Mirrored RAID on Linux?

Question

linux

How to properly set up a mirror raid on Linux?
Two disks in a mirror raid. And only one of them is installed OS. The second is connected as a folder and everything that is not related to the kernel is replicated to it.
While this raid should be seen in general as one disk.
Software raid.

Answer 1

during installation you create a raid, then install grub on both disks
in the system, you will see both disks, but if you look at the mounted disks, df, then the mount will lead to the md device