File-System Interface

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File-System Interface Implementation
Final-4
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File Attributes

• Name only information kept in human-readable
  form.
• 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 the directory
  structure, which is maintained on the disk.

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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.

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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

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Information in a Device Directory

• Name
• Type
• Address
• Current length
• Maximum length
• Date last accessed (for archival)
• Date last updated (for dump)
• Owner ID (who pays)
• Protection information (discuss later)

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Operations Performed on Directory

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

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Organize the Directory (Logically) to Obtain

• 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.
• Grouping logical grouping of files by
  properties, (e.g., all Java programs, all games,
  )

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Protection

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

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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.

owner
group
public
chmod
761
game
Attach a group to a file chgrp G
game
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File-System Structure

• File structure
• Logical storage unit
• Collection of related information
• File system resides on secondary storage (disks).
• File system organized into layers.
• File control block storage structure consisting
  of information about a file.

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Layered File System
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File System Implementation

• File systems store several important data
  structures on the disk
• A boot-control block, ( per volume ) a.k.a.
  the boot block in UNIX or the partition boot
  sector in Windows contains information about how
  to boot the system. This will generally be the
  first sector of the volume if there is a bootable
  system loaded on that volume, or vacant
  otherwise.
• A volume control block, ( per volume ) a.k.a.
  the master file table in UNIX or
  the superblock in Windows, which contains
  information such as the partition table, number
  of blocks on each filesystem, and pointers to
  free blocks and free FCB blocks.
• A directory structure ( per file system ),
  containing file names and pointers to
  corresponding FCBs. UNIX uses inode numbers, and
  NTFS uses a master file table.
• The File Control Block, FCB, ( per file )
  containing details about ownership, size,
  permissions, dates, etc. UNIX stores this
  information in inodes, and NTFS in the master
  file table as a relational database structure.

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A Typical File Control Block
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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

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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.
• Simple only starting location (block ) and
  length (number of blocks) are required.
• Random access.
• Wasteful of space (dynamic storage-allocation
  problem).
• Files cannot grow.

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Contiguous Allocation of Disk Space
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Linked Allocation

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

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Linked Allocation (Cont.)

• Simple need only starting address
• Free-space management system no waste of space
• No random access
• Mapping

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Linked Allocation
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Indexed Allocation

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

index table
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Example of Indexed Allocation
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Indexed Allocation (Cont.)

• Need index table
• Random access
• Dynamic access without external fragmentation,
  but have overhead of index block.
• Mapping from logical to physical in a file of
  maximum size of 256K words and block size of 512
  words. We need only 1 block for index table.

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Linked Free Space List on Disk
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Disk
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Disk Structure

• Disk drives are addressed as large 1-dimensional
  arrays of logical blocks, where the logical block
  is the smallest unit of transfer.
• The 1-dimensional array of logical blocks is
  mapped into the sectors of the disk sequentially.
• Sector 0 is 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, and
  then through the rest of the cylinders from
  outermost to innermost.

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Disk Structure
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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.)
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SCAN

• 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 (Cont.)
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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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RAID (redundant array of independent disks)
Structure

• RAID multiple disk drives provides reliability
  via redundancy.
• RAID is arranged into six different levels.

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RAID (cont)

• Several improvements in disk-use techniques
  involve the use of multiple disks working
  cooperatively.
• Disk striping uses a group of disks as one
  storage unit.
• RAID schemes improve performance and improve the
  reliability of the storage system by storing
  redundant data.
• Mirroring or shadowing keeps duplicate of each
  disk.
• Block interleaved parity uses much less
  redundancy.

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Standard levels

• RAID 0 comprises striping (but no parity or
  mirroring).
• RAID 1 comprises mirroring (without parity or
  striping).
• RAID 2 comprises bit-level striping with
  dedicated Hamming-code parity.
• RAID 3 comprises byte-level striping with
  dedicated parity.
• RAID 4 comprises block-level striping with
  dedicated parity.
• RAID 5 comprises block-level striping with
  distributed parity.
• RAID 6 comprises block-level striping with double
  distributed parity.

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RAID Levels
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RAID (0 1) and (1 0)
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Disk Attachment

• Disks may be attached one of two ways
• Host attached via an I/O port
• Network attached via a network connection

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I/O
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A Typical PC Bus Structure
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Interrupts

• CPU Interrupt request line triggered by I/O
  device
• Interrupt handler receives interrupts
• Maskable to ignore or delay some interrupts
• Interrupt vector to dispatch interrupt to correct
  handler
• Based on priority
• Some unmaskable
• Interrupt mechanism also used for exceptions

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Interrupt-Driven I/O Cycle
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Intel Pentium Processor Event-Vector Table
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Direct Memory Access

• Used to avoid programmed I/O for large data
  movement
• Requires DMA controller
• Bypasses CPU to transfer data directly between
  I/O device and memory

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Six Step Process to Perform DMA Transfer
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Characteristics of I/O Devices
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Blocking and Nonblocking I/O

• Blocking - process suspended until I/O completed
• Easy to use and understand
• Insufficient for some needs
• Nonblocking - I/O call returns as much as
  available
• User interface, data copy (buffered I/O)
• Implemented via multi-threading
• Returns quickly with count of bytes read or
  written
• Asynchronous - process runs while I/O executes
• Difficult to use
• I/O subsystem signals process when I/O completed

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Kernel I/O Subsystem

• Scheduling
• Some I/O request ordering via per-device queue
• Some OSs try fairness
• Buffering - store data in memory while
  transferring between devices
• To cope with device speed mismatch
• To cope with device transfer size mismatch
• To maintain copy semantics

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Kernel I/O Subsystem

• Caching - fast memory holding copy of data
• Always just a copy
• Key to performance
• Spooling - hold output for a device
• If device can serve only one request at a time
• i.e., Printing
• Device reservation - provides exclusive access to
  a device
• System calls for allocation and deallocation
• Watch out for deadlock