School of Computing Science

1

• School of Computing Science
• Simon Fraser University
• CMPT 300 Operating Systems I
• Chapters 10, 11, 12 File System and Disk
  Scheduling
• Dr. Mohamed Hefeeda

2
Objectives

• Understand how to store and manage information on
  secondary storage systems
• Understand file system
• Interface
• Structure
• Implementation
• Note file system is the most visible part of the
  OS to users

3
Secondary Storage Systems

• Various storage media
• Magnetic disks
• Magnetic tapes
• Optical disks
• .
• Each medium has different physical
  characteristics
• Storing bits on disks is different from storing
  them on CDs
• Yet, OS provides a uniform logical view of
  storage to users
• To efficiently store, locate, and retrieve data
  from a storage system, OS creates one or more
  file systems on it

4
File System Challenges

• File systems involve two design problems
• File system interface how file system looks to
  users
• Define a file, file attributes, operations on
  files, and how files are organized into
  directories
• File system implementation algorithms and data
  structures to map logical file system onto
  physical devices
• Block allocation, free-space management,
  searching a directory, data caching,

5
File System Layered Structure
Application Programs

• Interface file and directory structure
• Maintains pointers to logical block addresses

Logical File System

• Implementation block allocation,
• Maps logical into physical addresses

File-organization Module
Device Drivers

• Implementation device-specific instructions
• Writes specific bit patterns to device controller

Storage Devices
6
File System Interface File Concept

• From users perspective, a file is the smallest
  storage unit
• A file is a named collection of related
  information recorded on a secondary storage
• Information stored in a file could be of various
  types
• Text, numeric data
• Binary data
• Source code
• Executable programs
• ..

7
File Attributes

• Name only information kept in human-readable
  form
• Identifier unique tag (number) identifies file
  within file system
• 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 a directory,
  which is maintained on the disk as well
• Each file has an entry in the directory

8
File Operations

• Create
• Write
• Read
• Reposition within file
• Delete
• Truncate
• More operations (e.g., copy) can be composed of
  these primitives
• To perform these operations, we open the file
  (details later)

9
File System Interface Directory Concept

• Directory is a logical grouping of files
• A directory contains an entry for each file under
  it
• Some systems (UNIX) treat directories just as
  files
• In fact, UNIX treats everything as a file
• Operations on a directory
• Search for a file
• Create a file
• Delete a file
• List a directory
• Rename a file
• Traverse the file system

10
Directory Structure

• Design the directory structure to achieve
• 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
  (aliases, links)
• Grouping logical grouping of files by
  properties, (e.g., all Java programs, all games,
  )
• Tree-structured directories are the most common

11
Tree-Structured Directories
12
Tree-Structured Directories (contd)

• Efficient searching
• Grouping capability
• Things get complicated when we start adding links
  
• Directory is no longer a tree ? acyclic-graph
  structure

13
Acyclic-Graph Directories

• When file is deleted while some links still point
  to it ?
• Dangling pointers!

14
Acyclic-Graph Directories (contd)

• Solution for dangling links in Unix
• Symbolic link
• Just leave the dangling pointer for the user to
  delete
• Try
• ln s file.txt file_symLink.txt
• ls l
• rm file.txt
• ls l
• Hard link
• Keep a reference count on the file
• Only delete the physical file when all links to
  it are deleted
• Try
• ln file.txt file_link.txt
• ls l
• rm file.txt
• ls l
• Links may even create a cycle ? creating a
  general graph

15
General Graph Directory
16
General Graph Directory (contd)

• Suppose we are backing up the entire file system
  or searching for a file through the directory
• With links, we may visit the same subdirectory
  several times
• Very costly (remember directory is stored on
  disk)
• We may even loop for ever if we have cycles!
• Solution? Simply
• Bypass links during directory traversal!

17
File System Mounting

• A file system must be mounted before it can be
  accessed
• OS is given name of the device and a mount point
• OS checks device to make sure it has a valid file
  system
• Then, OS makes the new file system available

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Virtual File Systems

• Multiple file systems can be mounted at same time
  (typical)
• disk UFS (Unix), NTF (Windows), ext2 (Linux),
  ext3,
• CD iso 9660
• File systems on other machines, e.g., Network
  File System (NFS)
• Each file system has its own file and directory
  structure, allocation methods, algorithms and
  data structure,
• To shield users from all these differences, OS
  implements a virtual file system (VFS) layer
• VFS provides a common interface (API) to all file
  systems
• E.g., applications use open(), read(), write(),
  without worrying about which file system(s) is
  (are) being used

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Virtual File System
20
File System Implementation

• To implement a file system, we need
• On-disk structures, e.g.,
• directory structure, number of blocks, location
  of free blocks, boot information,
• (In addition to data blocks, of course)
• In-memory structures to
• improve performance (caching)
• manage file system

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On-disk Structures

• Boot block information to boot the OS
• Volume control block information about the
  volume (partition)
• number of blocks, block size, free block count,
• UFS calls it superblock
• NTFS calls it master file table (relational
  database)
• File control block (FCB) per file, details about
  the file, e.g.,
• size, location of data blocks, file permissions,
  ownership
• UFS calls it inode
• NTFS stores this info in the master file table
• Directory structure how files are organized into
  directories
• UFS uses inodes
• NTFS stores this info in the master file table

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On-disk Structures
Free block
Boot block
Superblock
Directory structure
File
File control block
Data block
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In-memory Structures

• Mount table info on each mounted volume
  (partition)
• Directory-structure cache info on recently
  accessed directories
• System-wide open-file table contains a copy of
  the FCB of each open file in the system
• And info on which process currently using which
  file
• Per-process open-file table contains an entry
  for each file opened by this process, which has
• a pointer to the corresponding entry in the
  system-wide open file table
• and info regarding the usage of the file by this
  process, e.g., current file pointer, open mode
  (read, write), ..

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Opening a File

• Search the directory to find the file control
  block
• May need to bring (from disk) multiple directory
  blocks into memory, if they are not already
  cached
• Consider the case open(/dir1/dir2/dir3/file.txt
  )
• Create an entry in the per-process open-file
  table (PFT)
• Check whether the system-wide open-file table has
  an entry for this file
• if it does
• increment its reference count
• Make the entry in PFT point to this entry
• If it does not
• Create a new entry, set its reference count to 1
• Make the entry in PFT point to the new entry
• Return a pointer (file descriptor) to the entry
  in PFT
• Successive file operations (read, write, ) use
  the file descriptor

25
Opening and Reading from a File
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Creating a File

• Allocate a new file control block (FCB)
• For faster file creation, FCBs are usually
  pre-allocated ?
• Find a free FCB
• Read relevant directory blocks in memory
• Update them to reflect the new file and write
  them back to disk
• Allocate free blocks for the data of to the file
• How do we allocate free blocks to files? And
• How do we know where the free blocks are?

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Allocation Methods

• Problem Allocate free blocks to files
• Given Disks allow random access of blocks
• Objectives Efficient disk space utilization, and
  fast file access
• Three common allocation methods
• Contiguous
• Linked
• Indexed

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Contiguous Allocation

• Each file occupies a set of contiguous blocks
• Needs only start address (block ) and length
  (number of blocks)
• Mapping of logical address (LA)
• Physical block Q start
• Offset within block R
• Block size 512

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Contiguous Allocation (contd)

• Pros
• Simple
• Supports random access efficiently
• Minimal disk head seeks ? fast
• Cons?
• External fragmentation
• Files may not be able to grow

30
Linked Allocation

• Each file is a linked list of blocks
• Blocks could be anywhere
• Each block has a pointer to the next block
• Need start block and end block (to append to
  file)

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Linked Allocation (contd)

• Mapping of logical addresses
• Physical block is at Qth location in the chain
• but, how do we get to it? Traverse the chain!
• Offset within block R 1
• Assume pointer takes 1 byte, and block size is
  512 bytes

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Linked Allocation (contd)

• Pros
• No waste of space (except for pointers)
• Simple need only start and end addresses
• Supports dynamic growing of files
• Cons
• No random access (or very costly to support)
• Reliability one block is corrupted, the chain is
  broken

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Indexed Allocation

• Bring all pointers together into an index block

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Indexed Allocation (cont'd)

• Mapping of logical addresses
• Q displacement into index block
• R offset within the block
• Pros
• Supports random access
• Supports dynamic growing of files
• No external fragmentation
• Cons
• Overhead of index blocks
• A file of one or a few data blocks needs an index
  block
• How do we choose the size of index blocks?

35
Indexed Allocation (cont'd)

• First, consider a file with one index block
• Assume each pointer takes 4 bytes, and block size
  is 512 bytes
• What is the maximum file size supported?
• Index block may have up to 512/4 128 entries ?
• max file size 128 512 64 KB
• Now how do we support larger files?
• Increase size of index blocks ? waste space for
  small files
• Better solutions?

36
Indexed Allocation (contd)

• Linked index blocks
• Last word in index block points to another index
  block
• May need to traverse the index linked list (long
  access time)
• Multilevel index
• First-level index block points to a set of
  second-level index blocks which refer to data
  blocks
• Shorter access time but more space overhead
• Combined (used in Unix File System)
• Multilevel and linked
• Each file has an index block (inode), which
  contains
• Pointers that point to data blocks directly (for
  small files)
• Pointers that point to index blocks, which in
  turn may point to either data blocks or another
  level of index blocks
• UNIX supports up to three level of index blocks

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Combined Scheme UNIX inode
Assume block size of 4KB, 4-byte pointers, 12
direct entries, 1 single, 1 double and1 triple
indirect, what is the max file size supported?
(12 1024 10241024 102410241024) 4KB
gtgt what the 32-bit file pointer can address
(4GB)!
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How Do We Know Where Free Blocks Are?

• Bit map
• Every block has a bit 0 occupied, 1 free
• 00011110 01100000 00001111 1..1
• Simple to implement
• Easy to find contiguous blocks
• Supported by hardware
• Single instruction to find offset of first bit
  with value 1 in a word (of 32 bits) ? fast
  searching
• Disadvantages
• Bit map is stored on disk ? slow to access
• Solution cache it in memory
• Bit maps are not small for large disks ? waste of
  space
• 40-GB disk with 1-KB blocks ? 40 M blocks ? 5-MB
  bitmap
• This makes it difficult to cache the entire
  bitmap

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How Do We Know Where Free Blocks Are?

• Linked List
• No waste of disk space
• But, not easy to get contiguous space

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Disk Scheduling

• Processes issue disk read/write requests
• Kernel maps these requests to physical block
  addresses
• These requests are sent to disk controller
• Problem If there are multiple outstanding
  requests (in a disk queue), which one should be
  serviced first?
• Objectives
• Fast disk access time
• High disk bandwidth (bytes/sec transferred
  between disk and memory)
• Fairness (may be!)
• Before presenting scheduling algorithms, let us
  understand the structure and operation of
  magnetic disks

41
Disk Physical Structure

• Several platters, each is divided into circular
  tracks, which are subdivided into sectors
• Head moves horizontally from one track to
  another
• Disk rotates at high speed (60--200 times/sec)
• Tracks accessed at same head position make a
  cylinder
• Drive can be directly attached to computer via
  I/O bus (EIDE, ATA, SCSI), or it could be
  attached through the network (ISCSI)

42
Disk Logical Structure

• Disk is viewed as a one-dimensional array of
  logical blocks
• The logical block is the smallest unit of
  transfer
• Block sector
• The array of blocks is mapped into sectors of the
  disk sequentially
• Block 0 is at the first sector of the first track
  on the outermost cylinder
• Mapping proceeds in order through that track,
• Then the rest of the tracks in that cylinder,
• Then through the rest of the cylinders from
  outermost to innermost
• Block Address ltcylinder, track, sectorgt

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Disk Operation

• Accessing (reading/writing) a block
• Move the head to desired track (seek time)
• Wait for desired sector to rotate under the head
  (rotational latency time)
• Transfer the block to a local buffer, then to
  main memory (transfer time)
• We try to minimize the seek time, which is
  proportional to the seek distance (distance moved
  by the head)

44
Disk Scheduling Algorithms

• Several algorithms exist to schedule the
  servicing of disk I/O requests
• FCFS
• SSTF
• SCAN, C-SCAN
• LOOK, C-LOOK
• We illustrate them with a request queue (0 -199
  cylinders)
• 98, 183, 37, 122, 14, 124, 65, 67
• Assume initial head position at cylinder 53

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First Come First Served

• Find the total head movements to service the
  request queue 98, 183, 37, 122, 14, 124,
  65, 67
• Let us work it out

Total head movements 640 cylinders
46
Shortest Seek Time First

• Select request with minimum seek time from
  current head position

• Total head movements 236 cylinders
• May cause starvation of some requests

47
SCAN

• Disk arm starts at one end and moves toward the
  other end, servicing requests
• When it gets to the other end, movement is
  reversed

• Total head movements 208 cylinders

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Circular SCAN (C-SCAN)

• Provides a more uniform wait time than SCAN
• The head moves from one end to the other,
  servicing requests as it goes
• When it reaches the other end, however, it
  immediately returns to the beginning of the disk,
  without servicing any requests on the return trip
• Treats cylinders as a circular list that wraps
  around from the last cylinder to the first one

49
C-SCAN (contd)
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C-LOOK

• Version of C-SCAN
• Arm only goes as far as the last request in each
  direction,
• Then reverses direction immediately

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Selecting a Disk-Scheduling Algorithm

• SSTF is common and has a natural appeal
• SCAN and C-SCAN perform better for systems that
  place a heavy load on the disk
• Performance depends on the number and types of
  requests
• Requests for disk service can be influenced by
  the file-allocation method
• The disk-scheduling algorithm should be written
  as a separate module, allowing it to be replaced
  with a different algorithm if necessary
• Either SSTF or LOOK is a reasonable choice for
  the default algorithm

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Summary

• File system interface File and Directory
  concepts
• Directory Structure tree and general graph
• Multiple file systems common interface using
  Virtual FS
• File system implementation
• On-disk structures directory structure, FCB,
  superblock,
• In-memory structures caches, open-file tables
• Details of opening, closing, accessing, and
  creating files
• Block allocation contiguous, linked, indexed
• Free-space management bitmap, linked list
• Disk structure cylinders, tracks, sectors,
  logical blocks
• Transfer time and positioning time (latency
  seek)
• Disk scheduling To minimize seek time (head
  movements)
• FCFC, SSTF, SCAN, LOOK