Mass-Storage Systems

Mass-Storage Systems

1. Tecnológico de Costa Rica, Carrera: Ingeniería en Computación, Sede: Cartago, Curso: Principios de sistemas operativos, Profesor: Esteban Arias Méndez, Estudiante: Jason David Espinoza Siles, Carné: 201223941, II semestre, 2016, Libro: Operating System Concepts: capítulo 10. Reporte #10

2. Disk Structure

   • Disk drives are addressed as large 1-dimensional arrays of logical blocks, where the logical block is the smallest unit of transfer
   • The 1-dimensional array of logical blocks is mapped into the sectors of the disk sequentially

3. Disk Attachment

   • SCSI itself is a bus, up to 16 devices on one cable, SCSI initiator requests operation and SCSI targets perform tasks
   • Host-attached storage accessed through I/O ports talking to I/O busses

4. Storage Array

   • Can just attach disks, or arrays of disks  Storage Array has controller(s), provides features to attached host(s)

5. Storage Area Network

   • Common in large storage environments
   • Multiple hosts attached to multiple storage arrays - flexible
   • SAN is one or more storage arrays
   • Storage made available via LUN Masking from specific arrays to specific servers

6. Network-Attached Storage

   • Network-attached storage (NAS) is storage made available over a network rather than over a local connection (such as a bus)

7. Disk Scheduling

   • The operating system is responsible for using hardware efficiently — for the disk drives, this means having a fast access time and disk bandwidth
   • Disk bandwidth is the total number of bytes transferred, divided by the total time between the first request for service and the completion of the last transfer

8. Shortest Seek Time First

   • selects the request with the minimum seek time from the current head position

9. SCAN

   • The disk arm starts at one end of the disk, and moves toward the other end, servicing requests until it gets to the other end of the disk, where the head movement is reversed and servicing continues.
     • SCAN algorithm Sometimes called the elevator algorithm
   • C-SCAN
     • Provides a more uniform wait time than SCAN  The head moves from one end of the disk to the other, servicing requests as it goes

10. CLOOK

    • Arm only goes as far as the last request in each direction, then reverses direction immediately, without first going all the way to the end of the disk

11. Disk Management

    • Low-level formatting, or physical formatting — Dividing a disk into sectors that the disk controller can read and write
    • To use a disk to hold files, the operating system still needs to record its own data structures on the disk
    • Raw disk access for apps that want to do their own block management, keep OS out of the way (databases for example)

12. 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 (raw)

13. RAID Structure

    • RAID – redundant array of inexpensive disks
    • Mean time to repair – exposure time when another failure could cause data loss  Mean time to data loss based on above factors  If mirrored disks fail independently, consider disk with 1300,000 mean time to failure and 10 hour mean time to repair

14. Stable-Storage Implementation

    • Write-ahead log scheme requires stable storage  Stable storage means data is never lost (due to failure, etc)
    • If failure occurs during block write, recovery procedure restores block to consistent state

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