If I make a ext4 filesystem and add about 20% of extra inodes, how would this effect the inode count if later I extend the filesystem? Would the extended filesystem also automatically add an extra 20% inodes? Or would it add a normal amount of inodes? Would the inodes be spread across the filesystem evenly after extending the filesystem?
The number of inodes on ext4 will grow proportionally to number of blocks (in ratio dependent on "-i" flag to mke2fs). See ext4 wiki for details. So if you specified 20% more inodes than default by using "-i ", than yes it would grow by same extra 20% more than default ratio. However, if you specified 20% more inode manually using "-N number-of-inodes", then no, number of inodes would grow only by default ratio.