Actuator Arm => Used to fetch data from Platter surface, Can move Back & Forth
Platter > Surface > Tracks > Sectors > Data
Disk size = 2(P x S x T x S x D)
Disk Access Time
Seek time (ST) => Time taken by R/W head to reach desired Track
Rotation Time (RT) => Time taken for one full Rotation (360o)
Rotational Latency => Time taken to reach desired Sector => Half of Rotation time
Transfer Time (TT) = Data to be Transferred/Transferred Rate
Transfer Rate/Data Rate = No. of Heads/Surface x Capacity of one Track x No. of Relations in one second
Disk Access Time = ST + RT + TT + Controller Time + Queue Time
Disk Scheduling Algorithms
First come First serve (FCFS) => Go to the point which came first
Calculate ST by adding the total distance travelled
No Starvation
Shortest Seek time First (SSTF) => Go to the shortest path from that point
On average gives Optimal result
Average response time is good
Starvation
SCAN/Elevator Algorithm => Go towards any one direction till the end and then towards other direction till the last request
In case of Dynamic, Starvation may occur
Look => Like SCAN but does not goes till the end during first direction
Circular Scan (CSCAN) => Go towards a direction till its end covering all request and then change direction to go to other end without covering request then again change direction and go & cover request till the last request
Circular Look (CLOOK) => Like CSCAN but not goes till end on either side if not required
System Threats and Security
Protection Problem => Ensure that each object is accessed correctly and only by those process which are allowed to do so