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Operating Systems

Operating Systems. File Management. What Is A File?. Named collection of data stored on hard disk, CD-ROM etc. Can be programs (both source and object forms) or data. The UNIX/Linux and Windows OS’s do not know anything about the file structure.

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Operating Systems

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  1. Operating Systems File Management

  2. What Is A File? • Named collection of data stored on hard disk, CD-ROM etc. • Can be programs (both source and object forms) or data. • The UNIX/Linux and Windows OS’s do not know anything about the file structure. • The application program associated with the file knows how to interpret the file. • Interpretation logged by OS

  3. What Is File Management? • The creation and deletion of files. • The creation and deletion of directories/ folders. • The manipulation of files and directories • The mapping of files onto disk storage. • Backup of files on stable (non volatile) storage.

  4. What is a Directory / Folder? • A logical grouping of files. • Actually a special type of file. • Directories can contain other directories known as sub-directories, creating a tree structure. • Hierarchical • UNIX/Linux uses the term directory, Windows uses the term folder.

  5. Example of a Directory Structure • Windows Explorer • The yellow icons are directories (folders). • The plus (+) sign next to the directory indicates that it contains sub-directories.

  6. Pathnames • In order to uniquely specify a file in a tree-structured directory, a pathname must be used. • A pathname prefixes the filename with a list of the directories from the base directory which have to be traversed in order to reach the file. • The pathname separator is the forward-slash (/) in UNIX/Linux and the backslash (\) in Windows. • At any instant, a process has a current directory, which is where the operating system will look for a file whose pathname is not specified.

  7. Pathname Examples • UNIX/Linux • Full pathname - /cprogs/work/file1.txt • Relative pathname - work/file1.txt • Windows • Full pathname - C:\CPROGS\WORK\FILE1.TXT • Relative pathname – WORK\FILE1.TXT

  8. Common Directory Commands

  9. File Naming • UNIX/Linux • File names are case-sensitive. • File name can be up to 255 characters in length. • File name can contain any character except /. • Characters that have special meaning to the shell (* > < ? \ ; : | [ ] ( )) should not be included in a file name. • Linux does allow spaces in the file name, other flavours of UNIX may not.

  10. File Naming • Windows • File names are not case sensitive, but the case will be preserved. • File name can be up to 255 characters in length. • File name can contain any character except / \ “ ; * ? < > |. • The portion of the file name after the last dot (.) is known as the extension. • The extension is used to specify the nature of the file. • Windows also maintains a short file name using the first 8 characters of the file name and the first three characters of the extension.

  11. File Naming • Both UNIX/Linux and Windows allow a file to be referenced using wildcard characters. • In both OS’s - * can be replaced by zero or more characters; ? can be replaced by one character. • UNIX/Linux also supports more complex wild-carding. • [a,b,c] - character can be either a, b or c. • [a-d] - character can be either a,b,c or d • [a-d,f] – character can be either a,b,c,d or f

  12. Hard Disk Partitioning • Partitioning is a means of sub-dividing a hard disk. • A hard disk must contain at least one partition. • There are two types of partitions – primary and extended. • A disk can contain up to 4 primary partitions or up to 3 primary and 1 extended partition. • One primary partition may be designated as active, this will be used for booting the computer. • Different operating systems can be installed on different primary partitions – allowing multi-booting. • An extended partition cannot be accessed directly by the operating system but is instead sub-divided into logical drives.

  13. Hard Disk Partitioning • In UNIX/Linux the fdisk command is used to manage hard disk partitions. • In Windows 2000+ use the DISKPART command or the Disk Management tool.

  14. Hard Disk Initialization • In order to be usable by the OS a partition must be initialized. • In UNIX/Linux the mkfs command is used, in Windows the command is FORMAT. • The initialisation process sets up the necessary structures on a disk partition. • In UNIX/Linux an initialized partition is referred to as a file system on Windows it is called a drive.

  15. File Space Allocation • Disk space is dynamically allocated to files. • Space is allocated in units of fixed size called allocation units or clusters. • When a new file is created, it is allocated one cluster. • As the file is extended the cluster eventually becomes filled and more clusters are assigned to the file.

  16. A A B A B C C B B Free cluster End of file B End of file A File Space Allocation • Ideally, successive file clusters should occupy contiguous space on the disk. • In general, the disk will contain many different files whose clusters are intermingled with each other. • Clusters are released when a file is deleted or shrunk, so free clusters may be intermingled with allocated clusters.

  17. UNIX/Linux File System • 4 classes of files: • regular file. • directory. • character or block special file: device such as a disk, printer, terminal. • named pipe: queuing buffer which passes the output of one process to the input of another.

  18. UNIX/Linux File System • A file is described by an information block called an inode. • Every file on disk has an associated inode. • Kernel memory contains a copy of the inode for every open file. • A directory entry holds the filename and a reference to its associated inode or its inode number.

  19. UNIX/Linux File System • The inode contains all information about a file, except its name. This information includes • File access and type information, collectively known as the mode. • File ownership information. • Time stamps for last modification, last access and last mode modification. • File size in bytes. • Addresses of physical blocks.

  20. UNIX/Linux File System - Inodes • The inode records which disk blocks/clusters have been allocated to a file. • This information is stored in 13 “pointers” in the inode structure. • The first 10 are direct pointers – they map directly to the physical address of a block of a file. • The other 3 pointers point to index blocks which themselves contain lists of pointers. • The pointers in the index blocks reference either data blocks or further index blocks.

  21. UNIX/Linux File System – Volume Structure • When the mkfs command is run to initialize a partition the following layout is created: • Boot block: first stage boot program. • Super block: contains details of disk size and allocation of inodes and data blocks. • Inodes: storage space for inodes. • Data Blocks: rest of disk available for directories and files.

  22. UNIX/Linux File System – Free Space Control • When the mkfs command is run it creates a chain of linked blocks. • The super block contains the start block in the chain. • Each block contains an array of free data block numbers and a pointer to the next block in the chain.

  23. Windows File System • Windows 2000+ supports 3 different file systems: • FAT also called FAT16 • FAT32 • NTFS • FAT and FAT32 are implemented similarly.

  24. FAT16/FAT32 File System • The FAT system maintains a table of entries known as the File Allocation Table (hence the name FAT). • The table consists of an array of 16-bit (FAT16) or 32-bit (FAT32) values. • The first value in the array identifies the disk system type and the second entry is set to all 1’s. • Each of the remaining entries maps to a cluster on the disk.

  25. FAT16/FAT32 File System • Each entry records one of five things: • the address of the next cluster in a chain • a special end of file (EOF) character that indicates the end of a chain • a special character to mark a bad cluster • a special character to mark a reserved cluster • a zero to note that that cluster is unused

  26. FAT16/FAT32 File System • Prior to any data being written to the disk, the FAT entries are all set to zero, indicating a free cluster. • As file space is allocated, the FAT entries contain pointers to the next entry in the chain. • When more file space is requested, the OS searches the FAT for an entry with a value of zero (unused) and allocates it to the file.

  27. FAT16/FAT32 File System – Volume Structure • When the FORMAT command is run on a partition to create a FAT file system, the following layout is created: • Boot sector: details about the disk’s characteristics and a bootstrap loader. • FAT: principal file allocation table. • Additional FAT(s): Duplicate table(s) which are updated at the same time as the principal FAT. • Root directory • File space: The rest of the disk.

  28. Disk Fragmentation • Over time, allocation of clusters becomes less contiguous. • Deletion and resizing of files may result in free clusters located in random positions on the disk. • Such a situation is called fragmentation and can result in reduced performance of the computer. • To correct this situation a defragmenting tool must be used which can put all the clusters of a file in contiguous locations and coalesce the free clusters.

  29. Disk Fragmentation • Windows comes with a disk defragmenting tool.

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