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file to find the indexing record with a value for SSN that is largest, but still less than or equal to the SSN being sought. Using the pointer from the indexing record, the system can now read the main file starting from a point near the record being sought. On average, the system will make 500 reads of the indexing file followed by 500 reads of the main file (one million records divided among 1000 index values means 1000 records per index, and on average the system must read half of them). So, indexing reduces the number of reads from 500,000 to 1000 improvement by a factor of 500. Files may contain information in both character form (ASCII, UNICODE, or EBCDIC) and binary form. Executable files, for example, are binary files in the special format required by the operating system. Executable files are process images ready to be brought in from the disk, stored in memory, and executed. UNIX takes the unique approach that any file is just a stream of bytes. If there s any structure to the information in the file, it s up to the application program to create the structure, and later to interpret it. In some applications this shifts some responsibility away from the file system manager to the application, but it is a simple, consistent conception. File System Units The file system relies on the disk hardware, of course. Disks almost always transfer multiples of a 512-byte sector. Even if your program tries to read a street address that is no more than 25 characters long, the disk will actually return to the operating system at least one sector, and somewhere within will be the street address. The sector is the unit of storage for the disk hardware. Usually the file system has a larger unit of storage than the sector. This minimum storage allocation for a file is called a block or cluster. Even a file consisting of a single character will require one cluster of space on the disk. The file system tracks free clusters, and allocates clusters of space to files as necessary. The wasted space within a file due to unused space in the last cluster is internal fragmentation with respect to the file system. Directories and Directory Entries Today almost all operating systems represent the file system organization as a tree of directories and subdirectories. The directory entry for a file will contain information such as the file name, permissions, time of last access, time of last modification, owner, file size, and location of the data on the disk. When a file is opened by a process, information from the directory is copied to an in-memory data structure often called a file control block or an open file descriptor. In addition to the information from the directory entry, the file control block may have information about read and write locks on the file, and the current state of the file (open or closed, for reading or writing). An interesting approach taken by UNIX was to make directories files. A directory is just a file that contains information about files. This sort of stunningly simple consistency is part of the charm of UNIX for its admirers. File Space Allocation There are several general approaches to allocating file space. One is contiguous allocation. With this conceptually simple approach, the file system allocates adjacent blocks or clusters for the entire file. Contiguous allocation has the merits that writing to and reading from the file are fast, because all the information is in the same place on the disk. Some real-time systems and high-speed data acquisition systems use contiguous file allocation for the reason of speed. One of the problems with contiguous allocation is that external fragmentation of the file system becomes a problem very quickly. The system must deliver a potentially large set of blocks to any new file, and the free space between existing files may be unusable because each is, in itself, too small. Another problem is that extending a file can be impossible due to lack of available space between allocations. Another type of allocation is linked allocation of file space. Each block in a file is linked to the next and the previous with pointers. This removes any requirement for contiguous allocation, eliminates external fragmentation as a cause of wasted space, and permits extending files at any time. The starting block of a file is all the system needs to know, and from there the system can follow the links between blocks. Free space can be maintained likewise in a linked list of blocks.
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