Establish the multipath device name and device mapper id
ls -l /dev/mapper/ 
Note the multipath device name and device mapper id (See Example)
Now find the sd<&letter> and make the scan lines 

find /sys -name "sd*"  -print  |egrep device mapper id |cut -d\/ -f8 |awk '{print "echo 1 > /sys/block/"$1"/device/rescan"}'



For Example

ls -l /dev/mapper/ 
total 0
lrwxrwxrwx 1 root root       7 Jul  5 09:10 cdb -> ../dm-4
lrwxrwxrwx 1 root root       7 Jul  5 09:10 cdc -> ../dm-3
crw------- 1 root root 10, 236 Jul  5 09:02 control
lrwxrwxrwx 1 root root       7 Jul  5 09:02 ol_rhel7lamp-home -> ../dm-2
lrwxrwxrwx 1 root root       7 Jul  5 09:02 ol_rhel7lamp-root -> ../dm-0
lrwxrwxrwx 1 root root       7 Jul  5 09:02 ol_rhel7lamp-swap -> ../dm-1
lrwxrwxrwx 1 root root       7 Jul  5 09:02 xstor-xstor -> ../dm-5
[root@adnix ~]#  find /sys -name "sd*"  -print  |egrep dm-3 |cut -d\/ -f8 |awk '{print "echo 1 >ls -l /dev/mapper/  /sys/block/"$1"/device/rescan"}'
echo 1 >ls -l /dev/mapper/  /sys/block/sdf/device/rescan
echo 1 >ls -l /dev/mapper/  /sys/block/sdi/device/rescan
echo 1 >ls -l /dev/mapper/  /sys/block/sdl/device/rescan
echo 1 >ls -l /dev/mapper/  /sys/block/sdr/device/rescan

/usr/sbin/multipathd -k'reconfigure'  ### a space is not missing k' is correct.

pvresize /dev/mapper/cdb