Changing filesystem resize patterns

> Modern filesystems like btrfs or bcachefs already have fully dynamic layouts, but I don't think we're ever going to get there with ext4 or XFS.

Do they need to be "fully dynamic" ?? Don't they just need internal structures to scale with size?

As I understand it (and my understanding is based on very old information that I haven't taken the trouble to update), these filesystem have various tables (Inodes, free block list) that are distributed across the device (in what were once called "cylinder groups"). Everything else is space used for file (and indirect block) storage, which is already fully dynamic. Have I over-simplified too much?

These tables should be smaller on smaller filesystems to avoid wastage (??) and larger on larger filesystems to improve sequential read time and to reduce fragmentation(??). And you cannot change the size of already-allocated tables when you grow the filesystem (because we don't have fully dynamic layouts).

So: could we scale these tables based on their location instead of the total size? So all inode groups aren't the same size, but their sizes and locations can be determined directly from data in the superblock. The calculation would be slightly more complex than it currently is, but it would just be a different calculation.

With this arrangement, most of the device would use a layout that is optimal for the device size (assuming the scale is closer to logarithmic than linear), and so you get most of the performance benefit.

Obviously this is still a format change and not to be done lightly. But it seems a lot less than "designing a new filesystem".