Disk fragmentation

[Super user]

On DOS PC's there are several programs to defragment the disk (i.e.
make files take up contiguous sectors/tracks and move then to the 
'beginning' of the disk).

Such a defragmentation option is not available on SCO XENIX/386.
(Is it available on ANY UNIX?)

Can fragmentation cause severe performance degradation on SCO XENIX?
I ask this specifically for SCO XENIX because in the (kernel) 'configure'
program there is an entry called 'disk block sorting'. Does that have
anything to do with fragmentation? And if so, is it effective enough to
replace a backup/delete/restore procedure one in a while?

(If not, what DOES the 'disk block sorting' do?)

Richard
(...!mcvax!neabbs!richard)