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 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 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

Roadmap File organisation and Access Overview 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 length should be known

The Pile Record access is by exhaustive search Used when data are collected and stored prior to processing or data are not easy to organize  Uses space well when data vary in size and structure  Adequate for exhaustive searches  Easy to update  Unsuitable for most applications

The Sequential File Fixed format used for records Records are of the same length same number of fixed-length fields in a particular order Only the values of fields need to be stored Field name and length are attributes of the file structure

The Sequential File Key field Uniquely identifies the record Records are stored in key sequence  Optimal for batch applications if they involve the processing of all the records  Easily stored on tape and disk  Poor performance for interactive applications considerable processing and delay due to the sequential search of the file for a key match

Indexed Sequential File An index is added to support random access The index is a sequential file An index record contains a key field and a pointer into the main file For searching Search the index to find the highest key value that is equal to or precedes the desired key value Search continues in the main file at the location indicated by the pointer

Indexed Sequential File An overflow file is added A new record is added to the overflow file and is located by following a pointer from its predecessor record The indexed sequential file is occasionally merged with the overflow file in batch mode

Indexed File Records are accessed only through their indexes no restriction on the placement of records allows variable-length records Uses multiple indexes for different key fields An exhaustive index contains one entry for every record in the main file A partial index contains entries to records where the field of interest exists

Indexed File When a new record is added to the main file, all of the index files must be updated. Used mostly in applications where timeliness of information is critical and data are rarely processed exhaustively examples: airline reservation systems and inventory control systems

Roadmap File Directories Overview File organisation and Access File Sharing Record Blocking

File Directory Contains information about files Attributes Location Ownership Directory itself is a file owned by the operating system

Directory Elements Basic Information Address Information File name: must be unique File type: e.g., text, binary File organization Address Information Volume: device on which file is stored Starting address: e.g., cylinder, track on disk Size used: in bytes, words or blocks Size allocated: maximum size of the file

Directory Elements Access Control Information Usage Information Owner: able to grant/deny access to other users and to change these privileges Access information: e.g., user’s name and password for each authorized user Permitted actions: controls reading, writing, executing, transmitting over a network Usage Information Date Created, Identity of Creator, Date Last Read Access, Identity of Last Reader, Date Last Modified

Hierarchical, or Tree-Structured Directory Master directory with user directories underneath it Each user directory may have subdirectories and files as entries Each directory and subdirectory can be organized as a sequential file

Naming The tree structure allows users to find a file by following a path from the root or master directory down various branches until the file is reached The series of directory names, culminating in the file name itself, constitutes a pathname for the file Duplicate filenames are possible if they have different pathnames

Naming Usually an interactive user or a process is associated with a current or working directory Files are referenced relative to the working directory unless an explicit full pathname is used

Roadmap File Sharing Overview File organisation and Access File Directories File Sharing Record Blocking

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

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 even know of the files existence Knowledge User can only determine that the file exists and who its owner is

Access Rights cont… Execution Reading Appending The user can load and execute a program but cannot copy it 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

Access Rights cont… Updating Changing protection Deletion 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

User Classes Owner Specific Users User Groups All 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

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

Roadmap Record Blocking Overview File organisation and Access File Directories File Sharing Record Blocking

Blocks and records Records are the logical unit of access of a structured file Blocks are the unit for I/O with secondary storage Three methods of blocking are common Fixed length blocking Variable length spanned blocking Variable-length unspanned blocking

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 fixed-length records

Fixed Blocking

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

Variable Blocking: Spanned  Difficult to implement  Records that span two blocks require two I/O operations

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

Variable Blocking: Unspanned

Describes the location of all files plus their attributes A Big Picture 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 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 Only authorized users are allowed to access particular files in particular ways