How mdadm react with disk corruption during check?

Hello,

I've searched a bit on the wiki but didn't find any clear answer.

So let assume we have a raid 1 with two disks : sda and sdb.
You can read and write on both disks without I/O error, so no drive are
going to be kicked out the array.
The only stuff is that sda will not read what has been written on some
sectors.

I know that mdadm can not detect integrity during normal usage, and as
read on the array will be performed by chunck randomly on the two disks
you get a partial corrupted reading.

Now, during checkarray command, mdadm is reading the whole disk, it will
detect that sda and sdb does not contain the same data (at least I hope
that checkarray is comparing data on both disks).

How does it decide which drive (sda or sdb) have correct data to write
it back the other disks?
Is there any messages available in such case?

And I've the same question with raid 1 on 3 disks and same behavior on sda. 

Thanks,

Aymeric.