RAID1 detection of faulty disks

Hi all,

just for my understanding of RAID1. When is a partition set faulty?
As soon as a read hits a bad block or only when a write attempts to
write to a bad block?

I'm a little bit confused as I read the thread 'Robust read patch for raid1'.
Does it mean that a read succeeds even if one disk is already gone?

Greetings B. Rieke