Information Management in Operating Systems: File System Concepts

Operating system Lecture three Dr jamal altuwaijari

3 - Information Management C/S can store information on several different types of physical media such as: a- Magnetic tape. b. Disk or drum. c. Optical disk. Each of these media has its own characteristics and physical organization. The 0/S provides a uniform logical view of information storage.

3-1 The File System The file system consists of three distinct parts.: a. A collection of files, each storing related data. B. A directory structure, which organizes and provides information about all the files in the system. c. A partitions which are used to separate physically or logically large collections of directories

3-2 File Concept A file is a named collection of related information that is recorded on secondary storage. - The data cannot be written to secondary storage unless they are within a file. - Commonly files represent programs (Both source and object forms) and data. - - All disk I/O is units of one block (physical record) and all blocks are the same size

3-3 Files Attributes In addition to file name, a file has certain other attributes which vary from' one 0/S to another but typically consists of these: Name, Type, location, Size, Protection, Time and Date, User identification 3-4 File Operations A file is an abstract data type, the 0/S provides system calls to create, write, read, reposition, delete, and truncate files, appending, renaming, and copying. When a file is open there are several pieces of information associated with an open file: a. File pointer. The 0/S must track the last read/ write location as a cullern- file position pointer. This pointer is unique to each process operating on the file. b. File open count. This counter tracks the number of opens and closes of each file. c. Disk location of the file. The information needed to locate the file on disk is kept in memory to avoid having to read it from disk for each operation

3-5 File Types A common technique for implementing file types is to include the type as a part of the file name. The name is split into two party a name and an extension usually separated by a period character. In this was the user and the 0/S can tell from the name alone what the type of a file is, see Table 3.1

3-6 File Structure. The file types may be used to indicate the internal structure of the file. If the 0/S support multiple file structures the resulting size of 0/S must be increased to contain the code to support these file structure. The locating an offset within a file can be complicated for the 0/S. All disk I/O is performed in units of one block (physical record) and all blocks are the same size. It is unlikely that the physical record size will exactly match the length of the logical record logical records may even vary in length Packing a number of logical records into physical record is a common Solution to this problem. For example, the UNIX 0/S defines all files to be simply a stream of bytes. Each byte is individually addressable by its offset form the beginning (or end) of the file. in this ease the logical record is 1 byte. The logical record size, physical block size, and packing technique determine how many logical records are in each physical block. The packing can be done either by the user's application program or by the 0/S_ In either case the file may be considered to be a sequence of blocks. All the basic I/O functions operate in terms of blocks

3-7 Access Methods Files store information. When is used this information must be accessed and read into computer memory. There are several ways that the information in the file can be accessed: 3-7-1 Sequential The simplest access method is sequential access information in the file is processed in order, one record after the other. This mode of access is by far the most common, for example, editors and compilers usually access files in this fashion. The bulk of the operations on a file are reads and writes. A read operation reads the next portion of the file and automatically advance a file pointer which tracks the 1/0 location. A write appends to the end of the file and advance, to the new end of file.

3-7-1 Sequential See figure 3.1, This method is based on a tape mode of a file and works as well on sequential access devices as it does on random access ones

3-7-2 Direct access Another method is direct access (or relative access). A file is made up of fixed length logical records that allow programs to read and write records rapidly in no particular order. This method is based on a disk model of a file since disks allow random access to any file block. A direct access allows arbitrary to be read or written. Thus we may read block 14 then read block 53 and then write block 7. The operations must be modified to include the block number as a parameter. Thus we have read n where n is the block number rather than read next and write next. The block number provided by the user to the 0/S is relative block number. A relative block number is an index relative to the beginning of the file, see Table 3.2

3-7-3 Other access Methods These methods can be built on top of a direct access method. They involve the construction of an index for the file. The index like an index in the block of a book contain pointer to the various blocks. To find an entry in the file we first search the index and then use the pointer to access the file directly and to find the desired entry, see figure 3.3 . We first make a binary search on the master index which provides the block number of the secondary index. This block is read in and again a binary search is used to find the block containing the desired record. Finally this block is searched sequentially. In this way any record can be located from its key by at most two direct access reads

8-3 Directory structure Some file systems store thousands of files on hundreds of giga bytes of disk. To manage all these data we need to organization them. This organization is usually done in two parts: a. The file system is broken into partitions (mini disks), each disk on a system contains at least one partition some areas within one disk defined in one partition, each treated a separate storage device, where as other systems allow partitions to be larger than a disk to group disks into one logical structure b.Each partition contain information about files within it. This information is kept in entries in a device directory or volume table of contents. The device directory records information such as name, location, size, and type for all files on that partition. See figure 34 shows the typical file system organization

8-3 Directory structure The directory can he viewed as a symbol table that translates file names into their directory entries. The following operation that are to performed on a directory: search 'Or a file, create a file, delete a file, list a directory, rename a file, and transverse the file system. The most common schemes for defining the logical structure of a directory are :

3-8-1 Singlelevel Directory It is the simplest structure. All files contained in the same directory, see figure 3.5 Advantages: It is easy to support and understand Disadvantages: When the number of files increases or when there is more than user. Since all files in the same directory they must have unique names

3-8-2 Two-level Directory In the two level directory structure, each user has his own user file directory (UFD). Each UFD a similar structure, but lists only the files of a single user, see figure 3.0 When a user logs in the system's master file directory, (MFD) is search. The MFD is indexed by user name or by account number, and each entry points to the UFD for that user.

3-8-2 Two-level Directory Advantages: 1. It is solves the name collision problem. 2. It fasts to locate certain file name. 3. We can use the same file name in two different UFD. Disadvantages: It requires a complicate search procedure where we need to join the user name with file name.

3.8.3 Tree -Structured Directories The two level directory is a two level tree we can extend this structure to a tree of arbitrary height. See figure 3.8. - This generalization allows user to create their own sub directories and to organize their file accordingly. The MS-DOS system for example is structured as tree - The tree has a root directory. Every file in the system has a unique path name. A path name is the path from the root through all the subdirectories to a specified file. - A directory (or subdirectory) contains a set of files or subdirectories. A directory is simply another file but it is treated in a special way. One bit in each directory defines the entry as a file (0) or as a subdirectory (1)

3.8.3 Tree -Structured Directories

3-8-4 AcyclicGraph Directories This organization provide more than one user to share all file or subdirectory. A share file or subdirectory will exist in the file system in two (or more ) places at once, see figure 3.9