How to do a persistent flat terrain in opensim ?

Very easy. :)

when you boot it up first - it will complain that it could not load the default terrain file. The file is called "default.r32". So - we need to create an appropriate file.

1) Generate the flat land at 21m height:

terrain fill 21

2) Save this as defailt terrain

terrain save F32 default.r32

Voila! Next time, opensim will find the file and will load the flatlands. Or any other landscape that you have made and saved.

UPDATE: well, the only side effect of this is a terrain multiplier. It is currently set to 60, so when loaded, the terrain ends up at 1260 meters. Good if you are used to living in the mountains, otherwise one would need to tweak it :-) I forgot where was it changeable, once I find it, I will update this post.