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After studying this chapter, you should be able to: Describe the basic concepts of files and file systems, understand the principal techniques for file organization and access, define B-trees, explain file directories, understand the requirements for file sharing.
Chapter 12 File Management • Overview • File organisation and Access • File Directories • File Sharing • Record Blocking Files • Files are the central element to most applications – file as an input to applications – file as an output for long-term storage and for later access • Desirable properties of files: – Long-term existence – Controlled sharing between processes – Structure that is convenient for particular applications File Structure Fields and Records • Fields – Basic element of data • e.g., student’s last name – Contains a single value – Characterized by its length and data type • Records – Collection of related fields • e.g., a student record – Treated as a unit File Structure File and Database • File – Collection of similar records – Treated as a single entity and may be referenced by name – Access control restrictions usually apply at the file level • Database – Collection of related data – Explicit relationships exist among elements – Consists of one or more files A Big Picture How to organize records in a file and access a particular record in a file? How to organize records as a sequence of blocks for I/O? individual block I/O requests must be scheduled for optimizing performance How to identify and locate a selected file? How to enforce user access control in shared systems? Roadmap • Overview • File organisation and Access • File Directories • File Sharing • Record Blocking File Organization • The basic operations that a user or application may perform on a file are performed at the record level – The file is viewed as having some structure that organizes the records • File organization refers to the logical structuring of records – Determined by the way in which files are accessed (access method) Criteria for File Organization • Important criteria include: – Short access time – Ease of update – Economy of storage – Simple maintenance – Reliability Criteria for File Organization • Priority will differ depending on the use – For batch mode file processing, rapid access for retrieval of a single record is of minimal concern • These criteria may conflict – Use of indexes (conflict with economy of storage) can be a primary means of increasing the speed of access to data The Pile • Data are collected in the order they arrive – No structure • Purpose is to accumulate a mass of data and save it • Records may have different fields – field should be self-describing (field name + value) – field length should be known (delimiters, subfield or default for a field type) 10 File Sharing • In multiuser system, there is almost always a requirement for allowing files to be shared among a number of users • Two issues – Access rights – Management of simultaneous access 28 Access Rights • A wide variety of access rights have been used by various systems – often as a hierarchy, with each right implying those that precede it • None – User may not know the existence of file by not allowing to read the user directory that includes this file • Knowledge – User can only determine that the file exists and who its owner is 29 Access Rights cont… • Execution – The user can load and execute a program but cannot copy it, e.g., proprietary programs • Reading – The user can read the file for any purpose, including copying and execution • Appending – The user can add data to the file but cannot modify or delete any of the file’s contents 30 Access Rights cont… • Updating – The user can modify, delete, and add to the file’s data • Changing protection – User can change access rights granted to other users • Deletion – User can delete the file 31 User Classes • Access can be provided to different classes of users – Owner: usually the files creator, has full rights and may grant rights to others – Specific users: individual users who are designated by user ID – User groups: a set of users identified as a group – All: all users who have access to this system 32 Simultaneous Access • When access is granted to append or update a file to more than one user, the OS or file management system must enforce discipline • User may lock the entire file or individual records during update • Mutual exclusion and deadlock are issues for shared access, ref readers/writers problem 33 Roadmap • Overview • File organisation and Access • File Directories • File Sharing • Record Blocking 34 Blocks and records • Records are the logical unit of access of a structured file • Blocks are the unit for I/O with secondary storage • For I/O to be performed, records must be organized as blocks • Three methods of blocking are common – Fixed length blocking – Variable length spanned blocking – Variable-length unspanned blocking 35 Fixed Blocking • Fixed-length records are used, and an integral number of records are stored in a block • Unused space at the end of a block is internal fragmentation • Common for sequential files with fixedlength records 36 Fixed Blocking 37 Variable Length Spanned Blocking • Variable-length records are used and are packed into blocks with no unused space • Some records may span multiple blocks – Continuation is indicated by a pointer to the successor block • Efficient for storage and does not limit the size of records 38 Variable Blocking: Spanned • Difficult to implement • Records that span two blocks require two I/O operations 39 Variable-length unspanned blocking • Uses variable length records without spanning • Wasted space in most blocks because of the inability to use the remainder of a block if the next record is larger than the remaining unused space • Limits record size to the size of a block 40 Variable Blocking: Unspanned 41 Revisit the Big Picture User views the file as having some structure that organizes the records; different access methods reflect different file structures Describes the location of all files plus their attributes Records must be organized as a sequence of blocks for output and unblocked after input individual block I/O requests must be scheduled for optimizing performance Only authorized users are allowed to access particular files in particular ways 42 ... systems and inventory control systems 18 Roadmap • Overview • File organisation and Access • File Directories • File Sharing • Record Blocking 19 File Directory • Contains information about files... locate a selected file? How to enforce user access control in shared systems? Roadmap • Overview • File organisation and Access • File Directories • File Sharing • Record Blocking File Organization... main file • requires on average 500 accesses to the index file + 500 accesses to the main file 15 Indexed Sequential File • An overflow file is added • A new record is added to the overflow file