Hi Doug and Rohit, thank you both for your input (and welcome :)),
I now get it we have a oplog problem that we’ll try to tackle by increasing the size and reducing as much as possible the writing during the sync.
We are also interested in learning a bit more about the snapshot solution, because the mongo documentation is a bit “light” at least for us non experts on that matter.
I understand we should shutdown a secondary, then copy its data files over to the new member, that’s the easy part. One question that stands is “What files should be copied over from S1 to S2 ?”. In /var/lib/mongod we have collection-XXX.wt indeed-XXX.wt WiredTiger*, a journal folder and a few others ?
Thanks again for your help, very much appreciated.
cheers,
Philippe