Without knowing any of the details of your environment: disk drive models, hardware configuration (e.g.: shared bus or separate buses), competing uses of the drives outside of RAID, the details of the array's configuration, and a complete I/O performance report ... I can only make guesses. Here are three of them:
Guess 1: Perhaps sd1 is being utilized for reads more often than sd0 is?
Guess 2: Perhaps you have active non-RAID partitions using sd1?
Guess 3: Perhaps there are significant performance differences between the two disk drives?
Your disk drive models and hardware configuration are in your
dmesg(8). If you use either of these drives outside of the
softraid(4) array, this should be clear from the drives'
disklabel(8). There's a lot of information in the iostat view of
systat(8):
Code:
iostat Display statistics about disk throughput. Statistics on disk
throughput show, for each drive, data transferred in
kilobytes, number of disk transactions performed, and time
spent in disk accesses (in fractions of a second).
I hope this helps.