EE512 System Programming

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EE512 System Programming
Lecture 9File Systems
Oct. 8, 2009 Prof. Kyu Ho Park http//core.kaist.a
c.kr
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• File Systems

1 Files 2 Directories 3 File system
implementation 4 Example file systems
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Long-term Information Storage

• Must store large amounts of data
• Information stored must survive the termination
  of the process using it
• Multiple processes must be able to access the
  information concurrently

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File Naming

• Typical file extensions.

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File Structure

• Three kinds of files
• byte sequence
• record sequence
• tree

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File Types

• (a) An executable file (b) An archive

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File Access

• Sequential access
• read all bytes/records from the beginning
• cannot jump around, could rewind or back up
• convenient when medium was mag tape
• Random access
• bytes/records read in any order
• essential for data base systems
• read can be
• move file marker (seek), then read or
• read and then move file marker

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File Attributes

• Possible file attributes

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File Operations

• Create
• Delete
• Open
• Close
• Read
• Write

• Append
• Seek
• Get attributes
• Set Attributes
• Rename

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An Example Program Using File System Calls (1/2)
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An Example Program Using File System Calls (2/2)
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DirectoriesSingle-Level Directory Systems

• A single level directory system
• contains 4 files
• owned by 3 different people, A, B, and C

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Two-level Directory Systems

• Letters indicate owners of the directories and
  files

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Hierarchical Directory Systems

• A hierarchical directory system

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Path Names

• A UNIX directory tree

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Directory Operations

• Readdir
• Rename
• Link
• Unlink

• Create
• Delete
• Opendir
• Closedir

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File System Implementation

• A possible file system layout

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File System Layout

• Sector 0 MBR to boot the computer
• End of the MBR partition table-gtStarting and
  ending address of each partition.
• When the computer is booted, the BIOS reads in
  and executes the MBR.
• The first work of the MBR is locating the active
  partition reads in its first block(boot block)
  and execute it.

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File System

• 5. The program in the boot block loads OS
  contained in that partition.

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Implementing Files (1)

• (a) Contiguous allocation of disk space for 7
  files
• (b) State of the disk after files D and E have
  been removed

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Implementing Files (2)

• Storing a file as a linked list of disk blocks

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Implementing Files (3)

• Linked list allocation using a file allocation
  table in RAM

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Implementing Files (4)

• An example i-node

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Implementing Directories

• When a file is opened , the OS uses the path name
  supplied by the user to locate the directory
  entry.
• The directory entry provides the information
  needed to find the disk blocks.
• Depending on the systems, the information may be
  the disk address of the entire files(contiguous
  allocation), the number of the first block, or
  the number of the i-node.

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Directory

• The main function of the directory system is to
  map the ASCII name of the file onto the
  information needed to locate the data.
• Attribute of a file
• files owner, creation time, modified time----

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Implementing Directories (1)

• (a) A simple directory
• fixed size entries
• disk addresses and attributes in directory entry
• (b) Directory in which each entry just refers to
  an i-node

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Implementing Directories (2)

• Two ways of handling long file names in directory
• (a) In-line
• (b) In a heap

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Shared Files (1)

• File system containing a shared file

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Shared Files (2)

• (a) Situation prior to linking
• (b) After the link is created
• (c)After the original owner removes the file

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Disk Space Management (1)
Block size

• Dark line (left hand scale) gives data rate of a
  disk
• Dotted line (right hand scale) gives disk space
  efficiency
• All files 2KB

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Disk Space Management (2)

• (a) Storing the free list on a linked list
• (b) A bit map

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Disk Space Management (3)

• (a) Almost-full block of pointers to free disk
  blocks in RAM
• - three blocks of pointers on disk
• (b) Result of freeing a 3-block file
• (c) Alternative strategy for handling 3 free
  blocks
• - shaded entries are pointers to free disk blocks

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Disk Space Management (4)

• Quotas for keeping track of each users disk use

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File System Reliability (1)
File that has not changed

• A file system to be dumped
• squares are directories, circles are files
• shaded items, modified since last dump
• each directory file labeled by i-node number

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Dumping Algorithm

• From the root directory, examine all the entries
  in it. All modified files are marked in its
  i-node bitmap. All directories including not
  modifed ones are also marked in its bitmap.
• Unmarking any directories that have no modified
  file or directories in them or under them.
• Dumping directories that are marked.
• Dumping files that are marked.

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File System Reliability (2)

• Bit maps used by the logical dumping algorithm

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Supplement to IPC
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Monitors

• A high-level abstraction that provides a
  convenient and effective mechanism for process
  synchronization
• Only one process may be active within the monitor
  at a time

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Schematic view of a Monitor
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Condition Variables

• Condition x, y
• Two operations on a condition variable
• x.wait () a process that invokes the operation
  is suspended.
• x.signal () resumes one of processes (if any)
  that invoked x.wait ()

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Monitor with Condition Variables
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Monitors (1)
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Monitors (2)
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Monitors (3)
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Interprocess Communication

• Processes
• Independent Processes or Cooperating Processes
• IPC issues
• How one process can pass information to another.
• Mutual Exclusion.
• Proper sequencing when dependencies are present.

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Interprocess Communication(IPC)

• Message Passing Shared Memory

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Producer-Consumer Problem

• Paradigm for cooperating processes, producer
  process produces information that is consumed by
  a consumer process
• unbounded-buffer places no practical limit on the
  size of the buffer
• bounded-buffer assumes that there is a fixed
  buffer size

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Message Passing (1)

• Figure 2-17. The producer-consumer problem with N
  messages.

. . .
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Message Passing (2)
. . .

• Figure 2-17. The producer-consumer problem with N
  messages.

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Message Passing

• Message system processes communicate with each
  other without resorting to shared variables
• Message passing facility provides two operations
• send(message) message size fixed or variable
• receive(message)
• If P and Q wish to communicate, they need to
• establish a communication link between them
• exchange messages via send/receive
• Implementation of communication link
• physical (e.g., shared memory, hardware bus)
• logical (e.g., logical properties)

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Implementation Questions

• How are links established?
• Can a link be associated with more than two
  processes?
• How many links can there be between every pair of
  communicating processes?
• What is the capacity of a link?
• Is the size of a message that the link can
  accommodate fixed or variable?
• Is a link unidirectional or bi-directional?

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Direct Communication

• Processes must name each other explicitly
• send (P, message) send a message to process P
• receive(Q, message) receive a message from
  process Q
• Properties of communication link
• Links are established automatically
• A link is associated with exactly one pair of
  communicating processes
• Between each pair there exists exactly one link
• The link may be unidirectional, but is usually
  bi-directional

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Indirect Communication

• Messages are directed and received from mailboxes
  (also referred to as ports)
• Each mailbox has a unique id
• Processes can communicate only if they share a
  mailbox
• Properties of communication link
• Link established only if processes share a common
  mailbox
• A link may be associated with many processes
• Each pair of processes may share several
  communication links
• Link may be unidirectional or bi-directional

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Indirect Communication

• Operations
• create a new mailbox
• send and receive messages through mailbox
• destroy a mailbox
• Primitives are defined as
• send(A, message) send a message to mailbox A
• receive(A, message) receive a message from
  mailbox A

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Indirect Communication

• Mailbox sharing
• P1, P2, and P3 share mailbox A
• P1, sends P2 and P3 receive
• Who gets the message?
• Solutions
• Allow a link to be associated with at most two
  processes
• Allow only one process at a time to execute a
  receive operation
• Allow the system to select arbitrarily the
  receiver. Sender is notified who the receiver
  was.

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Synchronization

• Message passing may be either blocking or
  non-blocking
• Blocking is considered synchronous
• Blocking send has the sender block until the
  message is received
• Blocking receive has the receiver block until a
  message is available
• Non-blocking is considered asynchronous
• Non-blocking send has the sender send the message
  and continue
• Non-blocking receive has the receiver receive a
  valid message or null

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Buffering

• Queue of messages attached to the link
  implemented in one of three ways
• 1. Zero capacity 0 messagesSender must wait
  for receiver (rendezvous)
• 2. Bounded capacity finite length of n
  messagesSender must wait if link full
• 3. Unbounded capacity infinite length Sender
  never waits

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Bounded-Buffer Message Passing Solution

• The Producer

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Bounded-Buffer Message Passing Solution

• The Consumer