File Management B Ramamurthy 12232021 B Ramamurthy 1

File Management B. Ramamurthy 12/23/2021 B. Ramamurthy 1

Introduction z. File system is the most visible aspect of an operating system. It provides the mechanism for on-line storage of and access to programs and data. It provides the persistent storage capability to a system. z. File systems consists of a collection of files, a directory structure, access methods, secondary storage management and partitions (which separate logical and physical collection of directories. ) 12/23/2021 B. Ramamurthy 2

Topics for Discussion z. File Attributes z. File operations and structures z. File Management z. File Organization z. File Directories z. Directory Structure z. File sharing 12/23/2021 B. Ramamurthy 3

File Attributes z Name: Symbolic reference for identifying the file object. z Type: Information that indicates the contents of the file. z Location: This information is a pointer a device and to the location of the file on that device. z Size : The current size of the file (in bytes, words, or blocks). z Protection: Access control information (RWX) z Time, date and user identification: This information may be kept for (1) creation, (2) last modification and (3) last use. Useful for protection, security and usage monitoring. 12/23/2021 B. Ramamurthy 4

File Operations and structures z A file is an abstract data type. z Operations: open, close, create, destroy, copy, rename, list, read, write, update, insert item, delete item, size, . . . z Open file table: Table containing information about open files. When a file operation is requested, an index into this table is used for locating the file. When a file is closed the entry is removed from the table. z Current file pointer: Last read/write location is kept as a current-file-position pointer. Each process using the file has a unique pointer. Where is it kept? z File open count: Number of opens done on a given file. To allow deletion from Open file table, once the count 12/23/2021 B. Ramamurthy 5 reaches 0.

File management (Fig. 12. 2) z Users and application programs interact with file system by means of commands for performing operations on files. z These commands are translated into specific file manipulation commands, after ensuring that the kind of access requested is allowed. z User view may be that of records or few bytes, but the actual IO is done in blocks. Data conversion to block “packing” is done. Optimized where applicable. z Now IO subsystems takes over by translating the file sub commands into IO subsystem (disk IO) commands. 12/23/2021 B. Ramamurthy 6

File organization z. File organization is the logical structuring of its contents as determined by the way in which they are processed. z. Why different organizations? Based on one or more of these requirements: y. Rapid access, Ease of update, economy, simple maintenance, Reliability. 12/23/2021 B. Ramamurthy 7

File organization (contd. ) z Some common file organizations: Fig. 12. 3 y. Pile : Data collected in the order in which they arrive: Variable length records. y. Sequential file: Fixed length records, sequential based on a key field. y. Indexed sequential file: Index file to allow random access and an overflow file. y. Indexed file: Multiple indices. No restriction on the placement of the record. Records of variable length. Partial or exhaustive index. z Discuss applications, advantages and disadvantages of each organization. 12/23/2021 B. Ramamurthy 8

File directories z. Directory is itself a file that contains information about the files, their attributes, location and ownership. z. Directory provides a mapping between file name and the actual files. z. Table 12. 2 for details kept in a directory. 12/23/2021 B. Ramamurthy 9

Directory structure z. Simplest form: one entry per file in a tabular form. Sequential list. z. Problems: y. File organization is difficult. y. Unique naming is required. y. Sharing becomes difficult. 12/23/2021 B. Ramamurthy 10

Directory structure (contd. ) z A more powerful, flexible approach is a hierarchical, tree-structured approach. z Issues: z Directory, sub-directory organization. z Naming: Use of tree-structured directory minimizes difficulty in assigning names: path name. z Path: absolute and relative, current working directory concept. z Navigation. See Fig. 12. 5 12/23/2021 B. Ramamurthy 11

File sharing z. Specific user, groups, world access. z. Different access privileges: read, write, execute. z. Changing protection. z. Locking : file locking or record level locking. 12/23/2021 B. Ramamurthy 12

Directory entries z(Most details given here pertain to unix file systems. ) z. A directory entry for a file is called a hard link or simply link to that file. z. A file may have more than one link. z. Besides hard link, unix file system provides symbolic link facility. 12/23/2021 B. Ramamurthy 13

Unix File Management (12. 7) z Unix kernel views all files as streams of bytes. z Four types of files are distinguished: y. Ordinary : regular files with data from user, or an application. y. Directory : Contains list of file names + pointers to associated inodes. y. Special : Terminals and other peripherals are abstracted as files. y. Named : named pipes. 12/23/2021 B. Ramamurthy 14

Operations z pathname seraching : lookup z name creation : creat, mknod, link, symlink, mkdir z name change/deletion: rename, remove, rmdir z attribute manipulation: access, getattr, setattr z object interpretation: open, readir, readlink, mmap, close z process control : advlock, ioctl, select z object management : lock, unlock, inactive, reclaim, abortop 12/23/2021 B. Ramamurthy 15

Inodes z. Inode (information node) is a structure that contains the key information needed for managing a file. z. Several files names may be associated with an inode. z. But each file contains exactly one file. 12/23/2021 B. Ramamurthy 16

Information in an inode z File mode (access and execution permissions) z Link count ( how many references) z Owner ID z Group ID z File Size z File Address : 39 bytes of address information as explained in the next slide z Last accessed time, last modified time, late inode modification time 12/23/2021 B. Ramamurthy 17

File address z 13 3 -byte addresses (39 bytes) z. Direct : 10 : direct pointer 10 1 K chunks of memory z. Single indirect: 1 : indirect block of 256 points each of which points to a 1 K : 256 K z. Double indirect: 1: 256 X 256 : 65 M z. Triple Indirect : 1: 256 X 256 : 16 G 12/23/2021 B. Ramamurthy 18

Directories z. Directories are allocated in units called chunks. z. Chunks are broken up into variable-length directory entries. z. A directory entry contains: index to inode structures, the size of entry in bytes, type of entry, length of filename, pointer to next entry. 12/23/2021 B. Ramamurthy 19