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There are some disadvantages of linked allocation. First, access to random records in the file is not possible directly. If an application needs the 57th record in a file using linked allocation, the system will have to read through the first 56 records to get to the 57th. It s also true that the pointers take up some space in the file system; that s overhead, although bits on the disk are cheap these days. Finally, if the blocks of a file really are scattered all over the disk, accessing the file contents will be slowed by all the mechanical delays of positioning the read/write heads again and again. Windows uses linked file allocation, and the defragmentation exercise brings the dispersed blocks of files together so that access will be faster. When the linked blocks are in fact adjacent to one another, reads and writes occur faster. A third approach to file allocation is indexed allocation, championed by UNIX. With indexed allocation, the file directory entry has pointers to the blocks containing the information in the file. The blocks need not be contiguous, and files can be extended by adding blocks at any time. Another advantage of indexed allocation is that the locations of all the blocks in the file can be determined from the directory entry, so random access is possible. A disadvantage is that any corruption of the directory entry can make it impossible to recover the information in the file. In UNIX a file directory entry is called an i-node or inode, for index-node. Among other things, the inode contains pointers to the file blocks, permissions, ownership information, and times of last access and modification. Since directories are just files in UNIX, each directory has an inode, too. When a program opens a file like /usr/work.txt, the system locates the inode for the root directory (almost always set to inode 2), and then reads the blocks of the root directory looking for something called usr. When it finds usr, it will also find the inode for usr. Then it can read the file that is the directory usr and look for something called work.txt. When it finds work.txt, it will also find the inode for work.txt. From the inode of work.txt the system will find pointers to all the blocks of the file. Journaling File Systems Some newer file systems incorporate logging or journaling of file system transactions. A transaction is a series of changes that must either all be successful, or none be successful. For example, if one moves a file from one location to another (cut and paste), the new file directory entry must be written and the old file directory entry must be erased. Both actions must occur, or neither must occur. A journaling file system first writes any intended changes to a write-ahead log file. Then it makes the required changes to the file system. If the changes to the file system fail for some reason (e.g., a total system failure at just the wrong moment), the file system can either recover the changes by redoing the transaction, or roll back the changes to the original state. The Windows NT File System (NTFS) incorporates journaling features, and so does Sun s UFS and Apple s Mac OS X. There are also several journaling file systems available for Linux. SUMMARY Operating systems are programs designed to make the use of the computer hardware easier, more efficient, and more secure. They manage the scheduling of processes, the allocation of memory and other resources to processes, and all input and output of data. In addition, operating systems provide a file system for managing programs and data, and various system software utilities for facilitating program development and execution. There are several varieties of operating systems, including interactive, batch, and real time. Each is designed to best meet the peculiar requirements of its intended use. Most modern operating systems support the concept of threads in addition to processes. A process represents a set of resources such as memory, open files, common variables, network connections, etc., and a thread represents a particular path of code execution. Threading allows for easy sharing of resources among the threads, which are essentially separate cooperating tasks. Multiprogramming, multiprocessing, and multithreading require careful consideration of thread and process synchronization techniques. These same operating system advances also introduce the possibility of deadlock. For process synchronization, most operating systems provide semaphores, a simple, effective construct first proposed by Edgar Dijkstra in 1965.
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