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Chapter 11 File-System Interface

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Outline

File ConceptAccess MethodsDirectory StructureFile System MountingFile Sharing Protection

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File Concept

Contiguous logical address spaceTypes: Data

numeric character binary

Program

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File StructureNone - sequence of words, bytesSimple record structure Lines Fixed length Variable length

Complex Structures Formatted document Relocatable load file

Can simulate last two with first method by inserting appropriate control characters.Who decides: Operating system Program

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File AttributesName – only information kept in human-readable form.Type – needed for systems that support different types.Location – pointer to file location on device.Size – current file size.Protection – controls who can do reading, writing, executing.Time, date, and user identification – data for protection, security, and usage monitoring.Information about files are kept in the directory structure, which is maintained on the disk.

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File OperationsCreateWriteReadReposition within file – file seekDeleteTruncateOpen(Fi) – search the directory structure on disk for entry Fi, and move the content of entry to memory.Close (Fi) – move the content of entry Fi in memory to directory structure on disk.

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File Types – Name, Extension

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Open a FileA file is open when it is first used. No search on directory is required for later usage. Two levels of tables

A table for each process (information dependent on the process)

index to system table current position pointer

A system open-file table (information independent from a process)

open count (for shared files) disk location access date file size

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OS and Disk I/OAdvantages for OS to know the file type avoid some mistakes (e.g., print a binary-obje

ct) TOPS-20: if source program has been modified,

running an old .obj file causes re-compiling automatically

Windows: doubling clicking a .doc file invokes MS Words

Disk I/O performs in units of one block logical records packing a physical block all file systems have internal fragmentation

larger block larger internal fragmentation

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Access MethodsSequential access: records are processed in order, e.g., tapes read/write (next) reset to beginning, skip forward/backword

Direct access (disks) position (seek) to the n-th record read/write

Other (direct access+index) an index table (can be put into memory); the index,

like an index in the back of a book, contains pointers to the various blocks.

file in disk: reduce I/O for searching on a large file

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Sequential-access

beginning endcurrent position

rewindread/write

Index and relative files

adams

asherarthur

smith

smith, john id age address

index filerelative file

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Directory Structure (1)A collection of nodes containing information about all files.

F 1 F 2F 3

F 4

F n

Directory

Files

Both the directory structure and the files reside on disk.Backups of these two structures are kept on tapes.

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Directory Structure (2)

Typical file-system organization disk partitions (IBM: minidisk, PC: volume)

files (including directories)

directory

directory

partition A

partition B

disk 1

directory

partition C

disk 2

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Directory Structure (3)Directory structures Single-level directory Two-level directory Tree-structured directory (more common) Acyclic-graph directory

(cycle-detection is expensive) General graph directory

(garbage collection is expensive)

flexible

complicated

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Operations Performed on Directory

Search for a fileCreate a fileDelete a fileList a directoryRename a fileTraverse the file system

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Organize the Directory (Logically) to Obtain

Efficiency – locating a file quickly.Naming – convenient to users. Two users can have same name for different

files. The same file can have several different

names.

Grouping – logical grouping of files by properties, (e.g., all Java programs, all games, …)

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Single-Level Directory

A single directory for all users.

Naming problem:1. difficult to remember all names 2. not easy to give a new name 3. confusion between different usersGrouping problem

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Two-Level DirectorySeparate directory for each user.

•Path name: user name + file name•Can have the same file name for different user•Efficient searching•No grouping capability

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Tree-Structured Directories

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Tree-structured directory each user has a current directory absolute path name vs. relative path name search path

stat mail dist test

root spell obj program

pe2 cc pas a

jaja ab cat bo a test

current

directory

mail\jaja, or

\spell\mail\jaja

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Acyclic-graph directory: 1. share subdirectory (file): the same file or directory may be in two different directories.

2. relative simplicity of the algorithms to traverse the graph and to determine when there are no more reference to a file.

: 1. Search/traverse the file system should be careful2. Deletion is complicated:

Dangling pointers to a deleted file. Solution: Keep a count of the number of refere

nces3. Detection of cycles is expensive

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cat mail dist pe2

root spell obj program

pe2 cc pasc a

cat bo a test

Acyclic-graph directory

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General graph directory: How do we guarantee no cycle?

1. Searching/traversing is difficult search a directory twice limit # of directories that can be accessed infinite loop

2. Deletion is more complicated Self-reference: a set of files with nonzero

reference-count that are impossible to refer, the possibility of self-referencing (a cycle) in the directory structure.

Solution: garbage collection (very expensive)

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cat mail dist pe2

root

spell obj program

pe2 cc pasc a

root bo a test

General graph directory

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File System Mounting

A file system must be mounted before it can be accessed.

A unmounted file system (I.e. Fig. 11-11(b)) is mounted at a mount point.

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(a) Existing. (b) Unmounted Partition

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Mount Point

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File SharingSharing of files on multi-user systems is desirable.

Sharing may be done through a protection scheme.

On distributed systems, files may be shared across a network.

Network File System (NFS) is a common distributed file-sharing method.

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ProtectionFile owner/creator should be able to control: what can be done by whom

Types of access Read Write Execute Append Delete List

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Access Lists and GroupsMode of access: read, write, executeThree classes of users

RWXa) owner access 7 1 1 1

RWXb) group access 6 1 1 0

RWXc) public access 1 0 0 1

Ask manager to create a group (unique name), say G, and add some users to the group.For a particular file (say game) or subdirectory, define an appropriate access. owner group public

chmod 761 game