empty AGs

[Christoph Hellwig <hch@infradead.org>]

On Wed, Oct 22, 2025 at 09:05:32AM -0700, Darrick J. Wong wrote:
> > I'm still missing what the overall plan is here.  For "normal" XFS
> > setups you'll always have inodes that we can't migrate.  Do you plan
> > to use this with inode32 only?
> 
> ...or resurrect xfs_reno?

That only brings up some vague memories.  But anything in userspace
would not be transactional safe anyway.

> 
> Data/attr extent migration might not be too hard if we can repurpose
> xfs_zonegc for relocations.

The zonegc code is very heavily dependent on not having to deal with
freespace fragmentation for writes, so I don't think the code is
directly reusable.  But the overall idea applies, yes.

> I think moving inodes is going to be very
> very difficult because there's no way to atomically update all the
> parents.
> 
> (Not to mention whatever happens when the inumber abruptly changes)

Yes.  That's my big issues with all the shrink plans, what are we
going to do about inodes?