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Chapter 13: Mass Storage Structure

Disk Structure

Disk Attachment

Disks may be attached one of two ways:

• Host attached via an I/O port
• Network attached via a network connection

Network-Attached Storage

Storage-Area Network

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.

Access time has two major components ：

• Seek time is the time for the disk are to move the heads to the cylinder containing the desired sector. 寻道

• Rotational latency is the additional time waiting for the disk to rotate the desired sector to the disk head. 旋转时延

  两者均为机械运动

Access time 中，还有transfer time,属于电子运动，时延很短

–>Minimize seek time–>Seek time ≈ seek distance -->

Disk Scheduling Several algorithms exist to schedule the servicing of disk I/O requests :

FCFS

SSTF (Shortest Seek Time First)

Selects the request with the minimum seek time from the current head position.

SSTF scheduling is a form of SJF scheduling; may cause starvation of some requests.

SCAN (elevator algorithm)

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.

C-SCAN (circular)

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. When it reaches the other end, however, it immediately returns to the beginning of the disk, without servicing any requests on the return trip.

C-LOOK

Version of C-SCAN

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.

Selecting a Disk-Scheduling Algorithm

The disk-scheduling algorithm should be written as a separate module of the operating system, allowing it to be replaced with a different algorithm if necessary.

Either SSTF or LOOK is a reasonable choice for the default algorithm.

Disk Formatting

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.

• Partition the disk into one or more groups of cylinders.
• Logical formatting or “making a file system”.

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