i just tried dd if=/dev/yourrootpartition of=/dev/null \ bs=1Mcount=howmuchmemoryyouwanttofill
it does not give me the control that you desire but it at least tries to use wasted memory
i just tried dd if=/dev/yourrootpartition of=/dev/null \ bs=1Mcount=howmuchmemoryyouwanttofill
it does not give me the control that you desire but it at least tries to use wasted memory