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COMP 314 - U3C11

Operating System Concepts, Seventh Edition

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COMP 314 – Unit 3 Chapter 11 (File System Implementation) File System Structure (411) Disks provide the bulk of secondary storage on which a file system is maintained. They have two characteristics: A disk can be rewritten in place. A disk can access directly any given block of information it contains. Thus, it is possible to access any given block sequentially or randomly. In order to improve efficiency, transfers between memory and disk are performed in blocks. (412) Each block has one or more sectors, whose size varies depending on the disk drive. To provide efficient and convenient access, the OS imposes one or more file systems to allow data to be stored, loacted, and then retrieved easily. A file system imposes two design problems: Defining how the file system should look to the user. This task involves defining a file and its attributes, operations allowed, etc. Creating algorithms and data structures to map the logical file system onto the physical secondary-storage devices. The file system itself is generally composed of many different levels. The lowest level, the I/O control, consists of device drivers and interrupt handlers to transfer information between the main memory and the disk system. Its input consists of high level commands such as “retrieve block 123.” Its output consists of low level, hardware specific instructions that are used by the hardware controller, which interfaces the I/O device to the rest of the system. The basic file system needs only to issue generic commands to the appropriate device driver to read and write physical blocks on the disk. Each physical block is identified by its numeric disk address (for example, drive 1, cylinder 73, track 2, sector 10). The file-organization module knows about files and their logical blocks, as well as physical blocks. (413) By knowing the type of file allocation used and the location of the file, the file-organization module can translate logical block addresses into physical block addresses for the basic file system to transfer. The file-organization module also includes the free-space manager, which tracks unallocated blocks and provides these blocks to the file-organization module when requested. The logical file system manages metadata information. Metadata includes all of the file-system structure except the actual data (or contents of the files). The logical file system manages the directory structure to provide the file-organization module with the information with the information the latter needs, given a symbolic file name. It maintains a file-structure via file-control blocks. A file control block (FCB) contains information about th efile, including ownership, permissions, and location of file
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COMP 314 - U3C11 - COMP 314 Unit 3 Chapter 11(File System...

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