Chapter 6 File Management

Objectives

To study the file service function of an OS as the lowest level provider of persistent storage. To consider typical file service interfaces and implementations. To study how the functions of a file service might be distributed.

Points to emphasise

• Naming. Pathnames and unique file identifiers. The file identifiers (sometimes called handles) given to users may or may not be the same as those used internally by the filing system.
• Sharing. Access control may be fine grained and complex to implement or crude and simple. Why it is useful to have links in addition to access control.
• Discuss the semantics of "write" and the trade-offs in its implementation. You can do a fast, unsafe write or a slow safe one.
• The file system implementor has to be concerned with the consistency of data structures in main memory and those on disk.
• There are environments for filing systems other than a university campus.
• The I/O bottleneck. Directions in which file services are likely to evolve.

Possible difficulties

The student may find it difficult to understand general issues when the local OS is very familiar. It may be useful to emphasise different application environments and requirements on future systems to store different types of data, such as multimedia, to create an open minded attitude and an approach based on reasoning rather than local knowledge.

Teaching hints

• Directories are a good example of (persistent) abstract data types. Who is the owner of a directory? Why cannot the owner read and write an arbitrary byte sequence from/to a directory? The owner is a client of the DS which is a client of the "byte-sequence file service" (FS).
• The DS knows the internal structure of directories which it stores as files on the FS. The FS does not know the internal structure of any files stored on it. Ask the students for other system services or utilities that use the FS in this way and themselves have clients. (e.g. mail, database management systems, management utilities which write error logs, spreadsheet packages).
• Ask the students to focus on what is in main memory and what is on disk.
  Consider a crash in which the contents of main memory are lost. Suppose a write request requires that several blocks are written out to disk and a crash occurs part way through. Suppose a crash occurs between the writing out of metadata and data.
  Consider the consequences of the loss of any individual disk or on-disk data structure. What are the implications of using a RAID scheme?
• Compare the projected performance n years ahead of disks with that of processors. How will future filing systems cope? Consider large client caches in main memory and on disk. How would the shared file service be used?
• Give the example that a one-minute video clip occupies 100 Mbytes and requires a sustained data transfer rate of 3 Mbits/s. Emphasise that the typical 4Kbyte block size and, for example, a table of pointers, with indirection after the first few pointers, is unsuitable for this kind of file.