Comment 8 for bug 1543233

here the answer:

man parted:

              minimal
                     Use minimum alignment as given by the disk topology information. This and the opt value will use layout information provided by the disk to align the logical partition table addresses to actual physical blocks on the disk. The min value is the minimum aligment needed to align the partition properly to physical blocks, which avoids performance degradation.

              optimal
                     Use optimum alignment as given by the disk topology information. This aligns to a multiple of the physical block size in a way that guarantees optimal
                     performance.

so, disk /dev/sda reported that its own optimal alignment is 1.5M, therefore parted applied it.

http://paste.openstack.org/show/486604/ gives the following results http://paste.openstack.org/show/486595/