why do scsi disk performance decline in SMP ?

Hi all,
I run three copy processes in redhat7.2 on SMP platform simultaneously,
which copy 1GB data from three windows client to linux server by samba, I found that the sector block number of the scsi command that my driver received is very discontiguous. so, it decline my scsi disk performance. but I run the same three copy processes in UP, the sector block number of the scsi command that my driver received is very contiguous. I don't know why? could anybody give me some suggestion? Any help is appreciative.