An Operating Systems Vade Mecum

Exercises 231(11) The process submits a Close service call (line 8). The file layer releases theopen-file descriptor for the file.We have ignored a few details here, such as how we ensure that the block layer does notdiscard one of its cached blocks while it is still in use by a higher layer, how room ismade for overflow file descriptors when the byte layer needs to bring them in, what happensif the file was only 400 bytes long at the outset, and what is different if the file is notstored on disk at all, but on a magnetic tape.8 FURTHER READINGGeneral discussions of file systems may be found in most of the textbooks cited at theend of Chapter 1. Individual file systems are discussed in a number of articles. The CAPfiling system (Needham and Birrell, 1977) is designed for the Cambridge CAP computer(Needham and Walker, 1977), whose architecture supports capabilities directly. TheIntel 432 computer also supports capabilities directly; its operating system, iMAX-432,takes advantage of that feature in its file structures (Pollack et al., 1981). In the Demosfile system for the Cray-1 computer, the file manager is a process instead of a module inthe kernel (Powell, 1977). Pilot, an operating system for a personal computer (Redell etal., 1980), has a flat directory structure, allows physical disks to be added and removed,provides mapped file access, and uses a label for each block that describes its use to promotesalvaging.File servers are machines dedicated to providing a file system for a collection ofautonomous computers. Some file servers that have been described in the literature areWFS (Swinehart et al., 1979), Felix (Fridrich and Older, 1981), the Xerox distributed filesystem (Mitchell and Dion, 1982), and the Cambridge file server (Birrell and Needham,1980).Several studies of file sizes have been reported, including one for the Los AlamosScientific Laboratory (Powell, 1977) and for a Tops-10 installation in an academicenvironment (Satyanarayanan, 1981).Database management is a large subject with close ties to the subject of operatingsystems. The special operating-system requirements of database management are discussedby Gray (1979) and Stonebraker (1981). Bernstein (1981) discusses a greatnumber of implementations of transactions for databases.We have shown only one way to organize the levels of a file manager. Other waysare given by Calingaert (1982), Shaw (1974), and Peterson and Silberschatz (1985).9 EXERCISES