操作系统还行的课件os012mass storage system

1、12.2Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 2005n文件系统从逻辑上来看，可以分成3大部分lCh10. 用户和程序所使用的文件系统接口lCh11.操作系统实现文件系统该接口所用到的数据结构和算法lCh12.最底层存储结构：次级和3级存储结构= todays topic12.3Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 2005Chapter 12: M

2、ass-Storage SystemsnOverview of Mass Storage Structure 大容量存储器据结构简介nDisk Structure 磁盘结构nDisk Attachment 磁盘与系统的连接（附属）nDisk Scheduling 磁盘调度nDisk Management 磁盘管理nSwap-Space Management 交换空间管理nRAID Structure RAID结构nStable-Storage Implementation 稳定存储实现nTertiary Storage Devices 三级存储结构12.4Silberschatz, Galvi

3、n and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 2005nMagnetic disks provide bulk of secondary storage of modern computers磁盘为现代计算机系统提供了大容量的外存lDrives rotate at 60 to 200 times per second 每秒旋转速度lTransfer rate is rate at which data flow between drive and computer 传输速度：在驱动器和计算机之间的数据传输速率lPos

4、itioning time (random-access time) is time to move disk arm to desired cylinder (seek time) and time for desired sector to rotate under the disk head (rotational latency)定位时间（随机访问时间）：寻道时间+旋转等待时间lHead crash results from disk head making contact with the disk surface 磁头碰撞：可能损坏磁盘4Thats badnDisks can be

5、 removablenDrive attached to computer via I/O bus 磁盘驱动器通过磁盘驱动器通过I/O总线与计算机相连总线与计算机相连lBusses vary, including EIDE, ATA, SATA, USB, Fibre Channel, SCSIlHost controller in computer uses bus to talk to disk controller built into drive or storage array 主机控制器通过总线与磁盘控制器相连12.5Silberschatz, Galvin and Gagne 2

6、005Operating System Concepts 7th Edition, Jan 1, 200512.6Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 2005nMagnetic tape磁带：早期使用，主要用于备份lWas early secondary-storage mediumlRelatively permanent and holds large quantities of datalAccess time slowlRandom access 1000 ti

7、mes slower than disklMainly used for backup, storage of infrequently-used data, transfer medium between systemslKept in spool and wound or rewound past read-write headlOnce data under head, transfer rates comparable to diskl20-200GB typical storagelCommon technologies are 4mm, 8mm, 19mm, LTO-2 and S

8、DLT12.7Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 2005nDisk drives are addressed as large 1-dimensional arrays of logical blocks, where the logical block is the smallest unit of transfer. 磁盘驱动器可以看作是一个一维的逻辑块的数组，逻辑块是最小的传输单位nThe 1-dimensional array of logical block

9、s is mapped into the sectors of the disk sequentially.一维逻辑块数组按顺序映射到磁盘的扇区lSector 0 is the first sector of the first track on the outermost cylinder.扇区0是最外面柱面的第一个磁道的第一个扇区lMapping proceeds in order through that track, then the rest of the tracks in that cylinder, and then through the rest of the cylind

10、ers from outermost to innermost.映射顺序是先按磁道内扇区顺序，再按柱面内磁道顺序，最后按从外到内的柱面顺序来排序的。12.8Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 2005nThe operating system is responsible for using hardware efficiently for the disk drives, this means having a fast access time and disk ba

11、ndwidth.操作系统的任务之一就是有效的使用硬件。对于磁盘驱动器来说，意味着较快的访问速度和较宽的磁盘带宽nAccess time has two major components访问时间分两部分lSeek time is the time for the disk are to move the heads to the cylinder containing the desired sector.寻道时间lRotational latency is the additional time waiting for the disk to rotate the desired sector

12、 to the disk head.旋转延迟nDisk 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.磁盘带宽：传递的总字节数/服务请求开始到最后传递结束的总时间12.9Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 2

13、005nSeveral algorithms exist to schedule the servicing of disk I/O requests. nWe illustrate them with a request queue (0-199).98, 183, 37, 122, 14, 124, 65, 67Head pointer 5312.10Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 2005Illustration shows total head moveme

14、nt of 640 cylinders.12.11Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 2005例：一个具有40个柱面的磁盘。假设一个读柱面11上的数据块的请求到达。当对柱面11寻道时，又顺序到达了新的请求，要求对柱面1，36，16，34，9，12上的数据进行访问。问磁头总共需要移动多少个柱面？答案：11112.12Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1

15、, 2005n最短寻道时间优先算法：nSelects the request with the minimum seek time from the current head position.选择与磁头位置最近的待处理请求nSSTF scheduling is a form of SJF scheduling; may cause starvation of some requests. 基本上是一种短作业有线调度，可能会产生饥饿nIllustration shows total head movement of 236 cylinders.12.13Silberschatz, Galvin

16、 and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 200512.4 Disk Scheduling磁盘调度磁盘调度初始情况初始情况：磁头位于53号磁道当前请求队列中离磁头位置最近的IO请求： 磁道65磁头移动距离 = 65-53 = 12当前总移动距离 =1212.14Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 200512.4 Disk Scheduling磁盘调度磁盘调度磁头位于65号磁道当前请求队列中

17、离磁头位置最近的IO请求： 磁道67磁头移动距离 = 67-65 = 2当前总移动距离 =12 + 2 = 1412.15Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 200512.4 Disk Scheduling磁盘调度磁盘调度磁头位于67号磁道当前请求队列中离磁头位置最近的IO请求： 磁道37磁头移动距离 = 67-37 = 30当前总移动距离 =14+30 = 4412.16Silberschatz, Galvin and Gagne 2005Operating Syst

18、em Concepts 7th Edition, Jan 1, 200512.4 Disk Scheduling磁盘调度磁盘调度磁头位于37号磁道当前请求队列中离磁头位置最近的IO请求： 磁道14磁头移动距离 = 37-14 = 23当前总移动距离 =44+23 = 6712.17Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 200512.4 Disk Scheduling磁盘调度磁盘调度磁头位于14号磁道当前请求队列中离磁头位置最近的IO请求： 磁道98磁头移动距离 = 98-

19、14 = 84当前总移动距离 =67+84 = 15112.18Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 200512.4 Disk Scheduling磁盘调度磁盘调度磁头位于98号磁道当前请求队列中离磁头位置最近的IO请求： 磁道122磁头移动距离 = 122-98 = 24当前总移动距离 =151 + 24 = 17512.19Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition

20、, Jan 1, 200512.4 Disk Scheduling磁盘调度磁盘调度磁头位于122号磁道当前请求队列中离磁头位置最近的IO请求： 磁道124磁头移动距离 = 124-122 = 2当前总移动距离 =175+2 = 17712.20Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 200512.4 Disk Scheduling磁盘调度磁盘调度磁头位于124号磁道当前请求队列中离磁头位置最近的IO请求： 磁道183磁头移动距离 = 183-124 = 59当前总移动距离

21、=177+59 = 23612.21Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 2005例：一个具有40个柱面的磁盘。假设一个读柱面11上的数据块的请求到达。当对柱面11寻道时，又顺序到达了新的请求，要求对柱面1，36，16，34，9，12上的数据进行访问。问磁头总共需要移动多少个柱面？访问序列：11,12,9,16,1,34,3612.22Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edit

22、ion, Jan 1, 2005nThe 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.磁臂从磁盘的一端向另一端移动，当磁头移过每一个柱面时，处理位于该柱面上的请求服务。当到达另一端时，改变方向继续处理nSometimes called the eleva

23、tor algorithm.12.23Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 200512.24Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 2005例：一个具有40个柱面的磁盘。假设一个读柱面11上的数据块的请求到达。当对柱面11寻道时，又顺序到达了新的请求，要求对柱面1，36，16，34，9，12上的数据进行访问。问磁头总共需要移动多少个柱面？访问序列：

24、（最初方向向大柱面）11,12,14,34,36,(40),9,112.25Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 2005nProvides a more uniform wait time than SCAN.提供了一种更均匀的等待时间nThe head moves from one end of the disk to the other. servicing requests as it goes. When it reaches the other end, ho

25、wever, it immediately returns to the beginning of the disk, without servicing any requests on the return trip.C-SCAN也是将磁头从磁盘一端移到另一端，随着移动不断的处理请求。但是，当磁头移到另一端时，会马上返回到磁盘开始，返回时不处理请求nTreats the cylinders as a circular list that wraps around from the last cylinder to the first one.将柱面当作一个环链，最后的柱面和第一个相连12.2

26、6Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 200512.27Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 2005nVersion of C-SCANnArm only goes as far as the last request in each direction, then reverses direction immediately, without

27、first going all the way to the end of the disk. 磁头只移动到一个方向上最远的请求为止，接着马上返回，而不是移动到磁盘的尽头12.28Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 200512.29Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 2005nSSTF is common and has a natural a

28、ppeal SSTF较为普通且有吸引力nSCAN and C-SCAN perform better for systems that place a heavy load on the disk.对于负荷较大的系统执行的好（无饥饿）nPerformance depends on the number and types of requests.性能主要依赖于请求的数量和类型nRequests for disk service can be influenced by the file-allocation method.受文件分配方法的影响nThe disk-scheduling algor

29、ithm should be written as a separate module of the operating system, allowing it to be replaced with a different algorithm if necessary.磁盘调度算法应作为操作系统的一个独立模块，这样必要的时候可以替换为另一个算法nEither SSTF or LOOK is a reasonable choice for the default algorithm. SSTF和LOOK是比较合理的默认算法12.30Silberschatz, Galvin and Gagne

30、2005Operating System Concepts 7th Edition, Jan 1, 2005n为了减少旋转延迟，对同一磁道上的连续读写信息进行合理分布称为旋转优化。n例：一个磁道上有10个扇区，有10个数据块A，B，。，J存放在各个扇区内。如果按照从A到J的顺序从0扇区开始依次存放，并且顺序处理这些数据。12.31Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 2005n当磁盘旋转速度为20毫秒（一周），处理程序读出每个记录后花4毫秒时间处理。则读出并处理数据块A后

31、，将转到数据块D的开始，所以，为了读出B，必须再转一周。于是处理10个数据块的总时间为：n10*(2（读）+4（处理）+16（旋转）)=220ms12.32Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 2005n如果采用旋转优化，采用这种顺序存放：A，H，E，B，I，F，C，J，G，D，则读出A后，恰好转到了B的开始。因此，总共的时间变成了n10*(2+4)=60ms12.33Silberschatz, Galvin and Gagne 2005Operating System

32、Concepts 7th Edition, Jan 1, 200512.34Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 2005习题习题12.35Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 2005n什么是文件的逻辑结构和物理结构？l文件的逻辑结构是从用户的角度看，一个文件是如何构成，一是把文件看成由若干个记录构成的文件；二是把文件看成流式文件。l物理结构是从系

33、统的角度来看，一个文件所占盘块在外存上的分布形式。有连续、链接、索引和混合等形式。12.36Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 2005n什么是绝对路径和相对路径？l在树型目录结构中，从根开始的路径名称为绝对路径；而相对于当前目录（工作目录）的路径名为相对路径。l假设当前目录/home/wang/testl需要访问当前目录下的hello.cpp文件l绝对路径为/home/wang/test/hello.cppl相对路径为./hello.cpp12.37Silbersch

34、atz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 2005n在linux系统中，把输入输出设备看作（）A、普通文件B、目录文件C、索引文件D、特殊文件n在linux中，按文件的内部构造和处理方式把文件分成3类：普通文件、目录文件和特殊文件。特殊文件是为了统一管理I/O设备而对设备的抽象。12.38Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 2005n（ ）不是文件系统的功能之一？A、方

35、便用户使用信息B、提供用户共享信息的手段C、提高信息安全程序D、分配磁盘的存储空间E、驱动外部设备n了解文件系统的基本功能；还包括文件操作和使用。E不是。12.39Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 2005n文件系统采用树型目录结构后，对于不同用户的文件，其文件名（）A、应该相同B 应该不同C、可以相同也可以不同D 受系统约束n答案：C12.40Silberschatz, Galvin and Gagne 2005Operating System Concepts 7

36、th Edition, Jan 1, 2005n从用户角度看，文件系统主要是实现（）单选A. 文件保护B. 文件保密C. 文件共享D. 按名存取n答案：D12.41Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 2005n在文件管理中可以利用位示图来实现（）A、磁盘的调度B、磁盘空间的管理C、文件的存取D、文件目录的查找n文件相应存储空间常用的方法包括：空闲表法、空闲链法、位示图和成组链接法n答案：B12.42Silberschatz, Galvin and Gagne 2005O

37、perating System Concepts 7th Edition, Jan 1, 2005n文件系统中用（ ）管理文件。A.作业控制块B.外页表C.目录D.软硬件结合的方法n答案：C12.43Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 2005n物理文件的组成方式是由（）确定的。A.应用程序B.主存容量C.外存容量D.操作系统n答案：D12.44Silberschatz, Galvin and Gagne 2005Operating System Concepts 7t

38、h Edition, Jan 1, 2005n若用8个字（字长32位）的位示图管理内存，假定用户归还一个块号为100的内存块，它对应的位示图的位置是（）A.字号为3，位号为5B.字号为4，位号为4C.字号为3，位号为4D.字号为4，位号为5n答案：B12.45Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 2005文件的二级目录结构由（）和（）组成。 A.根目录B.子目录C.主文件目录D.用户文件目录E.当前目录 n答案：C D12.46Silberschatz, Galvin a

39、nd Gagne 2005Operating System Concepts 7th Edition, Jan 1, 2005课堂作业课堂作业nP421 12.2 12.47Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 2005nLow-level formatting, or physical formatting Dividing a disk into sectors that the disk controller can read and write.低级格式化（物理格

40、式化）：把磁盘分成扇区，以便于磁盘控制器读和写n每个扇区都有其数据结构l头：包括组成扇区三维地址的三个数字：扇区所在的磁头（或盘面）、磁道（或柱面号）以及扇区在磁道上的位置即扇区号。头标中还包括一个字段，其中有显示扇区是否能可靠存储数据，或者是否已发现某个故障因而不宜使用的标记。有些硬盘控制器在扇区头标中 还记录有指示字，可在原扇区出错时指引磁盘转到替换扇区或磁道。最后，扇区头标以循环冗余校验（CRC）值作为结束，以供控制器检验扇区头标的读出情况， 确保准确无误。l数据区域l尾：保护数据的纠错码（ECC）。在初始准备期间，计算机用512个虚拟信息字节（实际数据的存放地）和与这些虚拟信息字节相应

41、的ECC数字填入这个部分。12.48Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 2005nTo use a disk to hold files, the operating system still needs to record its own data structures on the disk.lPartition the disk into one or more groups of cylinders.分区：由一个或多个柱面构成lLogical formattin

42、g or “making a file system”.逻辑格式化：安装文件系统12.49Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 2005n扇区到来时，磁盘控制器读出每个扇区的头标，把这些头标中的地址信息与期待检出的磁头和柱面号做比较（即寻道），然后，寻找要求的扇区号。n找到扇区后，磁盘控制器必须在继续寻找下一个扇区之前对该扇区的信息进行处理。如果是读数据，控制器计算此数据的ECC码，然后，把ECC码与已记录的ECC码相比较。如果是写数据，控制器计算出此数据的ECC码，与数

43、据一起存储。在控制器对此扇区中的数据进行必要处理期间，磁 盘继续旋转。12.50Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 2005nBoot block initializes system.lThe bootstrap is stored in ROM.自举保存在只读存储器（ROM）中lBootstrap loader program.自举加载程序l启动块（主引导记录MBR）：磁盘的固定位置，放置引导程序，还包含硬盘分区列表和说明系统引导分区的标志l引导分区：包含操作系统和

44、设备分配程序12.51Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 200512.52Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 2005磁盘有移动部件并且容错能力小，所以经常有缺陷扇磁盘有移动部件并且容错能力小，所以经常有缺陷扇区，坏扇区区，坏扇区对于简单磁盘如使用对于简单磁盘如使用IDE 控制器的磁盘控制器的磁盘, 坏扇区可手坏扇区可手工处理工处理 forma

45、t, chkdsk更复杂的磁盘如更复杂的磁盘如 SCSI 磁盘磁盘, 利用利用扇区备用扇区备用 或或 扇区滑扇区滑动动 来处理坏扇区来处理坏扇区扇区备用扇区备用 可能会使操作系统的磁盘调度算法无效可能会使操作系统的磁盘调度算法无效硬错误硬错误12.53Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 200553n交换空间交换空间:虚拟内存使用磁盘空间作为对主存的扩展虚拟内存使用磁盘空间作为对主存的扩展n交换空间可以两种方式实现交换空间可以两种方式实现l在普通文件系统上加以创建在普通

46、文件系统上加以创建4e.g. Windows family, Solaris 24简单，效率低简单，效率低l创建在独立的磁盘分区上创建在独立的磁盘分区上4e.g. Unix, Linux, Solaris 24优化速度优化速度12.54Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 200554nSolaris 2 仅在一页被交换出物理内存的时候分配交仅在一页被交换出物理内存的时候分配交换空间，而不是在虚拟内存页最初生成的时候换空间，而不是在虚拟内存页最初生成的时候12.55Sil

47、berschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 2005nLinuxLinux的内核态内存空间不允许换出的内核态内存空间不允许换出nLinuxLinux的交换空间：保存换出的页面的交换空间：保存换出的页面l使用整个块设备，称作交换设备使用整个块设备，称作交换设备l使用一个固定长度的文件，称作交换文件使用一个固定长度的文件，称作交换文件l交换设备和交换文件的内部格式是一致的交换设备和交换文件的内部格式是一致的l前前40964096个字节是一个以字符串个字节是一个以字符串“SWAP_SPAC

48、ESWAP_SPACE”结尾的结尾的位图位图l位图的每一位对应于一个交换空间的页面位图的每一位对应于一个交换空间的页面lLinuxLinux最多允许管理最多允许管理MAX_SWAPFILESMAX_SWAPFILES（缺省值为缺省值为8 8）个）个交换空间交换空间n交换设备远比交换文件更加有效交换设备远比交换文件更加有效12.56Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 200512.57Silberschatz, Galvin and Gagne 2005Operating

49、 System Concepts 7th Edition, Jan 1, 2005nRAID multiple disk drives provides reliability via redundancy.nRAID is arranged into six different levels.12.58Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 2005nSeveral improvements in disk-use techniques involve the use o

50、f multiple disks working cooperatively.nDisk striping uses a group of disks as one storage unit.nRAID schemes improve performance and improve the reliability of the storage system by storing redundant data.lMirroring or shadowing keeps duplicate of each disk.lBlock interleaved parity uses much less

51、redundancy.12.59Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 200512.60Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 200512.61Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 2005nWrite-ahead log sc

52、heme requires stable storage.nTo implement stable storage:lReplicate information on more than one nonvolatile storage media with independent failure modes.lUpdate information in a controlled manner to ensure that we can recover the stable data after any failure during data transfer or recovery.12.62

53、Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 2005nLow cost is the defining characteristic of tertiary storage.nGenerally, tertiary storage is built using removable medianCommon examples of removable media are floppy disks and CD-ROMs; other types are available.12.

54、63Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 2005nFloppy disk thin flexible disk coated with magnetic material, enclosed in a protective plastic case.lMost floppies hold about 1 MB; similar technology is used for removable disks that hold more than 1 GB.lRemovab

55、le magnetic disks can be nearly as fast as hard disks, but they are at a greater risk of damage from exposure.12.64Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 2005nA magneto-optic disk records data on a rigid platter coated with magnetic material.lLaser heat is u

56、sed to amplify a large, weak magnetic field to record a bit.lLaser light is also used to read data (Kerr effect).lThe magneto-optic head flies much farther from the disk surface than a magnetic disk head, and the magnetic material is covered with a protective layer of plastic or glass; resistant to

57、head crashes.nOptical disks do not use magnetism; they employ special materials that are altered by laser light.12.65Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 2005nThe data on read-write disks can be modified over and over.nWORM (“Write Once, Read Many Times”)

58、disks can be written only once.nThin aluminum film sandwiched between two glass or plastic platters.nTo write a bit, the drive uses a laser light to burn a small hole through the aluminum; information can be destroyed by not altered.nVery durable and reliable.nRead Only disks, such ad CD-ROM and DVD

59、, com from the factory with the data pre-recorded.12.66Silberschatz, Galvin and Gagne 2005Operating System Concepts 7th Edition, Jan 1, 2005nCompared to a disk, a tape is less expensive and holds more data, but random access is much slower.nTape is an economical medium for purposes that do not requi

60、re fast random access, e.g., backup copies of disk data, holding huge volumes of data.nLarge tape installations typically use robotic tape changers that move tapes between tape drives and storage slots in a tape library.lstacker library that holds a few tapeslsilo library that holds thousands of tap