Can someone explain me why if backup ,using dd, a primary partition to a primary partition on other disk the transfer rate (MB/s) is significantly slower compared to the the rate I have if the destination is a logical partition.Let's say that I have two disk sda and sdb and I want to backup sda1 a primary partion of sda, on sdb. I create a new partition on sdb that has the same size of sda1. If the partion on sdb is primary When I run dd  if=/dev/sda1 of=/dev/sdb1 the speed is 20 MB/s but if I launch using a logical partition as destinationdd if=/dev/sda1 of=/dev/sdb5the speed is 3 MB/s. To copy at the same speed I need to set the BS=4MB .I run the test more the one time also after rebooting the machine.
dd primary partition slow transfer rate
partition;backup;dd