File Systems

1
File Systems

• Chapter 6

6.1 Files 6.2 Directories 6.3 File system
implementation 6.4 Example file systems
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Long-term Information Storage

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

3
File Naming

• Typical file extensions.

4
File Structure

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

5
File Types

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

6
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

1. Create
2. Delete
3. Open
4. Close
5. Read
6. Write

1. Append
2. Seek
3. Get attributes
4. Set Attributes
5. 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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Memory-Mapped Files

• (a) Segmented process before mapping files
  into its address space
• (b) Process after mapping
• existing file abc into one segment
• creating new segment for xyz

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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

1. Create
2. Delete
3. Opendir
4. Closedir

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File System Implementation

• A possible file system layout

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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 (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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File System Reliability (2)

• Bit maps used by the logical dumping algorithm

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File System Reliability (3)

• File system states
• (a) consistent
• (b) missing block
• (c) duplicate block in free list
• (d) duplicate data block

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File System Performance (1)

• The block cache data structures

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File System Performance (2)

• I-nodes placed at the start of the disk
• Disk divided into cylinder groups
• each with its own blocks and i-nodes

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Disk Scheduling

• The operating system is responsible for using
  hardware efficiently for the disk drives, this
  means having a fast access time and disk
  bandwidth.
• Access time has two major components
• Seek time is the time for the disk are to move
  the heads to the cylinder containing the desired
  sector.
• Rotational latency is the additional time waiting
  for the disk to rotate the desired sector to the
  disk head.
• Minimize seek time
• Seek time ? seek distance
• Disk bandwidth is the total number of bytes
  transferred, divided by the total time between
  the first request for service and the completion
  of the last transfer.

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Disk Scheduling (Cont.)

• Several algorithms exist to schedule the
  servicing of disk I/O requests.
• We illustrate them with a request queue (0-199).
• 98, 183, 37, 122, 14, 124, 65, 67
• Head pointer 53

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FCFS
Illustration shows total head movement of 640
cylinders.
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SSTF

• Selects the request with the minimum seek time
  from the current head position.
• SSTF scheduling is a form of SJF scheduling may
  cause starvation of some requests.
• Illustration shows total head movement of 236
  cylinders.

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SSTF (Cont.)
40
SCAN/Elevator

• The disk arm starts at one end of the disk, and
  moves toward the other end, servicing requests
  until it gets to the other end of the disk, where
  the head movement is reversed and servicing
  continues.
• Sometimes called the elevator algorithm.
• Illustration shows total head movement of 208
  cylinders.

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SCAN/Elevator (Cont.)
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LOOK/Elevator

• Version of SCAN
• Arm only goes as far as the last request in each
  direction, then reverses direction immediately,
  without first going all the way to the end of the
  disk.

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C-SCAN

• Provides a more uniform wait time than SCAN.
• The head moves from one end of the disk 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 the cylinders as a circular list that
  wraps around from the last cylinder to the first
  one.

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C-SCAN (Cont.)
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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,
  without first going all the way to the end of the
  disk.

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C-LOOK (Cont.)
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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 of the operating system,
  allowing it to be replaced with a different
  algorithm if necessary.
• Either SSTF or LOOK/Elevator is a reasonable
  choice for the default algorithm.

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Log-Structured File Systems

• With CPUs faster, memory larger
• disk caches can also be larger
• increasing number of read requests can come from
  cache
• thus, most disk accesses will be writes
• LFS Strategy structures entire disk as a log
• have all writes initially buffered in memory
• periodically write these to the end of the disk
  log
• when file opened, locate i-node, then find blocks

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Example File Systems CD-ROM File Systems

• The ISO 9660 directory entry

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The CP/M File System (1)
BIOS had 17 I/O calls, OS in 3584 bytes, Shell in
2K, Zero page for h/w interrupt handling

• Memory layout of CP/M

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The CP/M File System (2)

• The CP/M directory entry format

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The MS-DOS File System (1)

• The MS-DOS directory entry

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The MS-DOS File System (2)

• Maximum partition for different block sizes
• The empty boxes represent forbidden combinations

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The Windows 98 File System (1)
Bytes

• The extended MOS-DOS directory entry used in
  Windows 98

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The Windows 98 File System (2)
Bytes
Checksum

• An entry for (part of) a long file name in
  Windows 98

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The Windows 98 File System (3)

• An example of how a long name is stored in
  Windows 98

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The UNIX V7 File System (1)

• A UNIX V7 directory entry

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The UNIX V7 File System (2)

• A UNIX i-node

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The UNIX V7 File System (3)

• The steps in looking up /usr/ast/mbox