New tricks for XFS

I am not sure I understand. What I was trying to say is that, as I understand it, there is a metadata structure that the copy-on-write mechanism can't share: space management. As you create subvolumes inside of subvolumes, each one of these "matryoshka" subvolumes will have to handle its internal space (eg the free space tree) in some way. At first, the CoW mechanism will share the blocks that store the free space information, of course. But once you start using the filesystems, they will differ quickly. For Nth subvolumes, you will have Nth free space trees for each one (plus the host).

Now let's imagine that you make a large file in the last subvolume. It will requires to allocate blocks for its image residing in the previous filesystem. If the allocation is big enough, it may require also to allocate blocks for that previous subvolume, and would require to allocate blocks for him too - all the way to the host filesystem. Allocation performance will suffer heavily. In short, XFS needs to replicate space management structures for each embedded subvolume (and potentially need to update *all* of them in some extreme cases). In ZFS/btrfs, space management is shared by all subvolumes so it does not have these problems.

This is what leads me to believe that excessive subvolume embedding would be a scalability issue for this model, but may be I'm not understanding it right