File Systems - New Mexico Institute of Mining and...

CSE325 Principles of Operating Systems

File Systems

David P. Duggan [email protected]

March 22, 2011

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External View of File Manager

Hardware

Application Program

File

Mgr

Dev

ice

Mgr

Mem

ory

Mgr

Pr

oces

s Mgr

UNIX Fi

le M

gr

Dev

ice

Mgr

Mem

ory

Mgr

Pr

oces

s Mgr

Windows

open() read()

close()

write()

lseek()

CreateFile() ReadFile() CloseHandle()

SetFilePointer()

WriteFile() mount()

Why do we need files?

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File Management •  File is a named, ordered collection of information •  File manager’s job

–  Storing the information on a device –  Mapping the block storage to a logical view –  Allocating/deallocating storage –  Organizing files efficiently

•  What abstraction should be presented to programmer?


Outline •  File Management Overview •  File and Directory Structures •  File Sharing & Protection •  Allocation Methods

–  Contiguous allocation –  Linked allocation –  Indexed allocation

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File Structure •  None - sequence of words, bytes •  Simple record structure

–  Lines –  Fixed length –  Variable length

•  Complex Structures –  Formatted document –  Relocatable load file

•  Who decides: –  Operating system –  Program

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Directories •  Set of files •  Used to organize files •  Translates external file names to internal file names (file

labels, i-nodes, file control blocks) •  Treated as a file but with special operations

•  Why do we need directories?

•  Operations –  Search for a file –  Create a file –  Delete a file –  List a directory –  Rename a file –  Traverse the file

system


Organize the Directory (Logically) •  Grouping – logical grouping of files by properties,

(e.g., all Java programs, all games, …) •  Efficiency – locating a file quickly (performance) •  Naming – convenient to users

–  Two users can have same name for different files –  The same file can have several different names

•  Directory structures?


Single-Level Directory

•  A single directory for all users

Grouping problem Naming problem

Good News? Problems?

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Two-Level Directory

•  Separate directory for each user

  Path name"  Can have the same file name for different user"  Efficient searching"  No grouping capability"

Advantages? Problems?


Tree-Structured Directories


Tree-Structured Directories (Cont.) •  Efficient searching

•  Grouping Capability

•  Current directory (working directory)

–  cd /spell/mail/prog –  type list

CSE325 - File Systems 12

Tree-Structured Directories (Cont.) •  Absolute or relative path name •  Creating a new file is done in current directory •  Delete a file

rm <file-name> •  Creating a new subdirectory is done in current

directory mkdir <dir-name>

Example: if in current directory /mail mkdir count

mail"

prog" copy" prt" exp" count"

Deleting “mail” ⇒ deleting the entire subtree rooted by “mail”"


Acyclic-Graph Directories •  Have shared subdirectories and files


Acyclic-Graph Directories (Cont.) •  Two different names (aliasing)

•  New directory entry type –  Link – another name (pointer) to an existing file –  Resolve the link – follow pointer to locate the file

•  If dict deletes count ⇒ dangling pointer Solutions:

–  Backpointers, so we can delete all pointers Variable size records a problem

–  Entry-hold-count solution


General Graph Directory


General Graph Directory (Cont.) •  How do we guarantee no cycles?

–  Allow only links to file not subdirectories –  Every time a new link is added use a cycle detection

algorithm to determine whether it is OK


Directory Implementation •  Linear list of file names with pointer to the data

blocks. –  simple to program –  time-consuming to execute

•  Hash Table – linear list with hash data structure. –  decreases directory search time –  collisions – situations where two file names hash to the

same location –  fixed size


File Sharing •  Sharing of files on multi-user systems is desirable

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

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

•  Sharing may be done through a protection scheme

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File Sharing – Remote File Systems •  Uses networking to allow file system access between systems

–  Manually via programs like FTP –  Automatically, seamlessly using distributed file systems –  Semi-automatically via the world wide web

•  Client-server model allows clients to mount remote file systems from servers –  Server can serve multiple clients –  Client and user-on-client identification is insecure or

complicated –  NFS is standard UNIX client-server file sharing protocol –  CIFS is standard Windows protocol –  Standard operating system file calls are translated into remote

calls


File Sharing – Consistency Semantics •  Consistency semantics specify how multiple

users are to access a shared file simultaneously –  Similar to process synchronization algorithms –  Unix file system (UFS) implements:

•  Writes to an open file visible immediately to other users of the same open file

•  Sharing file pointer to allow multiple users to share the pointer of current location into the file

–  Andrew File System (AFS) implemented complex remote file sharing semantics

–  AFS has session semantics •  Writes only visible to sessions starting after the file is

closed


Access Lists and Groups •  Mode of access: read, write, execute •  Three classes of access

RWX a) owner access 7 ⇒ 1 1 1

RWX b) group access 6 ⇒ 1 1 0 RWX c) 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"Attach a group to a file

chgrp G game

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Protection Mechanisms •  Files are OS objects: unique names and a finite set of

operations that processes can perform on them

•  Protection domain is a set of {object,rights}, where right is the permission to perform one of the operations

•  Each process runs in some protection domain.

•  How do we store all the protection domains ?

Domain"


Access Control Lists (ACL)

Example: Owner’s ID stored in PCB File owner’s ID and access permissions stored in inode


Capabilities List (C-List)

•  Each process has a capability for every resource it can access –  Kept with other process meta data –  Checked by the kernel on every access


Allocation Methods •  An allocation method refers to how disk blocks

are allocated for files:

–  Contiguous allocation

–  Linked allocation

–  Indexed allocation

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Contiguous Allocation •  Each file occupies a set of contiguous blocks on the

disk •  Maps the N file blocks into N contiguous blocks on the

secondary storage device

Head position 237 … First block 785 Number of blocks 25

File descriptor

•  Simple – only starting location (block #) and length (number of blocks) are required •  Random access

Good news?


Contiguous Allocation (cont.) •  Mapping from logical address to physical

LA/512"

Q"

R"

Block to be accessed = Q + starting address Displacement into block = R


Contiguous Allocation of Disk Space

•  Wasteful of space (dynamic storage-allocation problem)

•  Difficult to support dynamic file sizes (files cannot grow)

Drawbacks?

How do we solve this problem?


Linked Allocation •  Each file is a linked list of disk blocks: blocks may

be scattered anywhere on the disk. •  Each block contains a header with

–  Number of bytes in the block –  Pointer to next block

pointer"block ="


Linked Allocation

•  Simple – need only starting address

•  Free-space management system – no waste of space

Good news?

•  No random access •  Seeks can be slow •  Mapping of free space

Bad news?


File-Allocation Table


Indexed Allocation •  Brings all pointers together into the index block. •  Logical view.

index table"


Example of Indexed Allocation

•  Simplify seeks •  Random access •  Dynamic access without external fragmentation •  May link indices together (for large files)

Good news?

•  Overhead of index block.

Bad news?


Indexed Allocation – Mapping (Cont.)

"

outer-index"

index table" file"


Combined Scheme: UNIX (4K bytes per block)


Unallocated Blocks •  How should unallocated blocks be managed? •  Need a data structure to keep track of them

–  Linked list •  Very large •  Hard to manage spatial locality

–  Block status map (“bit map” or “bit vector”) •  Bit per block •  Easy to identify nearby free blocks •  Useful for disk recovery


Efficiency and Performance •  Efficiency dependent on

–  disk allocation and directory algorithms –  types of data kept in file’s directory entry

•  Performance –  disk cache – separate section of main memory for

frequently used disk blocks –  free-behind and read-ahead – techniques to optimize

sequential access –  improve PC performance by dedicating section of

memory as virtual disk, or RAM disk

File Systems - New Mexico Institute of Mining and...

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