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1、Chapter 14: Mass-Storage Systems,Disk Structure Disk Scheduling Disk Management S Management RAID Structure Disk Attachment Stable-Storage Implementation Tertiary Storage Devices Operating System Issues Performance Issues,1 9/27/2020,Disk Structure,Disk drives are addressed as large 1-dimensional ar
2、rays of logical blocks, where the logical block is the smallest unit of transfer. The 1-dimensional array of logical blocks is mapped into the sectors of the disk sequentially. Sector 0 is the first sector of the first track on the outermost cylinder. Mapping proceeds in order through that track, th
3、en the rest of the tracks in that cylinder, and then through the rest of the cylinders from outermost to innermost.,2 9/27/2020,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. 操作系统有责任高效地使
4、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。 Access time has two major components 访问时间包括两个主要部分 Seek time is the time 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。 Rotational latency is the additional time waiting for the disk to rotate the desired sector to the
5、 disk head. 旋转延迟是指等待磁盘上所需要的扇区旋转到磁头下面的时间。,3 9/27/2020,Disk Scheduling,Minimize seek time 最小寻道时间 Seek time seek distance 寻道时间 寻道距离 Disk 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
6、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。,4 9/27/2020,Disk Scheduling (Cont.),Several algorithms exist to schedule the servicing of disk I/O requests. We illustrate them with a request queue (0-199). 98, 183, 37, 122, 14, 124, 65, 67 Head pointer 53,5 9/27/2020,FCFS,Illustration shows total head movement of 640
7、cylinders.,6 9/27/2020,SSTF,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. Illustration shows total head movement of 236 cylinders.,7 9/27/2020,SSTF (Cont.),8 9/27/2020,SCAN,The disk a
8、rm 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. Sometimes called the elevator algorithm(电梯算法). Illustration shows total head movement of 208 cylinders.,9 9/27
9、/2020,SCAN (Cont.),10 9/27/2020,C-SCAN,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
10、return trip. Treats the cylinders as a circular list that wraps around from the last cylinder to the first one.,11 9/27/2020,C-SCAN (Cont.),12 9/27/2020,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 t
11、he way to the end of the disk.,13 9/27/2020,C-LOOK (Cont.),14 9/27/2020,Selecting a Disk-Scheduling Algorithm,SSTF is common and has a natural appeal SCAN and C-SCAN perform better for systems that place a heavy load on the disk. Performance depends on the number and types of requests. Requests for
12、disk service can be influenced by the method. 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.,15 9/27/2020,Disk Mana
13、gement,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
14、formatting or “making a ”. Boot block initializes system. The bootstrap is stored in ROM. Bootstrap loader program. Methods such as sector sparing used to handle bad blocks.,16 9/27/2020,MS-DOS Disk Layout,17 9/27/2020,S Management,S Virtual memory uses disk space as an extension of main memory. S c
15、an be carved out of the normal , more commonly, it can be in a separate disk partition. S management 4.3BSD allocates s when process starts; holds text segment (the program) and data segment. Kernel uses s to track s use. Solaris 2 allocates s only when a page is forced out of physical memory, not w
16、hen the virtual memory page is first created.,18 9/27/2020,4.3 BSD Text-Segment S,19 9/27/2020,4.3 BSD Data-Segment S,20 9/27/2020,RAID Structure,RAID multiple disk drives provides reliability via redundancy. RAID is arranged into six different levels.,21 9/27/2020,RAID (cont),Several improvements in disk-use techniques involve the use of multiple disks working cooperatively. Disk striping uses a group of disks as one storage unit.
