Changing filesystem resize patterns

Wouldn't it make sense to use the same logic for the journal as for the filesystem as a whole? I.e. allocate a huge address space for it but only use as much as makes sense for the amount that's appropriate for the size of the filesystem actually used. The "spare" wouldn't need to be backed by actual storage capacity as it's never touched.

That way, when the journal needs to grow to suit a growing filesystem, the address space has already been reserved to keep the journal contiguous and all that needs to change is that the address space is now backed by whatever provisioning setup is being used and the FS code now using that space.

I don't know much about filesystem design, but perhaps a similar approach could also apply to things like inodes - allocate lots of them but only use as many as are needed leaving the others unprovisioned.