$[$File System and Disk: $10\%]$

$(1)$ Except for single$-$level and two$-$level directories, what are the three popular schemes to define the logical structure of directories? $(3\%)$

$(2)$ Suppose that the range of a disk's cylinders is $1,200$ and the disk head currently stops at cylinder $55.$ Let the disk queue contain the requests of blocks on cylinders $,131,65,70.$ Please show how the disk head moves in the SSTF$,$ SCAN, C$-$SCAN, and CLOOK disk scheduling schemes. $(6\%)$

$(3)$ What is a component unit of the distributed file system? $(1\%)$