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- 2. • File systemsand IOCS • File Operations, File Organizations • Directory structures, File Protection • Interface between File system and IOCS • Allocation of disk space, Implementing file access Outline 2 Module-4 File system
- 3. 3 Overview of Filesystems
- 4. 4 4.1 File Systemand the IOCS
- 5. 5 4.1 File Systemand the IOCS File System and the IOCS facilities • File system views a file as a collection of data that is owned by a user, shared by a set of authorized users, and reliably stored over an extended period • IOCS views it as a repository of data that is accessed speedily and stored on I/O device that is used efficiently • Two kinds of data: file data and control data
- 6. 6 4.1 File Systemand the IOCS Logical organization of a file system
- 7. 7 4.1 File Systemand the IOCS File Processing in a Program • At programming language level: • File: object with attributes describing organization of its data and the method of accessing the data
- 8. 8 4.2 Files andFile Operations • File types can be grouped into two classes: • Structured files: Collection of records • Record: collection of fields • Field: contains a single data item • Each record is assumed to contain a unique key field • Byte stream files: “Flat” • A file has attributes, stored in its directory entry
- 9. 9 4.2 Files andFile Operations
- 10. 10 4.3Fundamental File Organizations •Records are stored in an ascending or descending sequence according to the key field • Record access pattern of an application is expected to follow suit • Two kinds of operations: • Read the next (or previous) record • Skip the next (or previous) record • Uses: • When data can be conveniently presorted into an ascending or descending order • For byte stream files Sequential File Organization
- 11. 11 4.3Fundamental File Organizations DirectFile Organization • Provides convenience/efficiency of file processing when records are accessed in a random order • Files are called direct-access files • Read/write command indicates value in key field • Key value is used to generate address of record in storage medium • Disadvantages: • Record address calculation consumes CPU time • Some recording capacity of disk is wasted • Dummy records exist for key values that are not in use
- 12. 12 4.3Fundamental File Organizations IndexSequential File Organization • An index helps determine location of a record from its key value • Pure indexed organization: (key value, disk address) • Index sequential organization uses index to identify section of disk surface that may contain the record • Records in the section are then searched sequentially
- 13. 13 4.3Fundamental File Organizations AccessMethods • Access method: IOCS module that implements accesses to a class of files using a specific file organization • Procedure determined by file organization • Advanced I/O techniques are used for efficiency: • Buffering of records • Records of an input file are read ahead of the time when they are needed by a process • Blocking of records • A large block of data, whose size exceeds the size of a record in the file, is always read from, or written onto, the I/O medium
- 14. 14 4.4Directory Structure File systemneeds to grant users: File naming freedom File sharing File system creates several directories Uses a directory structure to organize them Provides file naming freedom and file sharing
- 15. 15 4.4Directory Structure • Someconcepts: home directory, current directory • Path names used to uniquely identify files • Relative path name • Absolute path name
- 16. 16 4.4Directory Structure • Treestructure leads to a fundamental asymmetry in the way different users can access a shared file • Solution: use acyclic graph structure for directories • A link is a directed connection between two existing files in the directory structure Tree and Graph directory structure
- 17. 17 4.4Directory Structure • Mostfrequent operation on directories: search • Other operations are maintenance operations like: • Creating or deleting files • Updating file entries (upon a close operation) • Listing a directory • Deleting a directory • Deletion becomes complicated when directory structure is a graph • A file may have multiple parents • File system maintains a link count with each file Operations on Directories
- 18. 18 4.4Directory Structure • Therecan be many file systems in an OS • Each file system is constituted on a logical disk • i.e., on a partition of a disk • Files can be accessed only when file system is mounted Mounting of File Systems
- 19. 19 4.5 File Protection •Users need controlled sharing of files • Protection info field of the file’s directory entry used to control access to the file • Usually, protection info. stored in access control list • List of (<user_name>,<list_of_access_privileges>) • User groups can be used to reduce size of list • In most file systems, privileges are of three kinds: • Read • Write • Execute
- 20. 20 4.6 Interface BetweenFile System and IOCS • Interface between file system and IOCS consists of • File map table (FMT) • Open files table (OFT) • File control block (FCB)
- 21. 21 4.6 Interface BetweenFile System and IOCS
- 22. 22 4.6 Interface BetweenFile System and IOCS When alpha is opened: • File system copies FMTalpha in memory • Creates fcbalpha in the OFT • Initializes fields appropriately • Passes offset in OFT to process, as internal_idalpha
- 23. 23 4.7 Allocation ofDisk Space • Disk space allocation is performed by file system • Before contiguous memory allocation model • Led to external fragmentation • Now noncontiguous memory allocation model • Issues: • Managing free disk space • Use: free list or disk status map (DSM) • Avoiding excessive disk head movement • Use: Extents (clusters) or cylinder groups • Accessing file data • Depends on approach: linked or indexed
- 24. 24 4.7 Allocation ofDisk Space Linked Allocation • Each disk block has data, address of next disk block • Simple to implement • Low allocation/deallocation overhead • Supports sequential files quite efficiently • Files with nonsequential organization cannot be accessed efficiently • Reliability is poor (metadata corruption)
- 25. 25 4.7 Allocation ofDisk Space FAT (File Allocation table) • MS-DOS uses a variant of linked allocation that stores the metadata separately from the file data • FAT has one element corresponding to every disk block in the disk • Penalty: FAT has to be accessed to obtain the address of the next disk block • Solution: FAT is held in memory during file processing
- 26. 26 4.7 Allocation ofDisk Space • An index (file map table (FMT)) is maintained to note the addresses of disk blocks allocated to a file • Simplest form: FMT can be an array of disk block addresses Indexed Allocation
- 27. 27 4.8 Allocation ofDisk Space Indexed Allocation • Other variations: • Two-level FMT organization: compact, but access to data blocks is slower • Hybrid FMT organization: small files of n or fewer data blocks continue to be accessible efficiently
- 28. 28 4.9 Implementing fileAccess • File System Actions at open • Sets up the arrangement involving FCB and AFT • File System Actions during a File Operation • Performs disk space allocation if necessary • File System Actions at close • Updates directories if necessary
- 29. 29 4.9 Implementing fileAccess • Perform path name resolution • For each component in the path name, locate the correct directory or file • Handle path names passing through mount points • A file should be allocated disk space in its own file system • Build FCB for the file • Retain sufficient information to perform a close operation on the file • Close may have to update the file’s entry in the parent directory • It may cause changes in the parent directory’s entry in ancestor directories File system actions at open
- 30. 30 4.9 Implementing fileAccess File system actions at open
- 31. 31 4.9 Implementing fileAccess File system actions at File Operation • Each file operation is translated into a call: • < opn > (internal_id, record_id,< IO_areaaddr >); • Internal_id is the internal id of <file_name> returned by the open call • Record_id is absent for sequential-access files • Operation is performed on the next record • Disk block address obtained from record_id
- 32. 32 4.9 Implementing fileAccess File system actions at Close