Lab 5: Tutorial

This is ONLY a preview of the tutorial. If you would like to view the entire tutorial, you must purchase the tutorial.

Understanding the File System

A file system provides the mechanism for on-line storage of and access to information belonging to the OS and its users. Here, we first discuss the file system in very general terms and then explain how it is implemented in Nachos. Note that this is not an exhaustive discussion of a file system; it is rather an overview providing information sufficient for you to understand the Nachos file system.

The logical storage unit for information is files. These are mapped by the OS into physical devices. In general, they are treated as a sequence of bits, bytes, lines or records - the meaning is defined by the file's creator. Information about all the files is kept in a directory structure that resides on secondary storage along with the files.

The interface that the OS provides for the file system comprises, among other things, of operations to manipulate files, e.g., creating, opening, writing or truncating a file. Many of these file operations involving searching the directory structure for the entry associated with the file. Since searching the file entry for each operation performed on the file can be expensive, the OS maintains a small table containing information about all open files (the open-file table). When a file operation is requested, an index into this file table is used to get the appropriate entry. When the file is no longer being used, it is closed by the process and the operating system removes its entry from the open file table.

Each process also maintains a table of all the files that it has opened. This stores information about how the file(s) are being used, e.g., the current location of the file pointer. Each entry in this table points to the corresponding entry in the open-file table.

Organization of Files

To manage the data or information that the file system manages, it is organized in different levels. First, the file system is broken into partitions - typically, a disk on a system contains at least one partition, which is a low level structure in which files and directories reside. They may be used to provide separate areas within one disk, each being treated as a separate storage device, or partitions may be larger than a disk in order to allow grouping of disks in one logical structure. Each partition contains information about files it contains. This information is kept in a device directory which stores information such as name, location, size and type, for all files on that partition.

The directories themselves may be organized in several ways, the simplest being the single-level directory. All files are contained in the same directory. The two-level directory structure allows each user to have their own directory. The tree-structured directory is a generalization of the two-level structure, containing multiple levels.

File System Implementation

File Implementation The most important issue in file storage is keeping track of which disk blocks go with which file. Different operating systems use different methods - contiguous allocation and linked list allocation are important to know. In the former, each file is stored as a contiguous block of data on the disk, in the latter, the file is kept as a linked list of disk blocks - the first word of each block is used as a pointer to the next one. UNIX uses i-nodes to keep track of which blocks belong to each file. An i-node is a table that lists the attributes and disk addresses of the file's blocks. The first few disk addresses are stored in the i-node itself, so for small files, all the necessary information is in the i-node itself which is fetched from disk to main memory when the file is opened. For larger files, one of the addresses in the i-node is the address of a disk block called a single indirect block which contains additional disk addresses. If this is insufficient, another address called the double indirect block may contain the address of a block that contains a list of single indirect blocks.

Directory Implementation The simplest method is to use a linear list of file names with pointers to the data blocks. This requires a linear search to find a particular entry. Hash tables are also used by some operating systems - a linear list stores the directory entries but a hash function based on some computation from the file name returns a pointer to the file name in the list. Thus, directory search time is greatly reduced.

In UNIX, each entry in the directory contains just a file name and its i-node number. When a file is opened, the file system takes the file name and locates its disk blocks. The i-node is read into memory and kept there until the file is closed.

This is ONLY a preview of the tutorial. If you would like to view the entire tutorial, you must purchase the tutorial.

Each tutorial comes with the following:

• Assignment overview. Details on each topic in a general level and how they are implemented in Nachos.

• Walk-through. A complete step-by-step guide on how to implement the code necessary to complete each assignment.

• Frequently asked questions. Questions and answers to common problems students face while completing each assignment.

• Helpful links. Links to academic and other websites that offer Nachos assistance and documentation.

• Documentation. Detailed documentation listing all of the classes, structures, functions defined in Nachos.

• Roadmap. High-level overview of the source code, focusing on the big picture rather than on the details.

• Source code. The source code solution to each assignment.

This includes both documentation on understanding the concepts for each assignment and how to get started. Topics are accompanied with animations.

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