Can someone explain how I detect drive insertions and deletions when using the SG driver?

Do the interface cards (in this case QL23xx) provide any method of notifying when a drive is either plugged in or removed?

I know I can cause a rescan of the QL using

echo "scsi-qlascan" > /proc/scsi/qla2xxx/<cardnum>

But I don't know how to tell what this did.

Thanks,
Jeff