Computer Science 330
Operating Systems

Fall 2020, Siena College

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Agenda

• Announcements
  • The last zyBook assignment was due this morning, but anything you can complete by the end of the semester will be considered for grading
  • The next Term Project milestone is the paper drafts due on Monday - the more complete the better my feedback can be to improve your final version
  • Have not started grading shell projects yet
  • Today's office hours moved up a half hour to be 2-3 PM
• Disk Scheduling Algorithms
  • Recall: cost of a disk access includes seek time and rotational latency
  • We wish to minimize seek time by minimizing the distance the read/write head has to move in order to service the incoming requests by reordering requests based on cylinder number
  • This optimization may be done by the disk, the hardware controller, or by the operating system
  • Compare algorithms by examining their performance on a given request queue
  • First-Come First-Served (FCFS)
  • Shortest Seek Time First (SSTF)/Closest Cylinder Next
  • SCAN or Elevator Algorithm
  • LOOK Algorithm
  • Circular Algorithms
  • Comparing Disk Scheduling Algorithms
    • SSTF or LOOK are often reasonable for a default algorithm
    • SCAN and C-SCAN are better for heavily loaded systems where LOOK is unlikely to save much and SSTF runs the risk of starvation
    • performance depends on the frequency and types of requests
    • we may want to consider some of this when we decide how to organize file systems
  • FreeBSD's ufs filesystem uses an elevator algorithm (see /usr/src/sys/ufs/ufs/ufs_disksubr.c)
• RAID
  • Everyone know about parity and error correction?
• Disk cache
• Journaling filesystems
• Hierarchical storage
• Virtual file system layer

Links

• RAID on Wikipedia