Please excuse me if I am asking this question in the wrong place: I'm trying to network boot Ubuntu 22.04 on a Raspberry Pi 4B, and I'm not entirely clear where Raspberry ends and Ubuntu begins :-)

This activity is part of a longterm struggle I have taken up, not only to netboot Ubuntu on a Pi, but also to protect the root filesystem with overlayroot. To cut a long story short, netbooting works - overlayroot doesn't. The grisly details are available here for anyone who is interested.

I am currently pursuing the hypothesis that overlayroot will not work if the the root filesystem is an NFSv3 mount. What I wanted to ask here is:

  1. what is it that reads cmdline.txt and acts on nfsroot=...? (bootloader? something in Ubuntu? something else?)
  2. can that something be realistically made to work with something other then NFSv3? (e.g. NFS 4 or 4.1)

I have tried various combinations of (nfs)vers=4(.1) in cmdline.txt but they either elicit complaints or have no effect.


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