We had an argue today with my friend DeepM,
does making software raid1 with mdadm overwrites the partition or at least garbages its superblock / partition table / file system, if someone bumps to this question one day in ones life, here is the answer: NO, it does not.
I took two sata partitions sda3 and sdb3, mkfs.ext3, copy some stuff to both of them.
In picture 1, you can see the contents.
Next i create md device using both partitions with command:
mdadm -Cv /dev/md1 -l1 -n2 /dev/sda3 /dev/sdb3
Mdadm kindly warns me, that partition, contains
file system with (probably) files in it. Thank you, i know what i'm doing. Just press "y".
Resync process starts, 1.5G takes a few seconds to resync.
Picture 3 below: resync in progress.
After process is done, i remount newly created md1 device, check its contents and VOILA - everything is in place.