For configuring RAID both the partitions should be identical, you have to create the partition (for ex: sdb1) on sdb same like as sda2.
If sda fails or corrupted then obviously server doesn't come up but still data is there on sdb1
if you want to make sdb as bootable you have to configure raid during the time of installing o.s like
sda1+sdb1 for boot
sda2+sdb2 for / (root)
for better info google it