Does anyone know whether btrfs or XFS support reflinks from snapshot datasets?

I can confirm BTRFS yes, but note that source and destination need to be on the same mount point before kernel 5.18

XFS doesn't have snapshot support, so the short answer there is no.

Shows what I know about XFS. Thanks!

You can get psuedo-snapshots on XFS with a tool like https://github.com/aravindavk/reflink-snapshot

But, it still has to duplicate metadata which depending on the amount of files may cause inconsistency in the snapshot.

This is only a tangent given we are talking about snapshots and reflink, but just wanted to mention that LVM has snapshots, so if you need XFS snapshots, create the XFS filesystem on top of an LVM logical volume.

It's worth noting that, while LVM can support snapshots for XFS (or any other filesystem, really), the performance penalty is almost entirely unacceptable (one benchmark[0] shows an up to 60% drop in write throughput), so ideally your process should be something like

1. Snapshot LVM volume

2. Mount the LVM volume and back the snapshot up somewhere else

3. Delete the snapshot as soon as possible

Otherwise you're going to lose a ton of performance.

[0] https://www.percona.com/blog/disaster-lvm-performance-in-sna...

XFS doesn't have native snapshot support, though?