Do we have restriping under such configuration, i dout it, because the current data doesn't need to be restriped to the new disks.

No restriping will occur when the pool is expanded. New data will be written to the new disks until those disks have approximately the same utilization as the existing disks, after which new data will be written across all of the disks.

Richard J Anderson

Can you tell me what document can further elaborate on this?  Thanx.