操作系统第九次作业

操作系统第九次作业

问题一： Consider a file system where a file can be deleted and its disk space reclaimed while links to that file still exist. What problems may occur if a new file is created in the same storage area or with the same absolute path name? How can these problems be avoided?

答：

这将导致通过该路径渴望获得旧文件的用户访问得到的是新文件；

解决的办法可以是：

• 文件删除并且磁盘空间释放之后，立即删除所有相关的链接
• 如果该文件为共享文件，则对当前文件的共享用户数进行计数，当有一个用户删除该文件时，计数减一，当计数为零时真正删除该文件。

问题二： Consider a file system that uses a modified contiguous-allocation scheme with support for extents. A file is a collection of extents, with each extent corresponding to a contiguous set of blocks. A key issue in such system is the degree of variability in the size of the extents. What are the advantages and disadvantages of the following schemes?

​ a. All extents are of the same size, and the size is predetermined.

​ b. Extents can be of any size and are allocated dynamically.

​ c. Extents can be of a few fixed sizes, and these sizes are predetermined.

答：

问题三：What are the advantages of the variant of linked allocation that uses a FAT to chain together the blocks of a file?

答：

优点：

• 在访问存储在文件中间的块时，能通过追寻存储在FAT中的指针来确定位置，可以做到直接存取通常情况下，大部分的FAT可以被缓存在内存中，因此可以只通过内存访问来确定指针，而不必访问磁盘块，速度也会快很多。

问题四：Consider a system where free space is kept in a free-space list.

​ a. Consider a file system similar to the one used by UNIX with indexed allocation. How many disk I/O operations might be required to read the contents of a small local file at /a/b/c? Assume that none of the disk blocks is currently being cached.

​ b. Suggest a scheme to ensure that the pointer to the free space list is never lost as a result of memory failure.

答：

a、读取本地小文件/a/b/c的内容涉及4个磁盘操作:读取包含根目录所在的磁盘块找到a目录所在磁盘块，读取a目录所在磁盘块找到b目录所在磁盘块，读取b目录所在磁盘块找到文件c的FCB、读取包含文件c的磁盘块
到b目录所在磁盘块，读取b目录所在磁盘块找到文件c的FCB、读取包含文件c的磁盘块
b、
将空闲空间列表的指针存放在磁盘上，且可以备份几份。