CS345 11 - Disc Scheduling

CS345 11 - Disc Scheduling - Chapter 11 Disc Scheduling...

Info iconThis preview shows pages 1–11. Sign up to view the full content.

View Full Document Right Arrow Icon
Chapter 11 Disc Scheduling
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
BYU CS 345 Disc Scheduling 2 Topics to Cover… Disk Structure Disk Performance Disk Scheduling Objectives
Background image of page 2
BYU CS 345 Disc Scheduling 3 Disk Structure Addressed as a one dimensional array of logical sectors Logical mapping to physical sectors on disk Sectors are smallest addressable blocks (usually 512 bytes) Clusters composed of one or more sectors Simple, but……. . Defective sectors Hidden by substituting sectors from elsewhere Number of sectors per track is not constant 40% more sectors on outside track 0 512 bytes 1 512 bytes 2 512 bytes 3 512 bytes 4 512 bytes 5 512 bytes 6 512 bytes 7 512 bytes 8 512 bytes 9 512 bytes 10 512 bytes 11 512 bytes 12 512 bytes 13 512 bytes 14 512 bytes
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
BYU CS 345 Disc Scheduling 4 Disk Structure
Background image of page 4
BYU CS 345 Disc Scheduling 5 Disk speed
Background image of page 5

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
BYU CS 345 Disc Scheduling 6 Disk Performance Much slower than memory Typical disk speed: 4-10 ms (10 -3 s) Typical memory speed: 1-10 ns (10 -9 s) I/O bus Protocols EIDE – Enhanced Integrated Drive Electronics ATA – Advanced Technology Attachment SATA – Serial ATA USB – Universal Serial Bus FC – Fiber Channel SCSI – Small Computer System Interface SAS – Serial SCSI IDE – Integrated Disk Electronics
Background image of page 6
BYU CS 345 Disc Scheduling 7 Effective Transfer Rates Performance measures Seek Time – Time to move the heads Approximation (# of tracks × c) + startup/settle time Rotational Delay – Waiting for the correct sector to move under the head Average 1/2 rotation HD: 5400rpm 5.6ms, 10000rpm 3ms Floppy: 300 rpm 100ms Effective Times Access Time – Sum of seek time and rotational delay Transfer Time – Actual time needed to perform the read or write Time depends on locality
Background image of page 7

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
BYU CS 345 Disc Scheduling 8 Disk scheduling When a read/write job is requested, the disk may currently be busy All pending jobs are placed in a disk queue could be scheduled to improve the utilization Disk scheduling increases the disk’s bandwidth (the amount of information that can be transferred in a set amount of time)
Background image of page 8
BYU CS 345 Disc Scheduling 9 Disk Scheduling Random Select a random request to do next Worst possible performance, but useful for comparisons FIFO Do in the order they arrive Simple and fair in that all requests are honored Poor performance if requests are not clustered Especially true in multiprogramming systems Priority Do operations for high-priority processes first Not intended to optimize disk utilization, but to meet other objectives Users may try to exploit priorities
Background image of page 9

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
BYU CS 345 Disc Scheduling 10 Disk Scheduling LIFO (Last-In First-Out) Always take the most recent request Hope to catch a sequence of reads at one time (locality) Runs risk of starvation SSTF (Shortest Service Time First) Select the read closest to the current track next
Background image of page 10
Image of page 11
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 03/02/2012 for the course C S 345 taught by Professor Ericmercer during the Winter '12 term at BYU.

Page1 / 36

CS345 11 - Disc Scheduling - Chapter 11 Disc Scheduling...

This preview shows document pages 1 - 11. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online