The default parity rebuild occurs before fsck or mounting. You can ctrl-C, or modify /etc/rc, to have it run in the background.
The risk, as I understand it, for RAID1 is of a failure of one of the mirrors -during- the parity rebuild. If the array is not writeable during the failure, data integrity is assured. But, if the array is in active use, e.g. mounted r/w or in an fsck that is changing blocks -- you may be resorting to a backup for recovery.
I run RAIDframe,
|