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File Systems & Directory Structures: Access, Allocation & Free-Space Management, Study notes of Operating Systems

Drexel UniversityOperating Systems

An in-depth exploration of file systems and directory structures, covering access methods, allocation methods, and free-space management. Topics include contiguous, linked, and indexed allocation, as well as bit vector and linked free-space list. Understand the concepts behind file control blocks, virtual file systems, and file system implementation.

Typology: Study notes

Pre 2010

Uploaded on 08/19/2009

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koofers-user-1po 🇺🇸

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Operating Systems
File Concept
Access Methods
Directory Structure
File System Mounting
File Sharing - Protection
File Systems
File Concept
• Contiguous logical address sp ace
•Types:
–Data
• numeric
• character
•binary
–Program
File Structure
• None - sequence of words, bytes
• Simple record structure
–Lines
– Fixed length
– Variable length
• Complex Structures
– Formatted document
– Relocatable load file
• Can simulate last two with fi rst method by inserting
appropriate control characters.
• Who decides:
– Operating system
–Program
File Attributes
•Name
– only information kept in human-re
a
•Type – needed for systems that support different types.
•Location – pointer to file location on device.
•Size – current file size.
•Protection – controls wh o can do reading, writing,
executing.
•Time, date, and user identification
– data for protection,
security, and usage monitor ing.
• Information about files are kept in the directory structure,
which is maintained on the disk.
File Operations
• Create
•Write
•Read
• Reposition within file – file seek
• Delete
• Truncate
•Open(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.
File Types – Name, Extension
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pf4
pf5
pf8
pf9
pfa

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1

Operating Systems

File Concept Access Methods Directory Structure File System Mounting File Sharing - Protection

File Systems

File Concept

  • Contiguous logical address space
  • Types:
    • Data
      • numeric
      • character
      • binary
    • Program

File Structure

  • None - sequence of words, bytes
  • Simple 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

File Attributes

- Name - only information kept in human-rea - Type – needed for systems that support different types. **- Location

  • Size** – current file size. – pointer to file location on device. - 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.

File Operations

  • Create
  • Write
  • Read
  • Reposition within file – file seek
  • Delete
  • Truncate
  • Open( Fi ) – search the directory structure on disk for entry Fi , and move the content of entry to memory.
  • Close ( F (^) i ) – move the content of entry F (^) i in memory to directory structure on disk.

File Types – Name, Extension

Access Methods

- Sequential Access read next write next reset no read after last write (rewrite) - Direct Access read n write n position to n read next write next rewrite n n = relative block number

Sequential-access File

Simulation of Sequential Access on a

Direct-access File

Example of Index and Relative Files

Directory Structure

  • A collection of nodes containing information about all files.

F 1 F 2^ F 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.

A Typical File-system Organization

Tree-Structured Directories (Cont.)

  • Efficient searching
  • Grouping Capability
  • Current directory (working directory) - cd /spell/mail/prog - type list

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
  • Deleting “mail” ⇒ deleting the entire subtree rooted by “mail”.

mail prog copy prt exp count

Acyclic-Graph Directories

  • Have shared subdirectories and files.

Acyclic-Graph Directories (Cont.)

  • Two different names (aliasing)
  • If dict deletes list ⇒ dangling pointer. Solutions: - Backpointers, so we can delete all pointers. Variable size records a problem. - Backpointers using a daisy chain organization. - Entry-hold-count solution.

General Graph Directory General Graph Directory (Cont.)

  • How do we guarantee no cycles?
    • Allow only links to file not subdirectories.
    • Garbage collection.
    • Every time a new link is added use a cycle detection algorithm to determine whether it is OK.

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.

(a) Existing. (b) Unmounted

Partition

Mount Point File Sharing

  • Sharing 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.

Protection

  • File owner/creator should be able to control:
    • what can be done
    • by whom
  • Types of access
    • Read
    • Write
    • Execute
    • Append
    • Delete
    • List

Access Lists and Groups

  • Mode of access: read, write, execute
  • Three classes of users 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.

Schematic View of Virtual File

System

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

Allocation Methods

  • An allocation method refers to how disk blocks are allocated for files:
  • Contiguous allocation
  • Linked allocation
  • Indexed allocation

Contiguous Allocation

  • Each file occupies a set of contiguous blocks on the disk.
  • Simple – only starting location (block #) and length (number of blocks) are required.
  • Random access.
  • Wasteful of space (dynamic storage-allocation problem).
  • Files cannot grow.

Contiguous Allocation of Disk

Space

Extent-Based Systems

  • Many newer file systems (I.e. Veritas File System) use a modified contiguous allocation scheme.
  • Extent-based file systems allocate disk blocks in extents.
  • An extent is a contiguous block of disks. Extents are allocated for file allocation. A file consists of one or more extents.

Linked Allocation

  • Each file is a linked list of disk blocks: blocks may be scattered anywhere on the disk.

block = pointer

Linked Allocation (Cont.)

  • Simple – need only starting address
  • Free-space management system – no waste of space
  • No random access
  • Mapping
    • Block to be accessed is the Qth block in the linked chain of blocks representing the file.
    • Displacement into block = R + 1
  • File-allocation table (FAT) – disk-space allocation used by MS-DOS and OS/2.

LA/

Q R

Linked Allocation File-Allocation Table

Indexed Allocation

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

index table

Example of Indexed Allocation

Free-Space Management (Cont.)

  • Bit map requires extra space. Example: block size = 2^12 bytes disk size = 2 30 bytes (1 gigabyte) n = 2 30 /2 12 = 2 18 bits (or 32K bytes)
  • Easy to get contiguous files
  • Linked list (free list)
    • Cannot get contiguous space easily
    • No waste of space
  • Grouping
  • Counting

Free-Space Management (Cont.)

  • Need to protect:
    • Pointer to free list
    • Bit map
      • Must be kept on disk
      • Copy in memory and disk may differ.
      • Cannot allow for block[ i ] to have a situation where bit[ i ] = 1 in memory and bit[ i ] = 0 on disk.
    • Solution:
      • Set bit[ i ] = 1 in disk.
      • Allocate block[ i ]
      • Set bit[ i ] = 1 in memory

Linked Free Space List on Disk 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 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.

Various Disk-Caching Locations Page Cache

  • A page cache caches pages rather than disk blocks using virtual memory techniques.
  • Memory-mapped I/O uses a page cache.
  • Routine I/O through the file system uses the buffer (disk) cache.
  • This leads to the following figure.

I/O Without a Unified Buffer Cache Unified Buffer Cache

  • A unified buffer cache uses the same page cache to cache both memory-mapped pages and ordinary file system I/O.

I/O Using a Unified Buffer Cache Recovery

  • Consistency checking – compares data in directory structure with data blocks on disk, and tries to fix inconsistencies.
  • Use system programs to back up data from disk to another storage device (floppy disk, magnetic tape).
  • Recover lost file or disk by restoring data from backup.