Operating system concept (ninth edition) part 2
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Operating system concept (ninth edition) part 2
Part FourStorage ManagementSince main memory is usually too small to accommodate all the data and programs permanently, the computer system must provi Operating system concept (ninth edition) part 2 ide secondary storage to back up main memory. Modem computer systems use disks as the primary on-line storage medium for information (both programs and data). The file system provides the mechanism for on-line storage of and access to both data and programs residing on the disks. A file is a collect Operating system concept (ninth edition) part 2 ion of related information defined by its creator. The files are mapped by the operating system onto physical devices. Files are normally organized inOperating system concept (ninth edition) part 2
to directories for ease of use.The devices that attach to a computer vary in many aspects. Some devices transfer a character or a block of characters Part FourStorage ManagementSince main memory is usually too small to accommodate all the data and programs permanently, the computer system must provi Operating system concept (ninth edition) part 2 shared. They can be read-only or read-write. They vary greatly in speed. In many ways, they are also the slowest major component of the computer.Because of all this device variation, the operating system needs to provide a wide range of functionality to applications, to allow them to control all asp Operating system concept (ninth edition) part 2 ects of the devices. One key goal of an operating system’s I/O subsystem is to provide the simplest interface possible to the rest of the system. BecaOperating system concept (ninth edition) part 2
use devices are a performance bottleneck, another key is to optimize I/O for maximum concurrency.Mass-Storage StructureThe file system can be viewed lPart FourStorage ManagementSince main memory is usually too small to accommodate all the data and programs permanently, the computer system must provi Operating system concept (ninth edition) part 2 internal data structures and algorithms used by the operating system to implement this interface. In this chapter, we begin a discussion of file systems at the lowest level: the structure of secondary storage. We first describe the physical structure of magnetic disks and magnetic tapes. We then des Operating system concept (ninth edition) part 2 cribe disk-scheduling algorithms, which schedule the order of disk I/Os to maximize performance. Next, we discuss disk formatting and management of boOperating system concept (ninth edition) part 2
ot blocks, damaged blocks, and swap space. We conclude with an examination of the structure of RAID systems.CHAPTER OBJECTIVES•To describe the physicaPart FourStorage ManagementSince main memory is usually too small to accommodate all the data and programs permanently, the computer system must provi Operating system concept (ninth edition) part 2 s.•To evaluate disk scheduling algorithms.•To discuss operating-system services provided for mass storage, including RAID.10.1 Overview of Mass-Storage structureIn this section, we present a general overview of the physical structure of secondary and tertiary storage devices.10.1.1Magnetic DisksMagn Operating system concept (ninth edition) part 2 etic disks provide the bulk of secondary storage for modern computer systems. Conceptually, disks are relatively simple (Figure 10.1). Each disk plattOperating system concept (ninth edition) part 2
er has a flat circular shape, like a CD. Common platter diameters range from 1.8 to 3.5 inches. The two surfaces of a platter are covered with a magnePart FourStorage ManagementSince main memory is usually too small to accommodate all the data and programs permanently, the computer system must provi Operating system concept (ninth edition) part 2 Moving-head disk mechanism.A read-write head “flies” just above each surface of every platter. The heads are attached to a disk arm that moves all the heads as a unit. The surface of a platter is logically divided into circular tracks, which are subdivided into sectors. The set of tracks that are a Operating system concept (ninth edition) part 2 t one arm position makes up a cylinder. There may be thousands of concentric cylinders in a disk drive, and each track may contain hundreds of sectorsOperating system concept (ninth edition) part 2
. The storage capacity of common disk drives is measured in gigabytes.When the disk is in use, a drive motor spins it at high speed. Most drives rotatPart FourStorage ManagementSince main memory is usually too small to accommodate all the data and programs permanently, the computer system must provi Operating system concept (ninth edition) part 2 as two parts. The transfer rate is the rate at which data flow between the drive and the computer. The positioning time, or random-access time, consists of two parts: the time necessary to move the disk arm to the desired cylinder, called the seek time, and the time necessary for the desired sector Operating system concept (ninth edition) part 2 to rotate to the disk head, called the rotational latency. Typical disks can transfer several megabytes of data per second, and they have seek times aOperating system concept (ninth edition) part 2
nd rotational latencies of several milliseconds.Because the disk head flies on an extremely thin cushion of air (measured in microns), there is a dangPart FourStorage ManagementSince main memory is usually too small to accommodate all the data and programs permanently, the computer system must provi Operating system concept (ninth edition) part 2 damage the magnetic surface. This accident is called a head crash. A head crash normally cannot be repaired; the entire disk must be replaced.A disk can be removable, allowing different disks to be mounted as needed. Removable magnetic disks generally consist of one platter, held in a plastic case t Operating system concept (ninth edition) part 2 o prevent damage while not in the disk drive. Other forms of removable disks include CDs, DVDs, and Blu-ray discs as well as removable flash-memory deOperating system concept (ninth edition) part 2
vices known as flash drives (which are a type of solid-state drive).10.1 Overview of Mass-Storage Structure 469A disk drive is attached to a computer Part FourStorage ManagementSince main memory is usually too small to accommodate all the data and programs permanently, the computer system must provi Operating system concept (ninth edition) part 2 niversal serial bus (USB), and fibre channel (FC). The data transfers on a bus are carried out by special electronic processors called controllers. The host controller is the controller at the computer end of the bus. A disk controller is built into each disk drive. To perform a disk I/O operation, Operating system concept (ninth edition) part 2 the computer places a command into the host controller, typically using memory-mapped I/O ports, as described in Section 9.7.3. The host controller thOperating system concept (ninth edition) part 2
en sends the command via messages to the disk controller, and the disk controller operates the disk-drive hardware to carry out the command. Disk contPart FourStorage ManagementSince main memory is usually too small to accommodate all the data and programs permanently, the computer system must provi Operating system concept (ninth edition) part 2 at fast electronic speeds, occurs between the cache and the host controller.10.1.2Solid-State DisksSometimes old technologies are used in new ways as economics change or the technologies evolve. An example is the growing importance of solid-state disks, or SSDs. Simply described, an SSD is nonvolati Operating system concept (ninth edition) part 2 le memory that is used like a hard drive. There are many variations of this technology, from DRAM with a battery to allow it to maintain its state inOperating system concept (ninth edition) part 2
a power failure through flash-memory technologies like single-level cell (SLC) and multilevel cell (MLC) chips.SSDs have the same characteristics as tPart FourStorage ManagementSince main memory is usually too small to accommodate all the data and programs permanently, the computer system must provi Operating system concept (ninth edition) part 2 ey consume less power. However, they are more expensive per megabyte than traditional hard disks, have less capacity than the larger hard disks, and may have shorter life spans than hard disks, so their uses are somewhat limited. One use for SSDs is in storage arrays, where they hold file-system met Operating system concept (ninth edition) part 2 adata that require high performance. SSDs are also used in some laptop computers to make them smaller, faster, and more energy -ef ficien t.Because SSOperating system concept (ninth edition) part 2
Ds can be much faster than magnetic disk drives, standard bus interfaces can cause a major limit on throughput. Some SSDs are designed to connect direPart FourStorage ManagementSince main memory is usually too small to accommodate all the data and programs permanently, the computer system must provi Operating system concept (ninth edition) part 2 eplacement for disk drives, while others use them as a new cache tier, moving data between magnetic disks, SSDs, and memory to optimize performance.hi the remainder of this chapter, some sections pertain to SSDs, while others do not. l or example, because SSDs have no disk head, disk-scheduling algo Operating system concept (ninth edition) part 2 rithms largely do not apply, throughput and formatting, however, do apply.10.1.3Magnetic TapesMagnetic tape was used as an early secondary-storage medOperating system concept (ninth edition) part 2
ium. Although it is relatively permanent and can hold large quantities of data, its access time is slow compared with that of main memory and magneticPart FourStorage ManagementSince main memory is usually too small to accommodate all the data and programs permanently, the computer system must provi Operating system concept (ninth edition) part 2 for secondary storage.Gọi ngay
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