To persons who have interest for this topic,
what I did yesterday is,
pull the failed drive(18.2,15K) and pop in a new one(72.8G 10K) without reboot.
Followings are output when do command :
#hpacuali ctrl all show config
Smart Array 6i in Slot 0 (Embedded)
array A (Parallel SCSI, Unused Space: 0 MB)
logicaldrive 1 (17.0 GB, RAID 1, OK)
physicaldrive 2:0 (port 2:id 0 , Parallel SCSI, 72.8 GB, OK)
physicaldrive 2:1 (port 2:id 1 , Parallel SCSI, 18.2 GB, OK)
array B (Parallel SCSI, Unused Space: 0 MB)
logicaldrive 2 (67.8 GB, RAID 1, OK)
physicaldrive 1:2 (port 1:id 2 , Parallel SCSI, 72.8 GB, OK)
physicaldrive 1:3 (port 1:id 3 , Parallel SCSI, 72.8 GB, OK)
We lose some disk space and disk speed(which can be ignored),I think.