FILESFile system is the one of the most importantparts of the OS to a user.File is the collection of data created by user.
properties of files:- Long-term existence:: Files or stored on disk or other storage and do notdisappear when a user logs off. Sharable between processes:: Files have names and can have associated acccesspermission that permit controlled sharing Structure:: Files can be organized into hierarchical or morecomplex structure to refelect the relationships amongfiles.
FILE SYSTEMS:- It provide a means to store data organized as file as well as a collection of function that can be performed on file.Maintain a set ofattributes associated with the file. Operation include:- 1)create 2)delete 3)open 4)close 5)read 6)write
FILE STRUCTURE:- Files can be structured as a collection ofrecords or as a sequence ofbytes.Unix,Linux, WindowsMac OSs considerfile as a sequence of bytes.
Four terms are commonly used when discussing Files:Field Record File Database
Field:: It is a basic element of data .it contains a single value.itfixed or variable length. data base:: Collection of related data is called database.relationshipsamong elements of data are explicit. Files:: Files is the collection of similar records.it treated as asingle entity.may be refferenced by name. Record:: Record is a collection of related fields that can be treatedas a unit by some application programme.The one field isthe key -a unique identifier.
BASIC FILE SYSTEM:- It also referred to as the physical I/O level.It deals withblocks of data that are exchanged with disk or other massstorage device. placement of blocks on the secondarystorage device.buffering blocks in main memory.Itconsidered the part of the operating system. BASIC I/O SUPERVISOR:: It responsible for all file I/O initiation and termination.Itcontrol structures that deals with device I/O,scheduling,andfile status are maintained.It select the device on which I/O tobe performed.It concerned with scheduling disk and tapeaccessess to optimize performance.I/O buffers areassigned and secondary mamory is allocated at this level.
ACCESS CONTROL MODULE: Different access methods reflect different filestructures and different ways of accessing and processing the data. ACCESS CONTROL:: In a system with multiple user, its important toprotect one users object (file.directories) from otherusers.Tw o l e v e l s o f p r o t e c t i o n : :Logon verification:access determination:
USER ACCESS RIGHTS OWNER USER ALL Usually SPECIFI GROU the initial C PS All user creater of USER A set of who have the file access to individual user this Has full Users who are system rights who are not designat individu These May grant es by ally are public rights to user defined files another
LOGICAL FILE SYSTEM:: Logical I/O This level is the inter face betweenthe logical commands issued by aprogramme and thephysical detail required by the disk.
PHYSICAL FILE SYSTEM:: The physical organization of a file has to do with the way records are arranged and the characteristics of the medium used to store it.On magnetic disk,files can be organised in one of three ways::SEQUENTIAL,DIRECT,INDEXEDSEQUENTIAL.
THE PILE THETHE DIRECT SEQUENTIAOR HASHED L FILE FILE FIVE OF THE COMMON FILE THE ORGANIZA THE INDEXED TION ARE INDEXED SEQUENTIA FILE L FILE
THE PILE:: Least complicated form of file organization. Data are collected in the order they arrive. Each record consist of one burst of data. Purpose is simply to accumalate the mass of data and save it. Record access is by exhaustive search.
THE SEQUENTIAL FILE:: Most common form of file structure. A fixed format is used for records. Key field uniquely identifies the record and determines storage order Typically used in batch application Only organization that is easily stored on taps as well as disk.
INDEXED SEQUENTIALFILE:: Adds an index to the file to support random access. Adds an overflow file. Greatly reduces the time required to access a single record. Multiple levels of indexing can be used to provide greater efficiency in access.
INDEXED FILE::Records are accessed only through their indexes. Variable length record can be employed. Exhaustive index contain one entry for every record in the main file Partial index contain entries to record where the field of interest exists. Used mostly in application where timelines of information is critical. Example would be airline reservation system and inventory control systems.
DIRECT OR HASHED FILE:: Access directly any block of a known address. Makes uses of hashing on the key value. Often used where:: very rapid access is required. fixed length records are used. records are always accessed one at a
FILE DIRECTORY:: While writing fixed length record in keep track of the number of records and /or the number of bytes written in that file size is normally kept in the DIRECTORY where there is one entry for each file for file size. The MASTER FILE DIRECTORY (MFD) is stored immediately after the volume descriptor and lists the names and characteristics of every file name in the MFD can refer to program files.Early operating system supported only a single directory per volume.This directory was created by the file manager and contained the names of files.Usually organized in alphabetical.Spatical or chronological order.
OPERATIONS PERFORMAD ON A DIRECTORY:: CREATE DELETE LIST SEARCH FILE FILES DIRECTORY UPDATE DIRECTORY