1. CS246 Data & File Structures Lecture 1 Introduction to File Systems Instructor: Li Ma Office: NBC 126 Phone: (713) 313-7028 Email: malx@tsu.edumalx@tsu.edu Webpage: http://itscience.tsu.edu/ma Department of Computer Science Texas Southern University, Houston January, 2007

2. Spring 2006by Li Ma, TSU - cs344 Motivation Why do we need file structure design? What are most computers used to for? –Examples: Editing document Internet surfing or Composing email Programming, etc. –Data processing: storage, organization, access, and operation on data

3. Spring 2006by Li Ma, TSU - cs344 Computer Architecture Main Memory Data Transfer Secondary Memory Data is manipulated here Data is stored here

4. Spring 2006by Li Ma, TSU - cs344 Advantages of Memories Programs are executed in main memory since it is fast Data is stored in secondary memories since –Data is not lost during power failures – stable –Affordable – large and cheap

5. Spring 2006by Li Ma, TSU - cs344 Disadvantages of Memories Main memory is small and expensive –Many programs are too large to fit in main memory Main memory is volatile, Data is lost during power failure Secondary storage is slow (10,000 times slower than main memory)

6. Spring 2006by Li Ma, TSU - cs344 Solution Secondary storage provides reliable, long- term storage for large volume of data Data we are interested in (a small portion of data) is always in Main memory since it can be rapidly manipulated and processed Data can be transferred automatically between the Main memory and the secondary storage

7. Spring 2006by Li Ma, TSU - cs344 Problem with the Approach Transferring data between Main memory and secondary memory is slow because Secondary memory is slow An important aspect of file system management is to minimize the amount of data transfer, or eliminate unnecessary transfers

8. Spring 2006by Li Ma, TSU - cs344 File Systems Examples –Telephone book (primary index: name?) –Library (primary index: number, secondary indices: author, title, and subject) A file system provides a convenient method for organizing and storing files

9. Spring 2006by Li Ma, TSU - cs344 File Systems (cont) File systems are software programs that allow us to efficiently organize data and operate on data –Data is organized into files on hard disks or other physical media –A file is divided into records –Records are composed of fields

10. Spring 2006by Li Ma, TSU - cs344 Example of A File A student file may be a collection of student records, one record for each student Each student record may have several fields, such as: –Name –Address –Student Number –Sex –Age –Grade point average (GPA).

11. Spring 2006by Li Ma, TSU - cs344 Structure of A File Typically, each record in a file has the same fields Files are large and are stored in secondary storage Records we are currently interested in are copied to Main memory Organizing the records of a file, and getting the records we are interested in are the main topics of this course

12. Spring 2006by Li Ma, TSU - cs344 File Properties Persistence: Data written into a file persists after the program stops, so the data can be used later Sharability: Data stored in files can be shared by many programs and users simultaneously Large Size: Data files can be very large that, typically, they cannot fit into Main memory

13. Spring 2006by Li Ma, TSU - cs344 File Structure Design A file structure is a combination of representation for data in files and operations for accessing the data The intension between a disk’s relatively slow access time and its enormous, non-volatile capacity is the driving force behind file structure design

14. Spring 2006by Li Ma, TSU - cs344 File Structures Sequential file structures – files on tape Indexed file structures – files on disk, with key and pointer Tree structures: AVL tree, B-tree, B+ tree Hash file re-organization

15. Spring 2006by Li Ma, TSU - cs344 Course Content & Outline This course covers data processing from a computer science perspective: –Storage of data – storage devices (disk & tape) –Organization of data – file organization sequential - Tape direct (hashing) indexed sequential (B-trees) multi-key (secondary indices) –Access to Data – file systems –Processing of Data – database system