I want to mount my root filesystem as read-only on boot, and use separate writable partition for logs.
Is it necessary to create a ramfs image for such task?
How can I do this?
Yes, typically you would not want to mount / as read-only. I would recommend putting /usr in it's own partition and then mounting that read-only since it contains the majority of your executable files, which you would want to protect against manipulation of binaries. You could also run lsof to determine what processes are writing to the directories under /.