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1、Silberschatz, Galvin and Gagne 200213.1Chapter 14: Mass-Storage Systems 海量存储器系统n14.1 Disk Structure 磁盘结构n14.2 Disk Scheduling 磁盘调度n14.3 Disk Management 磁盘管理n14.4 Swap-Space Management 交换空间管理n14.5 RAID Structure RAID结构n14.6 Disk Attachment 磁盘连接n14.7 Stable-Storage Implementation 稳定存储实现n14.8 Tertiary
2、Storage Devices 三级存储设备nOperating System Issues 有关操作系统的问题nPerformance Issues 有关性能的问题Silberschatz, Galvin and Gagne 200213.214.1 Disk Structure磁盘结构nDisk drives are addressed as large 1-dimensional arrays of logical blocks, where the logical block is the smallest unit of transfer.磁盘设备是以一种逻辑块的一维大数组的形式编址
3、的，这里 的逻辑块是传输的最小单位。 nThe 1-dimensional array of logical blocks is mapped into the sectors of the disk sequentially.逻辑块的一维数组映射到磁盘上一些相连的扇区。 FSector 0 is the first sector of the first track on the outermost cylinder.0扇区是最外边柱面的第一个磁道的第一个扇区。 FMapping proceeds in order through that track, then the rest of t
4、he tracks in that cylinder, and then through the rest of the cylinders from outermost to innermost. 数据首先都映射到一个磁 道，其余的数据映射到同一柱面的其他磁道，然后按照 从外向里的顺序映射到其余的柱面。Silberschatz, Galvin and Gagne 200213.3Disk Structure磁盘结构nConstant linear velocity(CLV) (恒定线速度） FDensity of bits per track is uniform FThe farther
5、a track is from the center of the disk, the greater its length, so the more sectors it can hold. F磁头越往中心移动，转速越快，以保持数据速率不变 FCD-ROM和DVD-ROM采用这种方法 nConstant angular velocity(CAV) (恒定角速度） F磁头转速不变 F为保持数据速率不变，从中心往外，数据密度由大变小 F硬盘等采用这种方法 nLow-level formatted FBlock size:512 bytes or 1024Silberschatz, Galvin
6、and Gagne 200213.414.2 Disk Scheduling磁盘调度nThe operating system is responsible for using hardware efficiently for the disk drives, this means having a fast access time and disk bandwidth.操作系统任务就是高效地使用硬件对于磁盘设备，这意 味着很短的访问时间和磁盘带宽。 nAccess time has two major components访问时间包括两个主要部分 FSeek time is the time
7、 for the disk are to move the heads to the cylinder containing the desired sector.寻道时间是指把磁头移到所需柱面的时间。 FRotational latency is the additional time waiting for the disk to rotate the desired sector to the disk head. 旋转延迟是指将磁头旋转到磁盘上指定扇 区所需的等待时间。Silberschatz, Galvin and Gagne 200213.5Disk Scheduling (Con
8、t.)nMinimize seek time 最小寻道时间nSeek time seek distance 寻道时间 寻道距离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，是用传输的总位数，除以第一个服务请求与最 后传输完成之间的总时间。（也就是单位时间内数据传输 量）Silberschatz, Galvin
9、and Gagne 200213.6Disk Scheduling (Cont.)nSeveral algorithms exist to schedule the servicing of disk I/O requests.有几种磁盘I/O请求的服务调度算法nWe illustrate them with a request queue (0-199). 98, 183, 37, 122, 14, 124, 65, 67 Head pointer 53Silberschatz, Galvin and Gagne 200213.714.2.1 FCFS Scheduling先来先服务调度nS
10、implest form. Illustration shows total head movement of 640 cylinders.如下图所示，磁头总共移动了 640个柱面的距离。Silberschatz, Galvin and Gagne 200213.814.2.2 SSTF Scheduling 最短寻道时间优先调度nShortest-seek-time-first(SSTF) algorithm nSelects the request with the minimum seek time from the current head position.选择从当前磁头位置所需寻道
11、时间最短的请求。nSSTF scheduling is a form of shortest-job-first(SJF) scheduling; SSTF是SJF调度的一种形式；nmay cause starvation of some requests.有可能引起某些请求的饥饿。nIllustration shows total head movement of 236 cylinders.如图所示，磁头移动的总距离是236柱面。nIts not optimal, 最佳磁头移动的总距离是208柱面Silberschatz, Galvin and Gagne 200213.9SSTF (Co
12、nt.)Silberschatz, Galvin and Gagne 200213.1014.2.3 SCAN Scheduling 扫描调度nThe 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.磁头从磁盘的一端开始向另一端移动，沿途响应访问请求 ，直到
13、到达了磁盘的另一端，此时磁头反向移动并继续响应 服务请求。nSometimes called the elevator algorithm.有时也称为电梯算法。nIllustration shows total head movement of 236 cylinders. 如图所示，磁头移动的总距离是236柱面。Silberschatz, Galvin and Gagne 200213.11SCAN (Cont.)Silberschatz, Galvin and Gagne 200213.1214.2.4 C-SCAN SchedulingnCircular SCAN(C-SCAN) sch
14、eduling provides a more uniform wait time than SCAN. 提供比扫描算法更均衡的等待时间。nThe head moves from one end of the disk to the other. servicing requests as it goes. When it reaches the other end, however, it immediately returns to the beginning of the disk, without servicing any requests on the return trip.磁头
15、从磁盘的一端向另一端移动，沿途响应请求。当它到了另一端，就 立即回到磁盘的开始处，在返回的途中不响应任何请求。nTreats the cylinders as a circular list that wraps around from the last cylinder to the first one.把所有柱面看成一个循环的序列，最后一个柱面接续第一个柱面。n磁头移动的总距离是183（*在从一边到另一边的变化过程中不接受任何 请求）柱面。Silberschatz, Galvin and Gagne 200213.13C-SCAN (Cont.)Silberschatz, Galvin and Gagne 200213.1414.2.5 LOOK/C-LOOK SchedulingnSCAN和C-SCAN总是将磁臂在整个盘面宽度上移动，其实 这样做并不实用。nSCAN和C-SCAN的一种改进形式：LOOK和C-LOOK。nC-LOOK: 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 th
