[CentOS] question on software raid

dmesg is not reporting any issues.

The /proc/mdstat looks fine.
md0 : active raid1 sdb1[1] sda1[0]
    X blocks [2/2]  [UU]

however /var/log/messages says:

smartd[3392] Device /dev/sda 20 offline uncorrectable sectors

The machine is running fine.. raid array looks good - what
is up with smartd?

THanks,

Jerry