Plenty of flash, but only 20MB usable?

Issue: I have a device with 128MB (or more) flash, but after a fresh installation, I have only 20MB left for package installation. Where has the rest gone? How can I use the entire flash?

Solution:

• For Netgear R7800 → https://forum.openwrt.org/t/ipq806x-r7800-how-use-entire-area/6862
• For Netgear R6220 → R6220 - Make use of reserved 60 MiB partition