Mass Storage Structure Two - Computer Operating Systems - Lecture Slides, Slides of Operating Systems

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Lecture 20

Ch. 12: Mass Storage Structure

Ch 13: I/O Systems

Moving-head Disk Mechanism

• Booting from a Disk in Windows

Swap-Space Management

• Swap-space
  • Virtual memory uses disk space as an extension of main memory
• Swap-space can be carved out of the normal file system, or,
  • more commonly, it can be in a separate disk partition
• Swap-space management
  • BSD allocates swap space when process starts;
    • holds text segment (the program) and data segment
  • Kernel uses swap maps to track swap-space use
  • Solaris 2 allocates swap space only when a page is forced out of physical memory, - not when the virtual memory page is first created.

RAID Structure

• Redundant Array of Independent Disks
  • multiple disk drives provides reliability via redundancy
• Several improvements in disk-use techniques involve the use of multiple disks working cooperatively.
• Disk striping uses a group of disks as one storage unit.
• RAID schemes improve performance and improve the reliability of the storage system by storing redundant data. - Mirroring or shadowing keeps duplicate of each disk. - Block interleaved parity uses much less redundancy.

RAID Levels

Tertiary Storage Devices

• Low cost is the defining characteristic of tertiary storage.
• Generally, tertiary storage is built using removable media
• Common examples of removable media are
  • floppy disks and CD-ROMs

Removable Disks

• Floppy disk
  • thin flexible disk coated with magnetic material
  • enclosed in a protective plastic case
• magneto-optic disk
  • records data on a rigid platter coated with magnetic material
  • Laser heat is used to amplify a large, weak magnetic field to record a bit.
  • Laser light is also used to read data (Kerr effect)
  • magneto-optic head flies much farther from the disk surface than a magnetic disk head,
  • magnetic material is covered with a protective layer of plastic or glass; resistant to head crashes
• Optical disks do not use magnetism; they employ special materials that are altered by laser light.

Tapes

• Compared to a disk, a tape is less expensive and holds more data, - random access is much slower
• Tape is an economical medium for purposes that do not require fast random access, - e.g., backup copies of disk data, holding huge volumes of data
• Large tape installations typically use robotic tape changers that move tapes between tape drives and storage slots in a tape library Docsity.com

Operating System Issues

• Major OS jobs are to manage physical devices and to present a virtual machine abstraction to applications
• For hard disks, the OS provides two abstraction: - Raw device - an array of data blocks. - File system - OS queues and schedules the interleaved requests from several applications.

Tape Drives

• The basic operations for a tape drive differ from those of a disk drive
• locate positions the tape to a specific logical block, not an entire track - corresponds to seek
• The read position operation returns the logical block number where the tape head is
• The space operation enables relative motion
• Tape drives are “append-only” devices; Docsity.com

File Naming

• The issue of naming files on removable media is especially difficult when we want to write data on a removable cartridge on one computer, and then use the cartridge in another computer.
• Contemporary OSs generally leave the name space problem unsolved for removable media, - depend on applications and users to figure out how to access and interpret the data. Docsity.com

Speed

• Two aspects of speed in tertiary storage are bandwidth and latency
• Bandwidth is measured in bytes per second.
  • Sustained bandwidth
    • average data rate during a large transfer; # of bytes/transfer time
    • Data rate when the data stream is actually flowing.
  • Effective bandwidth

Speed (Cont.)

• Access latency – amount of time needed to locate data. - Access time for a disk requires to move the arm to the selected cylinder and wait for the rotational latency - Access on tape requires winding the tape reels until the selected block reaches the tape head - Generally say that random access within a tape cartridge is about a thousand times slower than random access on disk.