What is the X in X-mas?
(Or sector map) The mapping from logical to physical sector numbers on a magnetic disk designed to optimise sequential reads and writes. Data is usually transferred to and from the disk in blocks or sectors where one sector lies within a continuous range of rotational angle of the disk. If logical sectors are assigned sequentially to physical sectors (0,1,2,...) then by the time one sector has been read and processed (e.g. writen to main memory) the start of the next logical sector will have passed the read/write head and will not be accessible until the disk's rotation brings it back under the head.
Staggering the physical sectors (e.g. 0,3,6,1,4,7,2,5,8) aims to allow just enough time deal with one sector before the next is accessible. This obviously depends on the relative speed of the rotation of the disk, sector size, sectors per track and the speed of transfer of sectors to main memory.