Operating Systems Internals and Design Principles 6E William

Operating Systems: Internals and Design Principles, 6/E William Stallings Chapter 12 File Management Patricia Roy Manatee Community College, Venice, FL © 2008, Prentice Hall 1

File Management • File management system consists of system utility programs that run as privileged applications • Concerned with secondary storage • Long-term existence • Shared between processes • Structure (internal or directories) 2

File Attributes Possible file attributes 3

File System Calls • Principle Win 32 API functions for file I/O • Second column gives nearest UNIX equivalent 4

Files System Software Architecture 5

Elements of File Management 6

File Management Functions • Identify and locate a selected file • Use a directory to describe the location of all files plus their attributes • On a shared system describe user access control 7

Criteria for File Organization • Quick random access – Needed when accessing a single record • Ease of update • Economy of storage – Should be minimum redundancy in the data – Redundancy can be used to speed access such as an index • Simple maintenance • Reliability 8

The Pile 9

The Sequential File 10

Indexed Sequential File 11

Indexed File 12

File Organization • The Direct or Hashed File – Directly access a block at a known address – Key field required for each record 13

Performance 14

File Directories • Contains information about files – Attributes – Location – Ownership • Directory itself is a file owned by the operating system • Provides mapping between file names and the files themselves 15

Information Elements of a File Directory 16

Information Elements of a File Directory 17

Hierarchical, or Tree-Structured Directory • Master directory with user directories underneath it • Each user directory may have subdirectories and files as entries 18

Tree-Structured Directory 19

Example of Tree-Structured Directory 20

Hierarchical, or Tree-Structured Directory • Files can be located by following a path from the root, or master, directory down various branches – This is the pathname for the file • Can have several files with the same file name as long as they have unique path names • Current directory is the working directory • Files are referenced relative to the working directory 21

Directory System Calls • Principle Win 32 API functions for directory management • Second column gives nearest UNIX equivalent, when one exists 22

File Sharing • In multiuser system, allow files to be shared among users • Two issues – Access rights (read, write, execute) – Management of simultaneous access (cf. interprocess communication, mutual exclusion) 23

Access Matrix 24

Access Control List 25

Capability Lists 26

File System Implementation A possible file system layout 27

Secondary Storage Management • Space must be allocated to files • Must keep track of the space available for allocation 28

Disk Space Management Block size • Dark line (left hand scale) gives data rate of a disk • Dotted line (right hand scale) gives disk space efficiency • All files 2 KB 29

Contiguous Allocation • Single set of blocks is allocated to a file at the time of creation • Only a single entry in the file allocation table – Starting block and length of the file • External fragmentation will occur – Need to perform compaction 30

Contiguous File Allocation 31

Contiguous File Allocation 32

Chained Allocation • Allocation on basis of individual block • Each block contains a pointer to the next block in the chain • Only single entry in the file allocation table – Starting block and length of file 33

Chained Allocation • No external fragmentation • Best for sequential files • No accommodation of the principle of locality 34

Chained Allocation 35

Chained Allocation 36

Indexed Allocation • File allocation table contains a separate onelevel index for each file • The index has one entry for each portion allocated to the file • The file allocation table contains block number for the index 37

Indexed Allocation 38

The MS-DOS File System (1) The MS-DOS directory entry 39

The MS-DOS File System (2) • Maximum partition for different block sizes • The empty boxes represent forbidden combinations 40

Inodes • Index node • Control structure that contains key information for a particular file: attributes and location of blocks 41

Free. BSD File Allocation 42

UNIX Directories and Inodes 43

The UNIX File System The steps in looking up /usr/ast/mbox 44

Shared Files (1) File system containing a shared file 45

Shared Files (2) (a) Situation prior to linking (b) After the link is created (c) After the original owner removes the file 46

Windows 2000 (1) The NTFS master file table 47

Windows 2000 (2) An MFT record for a three-run, nine-block file 48

Windows 2000 (3) A file that requires three MFT records to store its runs 49

Linux Virtual File System • Uniform file system interface to user processes • Represents any conceivable file system’s general feature and behavior • Assumes files are objects that share basic properties regardless of the target file system 50

Linux Virtual File System Context 51

Linux Virtual File System Concept 52