TAILIEUCHUNG - Abraham Silberschatz-Operating System Concepts (9th,2012.12)2
Part main memory is usually too small to accommodate all the data permanently, the computer system must provide to back up main memory. Modern computer systems use the primary on-line storage medium for information (both data). The file system provides the mechanism for on-line and access to both data and programs residing on the disks. A a collection of related information defined by its creator. The files by the operating system onto physical devices. Files are into directories for ease of useThe devices that attach to a computer vary in many aspects. transfer a character or a block of characters at a time. be accessed only sequentially, others randomly. Some synchronously, others asynchronously. Some are dedicated, . They can be read-only or read – write. They vary greatly in speedIn many ways, they are also the slowest major component of of all this device variation, the operating system needs a wide range of functionality to applications, to allow them all aspects of the devices. One key goal of an operating system’ subsystem is to provide the simplest interface possible to the rest system. Because devices are a performance bottleneck, another to optimize I/O for maximum file system can be viewed logically as consisting of three parts. In , we examine the user and programmer interface to the file system. 12, we describe the internal data structures and algorithms used operating system to implement this interface. In this chapter, we begin of file systems at the lowest level: the structure of secondary storageWe first describe the physical structure of magnetic disks and magnetic tapesWe then describe disk-scheduling algorithms, which schedule the order I/Os to maximize performance. Next, we discuss disk formatting of boot blocks, damaged blocks, and swap space. We an examination of the structure of RAID OBJECTIVES.• To describe the physical structure of secondary storage devices and on the uses of the devices• To explain the performance characteristics of mass-storage devices• To evaluate disk scheduling algorithms• To discuss operating-system services provided for mass storage, Overview of Mass-Storage this section, we present a general overview of the physical structure and tertiary storage DisksMagnetic disks provide the bulk of secondary storage for modern . Conceptually, disks are relatively simple (Figure ). Each has a flat circular shape, like a CD. Common platter diameters to inches. The two surfaces of a platter are covered with a . We store information by recording it magnetically on the 10 Mass-Storage tspindlearm Moving-head disk read –write head “flies” just above each surface of every platter. are attached to a disk arm that moves all the heads as a unit. The a platter is logically divided into circular tracks, which are subdivided . The set of tracks that are at one arm position makes up a cylinderThere may be thousands of concentric cylinders in a disk drive, and each contain hundreds of sectors. The storage capacity of common disk measured in gigabytesWhen the disk is in use, a drive motor spins it at high speed. Most 60 to 250 times per second, specified in terms of rotations per minute.(RPM). Common drives spin at 5,400, 7,200, 10,000, and 15,000
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