Device Management Functions:
  Tracking the status of each device (disk drives, printers, modems, etc.)
  Uses policies to determine which process gets a device and for how long
    Allocating the devices
    Deallocating devices


Peripheral devices are categorized as follows:
   Dedicated
   shared
   virtual

Most important differences among devices
   Speed
   Degree of sharability


Shared devices
   Assigned to several processes (DASD)
   time by interleaving their requests
   Interleaving must be carefully controlled by Device Manager
   conflicts must be resolved based on pre-determined policies

DASDs: 
   Any devices that can directly read or write to specific place on a disk

 

Physical disk geometry - readable by the system BIOS
      Track - concentric circle containing data
      Sector - part of a track


 
     
Cylinder - all tracks readable from one head position
Seek - move read/write heads between tracks




Physical formatting (low-level formatting)
   Usually done at the factory. 
   Must be done before logical formatting
   Defines the tracks, sectors, and cylinders
Logical formatting
   Lets disk to be accessed using sequentially numbered logical sectors
   Installs a file system (ex: NTFS)


Fixed head - faster but $$




One read/write head floats over the surface of each disk, e.g., 
   Can be a single platter
   Can be a part of a disk pack (stack of platters)
   Disk Pack:
      Each platter has two surfaces for recording (except top and bottom)
      Each surface is formatted with concentric tracks 

Number of tracks ranges to a thousand or more on a high-capacity hard disk
   Track 0 identifies the outermost track
   highest-numbered track is in the center

The arm moves all of the heads in unison
To access any given record, the system needs:
   Cylinder number     Surface number    Record number
 

Time required to access a file depends on:
1. Seek time: Time to position read/write head - Slowest of three factors
2. Search time (rotational delay): 
      Time to rotate DASD until desired record is under read/write head
3. Transfer time: Time to transfer data from SS to RAM - Fastest of three


   Access time = Seek time + Search time + Transfer time


Seek Strategy:
==============
Policy to determine order in which processes get device
   Seek time is largest delay in disk access
   Goal is to keep seek time to a minimum

Types of seek strategies:
   1. First come, first served (FCFS), 
   2. Shortest seek time first (SSTF), 
   3. SCAN (including LOOK, N-Step SCAN, C-SCAN, & C-LOOK)


   Every scheduling algorithm should:
   1. Minimize arm movement
   2. Minimize mean response time
   3. Minimize variance in response time


FCFS: On average, doesn’t meet any of three goals of a seek strategy    
      Disadvantage: Extreme arm movement



While retrieving data from Track 15, the following requests arrive:
   Track 4, 40, 11, 35, 7, and 14. 
   It takes 1 ms to travel from one track to next


Shortest Seek Time First (SSTF):
  Request with track closest to one being served is satisfied next
  Minimizes overall seek time
  Postpones traveling to those that are out of way 



While retrieving data from Track 15, the following requests arrive:
   Track 4, 40, 11, 35, 7, and 14. 
   It takes 1 ms to travel from one track to next


SCAN (LOOK: )
  Arm doesn’t go all the way to either edge unless there are requests 
  Eliminates possibility of indefinite postponemenT



While retrieving data from Track 15, the following requests arrive:
   Track 4, 40, 11, 35, 7, and 14. 
   It takes 1 ms to travel from one track to next


Which strategy is the best?
FCFS 
   works well with light loads
   service time becomes unacceptably long under high loads
SSTF 
   works well with moderate loads
   has problem of localization under heavy loads
LOOK 
   works well with light to moderate loads
   eliminates problem of indefinite postponement 
   similar to SSTF in throughput and mean service times

Rotational ordering: 
  Optimizes search times by ordering requests once heads are positioned
  Reduces rotational delay (second factor in time delay)
  Time spent moving read/write head is hardware dependent
  Amount of time wasted on rotational delay can be reduced:
      Arrange requests so next sector to be read is closer to head position




FCFS
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ORDERED
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