but installing them as well did not help but at least I got a different error message and I then I realised there was no swap on the new DO server so I created a 2Gb swap file and then everything installed OK!
but I have not had to build a kernel for many, many years - I think I am well past that interest . .
‘aufs was rejected merging into mainline Linux. Its code was criticized for being “dense, unreadable, and uncommented”. Instead, OverlayFS was merged in the Linux kernel  After several attempts, the author has given it up’
which does not give me any enthusiasm for aufs either . .
Device mapper issues have nothing to do with Discourse.
Redhat team get very upset when we mention that it just does not work for us, but release after release they say there are no bugs left, and each time we keep seeing Discourse users complain about corruption due to device mapper.