STORAGE MANAGEMENT

• Storage management is defined as referring to data storage equipment management that is used to store data generated by user/computer. Hence, it is an administrator's tool or collection of processes used to keep the data and storage equipment secure.
• Storage management is a mechanism for users to maximize the usage of storage resources and maintain data integrity for any media they reside on, and the storage management category typically includes various types of sub-categories covering issues such as security, virtualization, and more.
• Disk scheduling


  Scheduling I/O requests arriving for the disk, also known as I/O scheduling
• Importance of disk scheduling

  • Multiple I/O requests may arrive by different processes and only one I/O request can be served at a time by the disk controller, so other I/O requests need to wait in the waiting queue and need to be scheduled
  • Two or more request may be far from each other so can result in greater disk arm movement
  • Hard drives are one of the slowest parts of the computer system and thus need to be accessed in an efficient manner
• Seek Time


  Time taken to locate the disk arm to a specified track where the data is to be read or write
• Rotational Latency


  Time taken by the desired sector of disk to rotate into a position so that it can access the read/write heads
• Transfer Time

  Time to transfer the data, depends on the rotating speed of the disk and number of bytes to be transferred
• Disk Access Time

  Seek time + Rotational Delay + Controller Overhead + Queuing delay
• Disk Response Time


  Average time spent by a request waiting to perform its I/O operation, and the variance of response time for individual requests
• Disk Scheduling Algorithms


  • First Come First Serve (FCFS)
  • Shortest Seek Time First (SSTF)
  • SCAN
  • LOOK
  • Circular SCAN (C-SCAN)
  • Circular LOOK (C-LOOK)
• First Come First Serve (FCFS) Scheduling Algorithm


  1. Services disk locations based on the arrival order of the requests

  2. Causes wild swings from the innermost to the outermost tracks of the disk or vice versa
• Shortest Seek Time First (SSTF) Scheduling Algorithm

  Proceeds to the track that is closest to its current position, may cause starvation of some requests
• SCAN Scheduling Algorithm


  Moves the access arm back-and-forth the innermost and outermost track, servicing all requests in the direction it is headed, also known as the Elevator Algorithm
• LOOK Scheduling Algorithm


  Similar to SCAN, but the read/write head only goes to the farthest location in need of servicing, instead of reaching the end-to-end
• Circular SCAN (C-SCAN) Scheduling Algorithm

  A modified version of SCAN, sweeps the disk from end-to-end, but as soon as it reaches one end it moves to the other end without servicing any requests, then starts servicing requests
• Circular LOOK (C-LOOK) Scheduling Algorithm

  Like C-SCAN, but the read/write head only goes to the farthest location in need of servicing, instead of reaching the end-to-end
• Disk scheduling


  It is usually done by the OS to schedule I/O requests arriving for the disk
• Goal of disk scheduling

  • To minimize seek time (time taken to reach up to desired track)
• Multiple I/O requests may arrive by different processes and only one I/O request can be served at a time by the disk controller
• Two or more requests may be far from each other resulting to greater disk arm movement
• Hard drives are one of the slowest parts of the computer system and thus need to be accessed in an efficient manner
• Disk scheduling algorithm objectives

  • Less traveling head time
  • Fairness
  • High throughput
• FCFS (First-Come-First-Serve) or FIFO (First-In, First-Out)


  The easiest disk scheduling algorithm among all the scheduling algorithms
• FCFS/FIFO disk scheduling algorithm


  Each input/output request is served in the order in which the requests arrive
• In FCFS/FIFO, starvation does not occur because FCFS/FIFO address each request
• SSTF (Shortest Seek Time First)


  The job which has less seek time will be executed first
• SSTF disk scheduling


  Calculate the seek time first, then serve each request based on seek time, executing the request closest to the disk arm first
• SCAN disk scheduling algorithm


  Move the disk arm in a specific direction (towards larger or smaller values), addressing each request in its path, then reverse direction
• SCAN disk scheduling algorithm is also called an elevator algorithm because its working is like an elevator
• 1. SCAN (Circular-SCAN)


  An enhanced version of SCAN disk scheduling, where the disk head starts at one end, moves to the other end servicing requests, then jumps back to the first end without servicing any requests on the return
• LOOK disk scheduling


  Same as SCAN but instead of going to the last track, it goes only to the last request and then changes direction
• 1. LOOK (Circular Look)


  Takes advantages of both C-SCAN and LOOK, moving the disk arm to the highest request, then jumping to the lowest cylinder without servicing any requests, and continuing to service the remaining requests
• Total Head Movement (THM)


  The total distance the disk head moves to service all requests
• Seek Time


  THM * Seek Rate (3ms per cylinder move)