Hard links are to deduplicate identical tiles, right? How much does that save and how close is the tile with most links (empty one(s)?) to the filesystem limits? Inquiring minds want to know...
In total, there are 271 million hard links. So out of 300 million files 271 are hard links!
The file system limit is 64k hard-links for the same file, so I have to handle the case when it's reached and then start a new file for the next 64k.
I had never heard that, so I went sniffing around and it seems to be ext4 specific[1] but I wasn't able to easily get the limits for ZFS (or xfs, etc), so depending on how much glucose one wished to spend it may be better to use a different FS than all that renaming work around