• Dates: 4/1-4/8:
• Reading: Ch11, Ch12 (only briefly), Ch13, Ch14.

File systems: concept

• basic concept: online storage
• file system: files and directories
• contrast user/system view:
  • user: logical view of storage = file
  • system: physical storage = disk, tape, etc.
  • physical media: persistent or non-volatile
  • execution and access: related to process/memory management
    PCB contents?
    loading
    swapping
    virtual memory
    interaction between memory and backing store
• file system organization:
  user applications to physical devices
• layers in file systems
• access methods:
  • sequential access
  • direct access
  • more later
• related: allocation methods
  • contiguous, list, index
  • similarity/comparison to memory allocation
  • in connection with access methods later

File systems: Attributes and Information

• file types and attributes
• operations based on types and attributes
• basic attributes
  • name
  • type: basic types, program or data,
    detailed common types: Fig 11.1 (p.377)
  • location (mostly for system)
  • size
  • protection: access control
  • time, date, user identification: creation/modification/use
• information kept in the directory structure
• example information:
  UNIX example: Fig 11.13 (p.406)
  description of the fields 11.6.4 (p.406 too)

File systems: Operations and Assess Methods

• file operations
• files as an ADT (abstract data type)
• ADT = content and pre-defined operations
• basic operations
  • creating
  • writing
  • reading
  • repositioning
  • deleting
  • truncating
• others as combinations of the the above
  copying, appending, renaming, etc.
• information for management/operations
  • file status: open, closed, etc.
  • file pointer
  • open count
  • disk location
• file type => (internal) file structure and interpretation (semantics)

• access methods
• to support different operations
• affect different implementations
• basic access methods
  • sequential access: tape, etc.
    current position for access, reposition
  • direct access: more flexible
• other access build on basic access methods
• protection and access (later)
• other access methods:
  • use of index
    example Fig 11.4 (p.382)
  • file hierarchy
  • DBMS vs files

File systems: Directories

• directories and directory structure
• motivation: too many files
• usability and efficiency concerns
• collection of files as a directory
• generalization: tree-structured directories
  example: Fig 11.8 (p.388)
  single-/two-level directory as special cases of trees
  (Figs 11.6 and 11.7)
• sharing: acyclic graph instead of tree
  example: Fig 11.9 (p.390)
• general graph, not commonly used

• files and directories: implementation issues
  • physical view:
    • directory as index (title/ToC)
    • followed by individual files
    • example: Fig 11.5 (p.383)
  • sequential vs direct access impact on implementation
  • allocation: contiguous, list, index
  • free-space management: similar to free memory management
  • directory tables/lists
  • efficiency
  • performance
  • reliability and quality (consistency)
• many implementation issues covered in Ch.12

File systems: Other issues

• mounting file system:
  similar to opening individual files
  for a sub-tree
• sharing
  • client-server model
  • ftp, web, etc.
• protection: authorized vs. unauthorized access
  • directory/read/write/execute
  • general access protection: rwx
    for who?
  • UNIX example: user-group-universal rwx
    again, Fig 11.13 (p.406)
  • other types of access: append, delete, list, etc.
• discussed in book in more details

I/O systems

• I/O and I/O systems
• basic concepts
• I/O operation handling
  • polling
  • interrupt
  • DMA
• hardware: architecture class
• device driver:
  part of OS, but device specific

• I/O management through polling
• basic ideas
  • CPU controlled
  • passive devices
  • if I/O needed during polling, service it
• consequences
  • easily managed
  • busy waiting by CPU
  • no immediate service for urgent request
• => other ideas of I/O management

• interrupt based I/O management
• basic ideas
  • request I/O, generate interrupt
  • active devices
  • interrupt handling: similar to procedure/function calls
  • compare to Fig 13.3 (p.461)
• interrupt types and priority
  • maskable vs. nonmaskable
  • hardware vs. software
  • priority
  • handling priority: selective masking
• advantages over polling
  • no busy waiting
  • priority
• but requires more management overhead
• more commonly used

• interrupt for purposes other than I/O
  • basic operations/utilities: clock
  • administrative purposes: monitoring etc.
  • power and other disturbances
  • RR implementation example

• I/O through DMA
• direct memory access
• if no CPU involvement is constantly needed
• improved efficiency/parallelism
• general sequence: set-up, DMA, clear-up
• example: Fig 13.5 (p.465)

Mass-storage management

• main topics in mass-storage management
  • storage hierarchy: back in Fig 2.6 (p.39)
  • mass-storage: secondary and tertiary storage
  • main secondary device: disks
  • main tertiary device: tapes
  • speed differences
  • disk scheduling algorithms
  • other mass-storage management

• disk access
  • logical block: typically 512, 1024 bytes
  • mapped to sectors
  • tracks and cylinders
  • logical address => physical address (cylinder/track/sector)
  • access: seek time + rotational latency
  • seek time: head to correct cylinder
  • rotational latency: spin/rotate to correct sector
  • seek time dominant
  • scheduling: cylinders in the disk queue

Mass-storage management: Disk Scheduling

• disk scheduling algorithms
  • FCFS (or FIFO)
  • SSTF (shortest seek time first)
  • SCAN or elevator algorithm
  • C-SCAN (circular SCAN)
  • LOOK and C-LOOK

• FCFS (or FIFO)
  • simple to implement
  • but efficiency problem
  • head movement (seek time)
  • back and forth, large movements
  • example

• SSTF (shortest seek time first)
  • to improve FCFS performance
  • back and forth, small movements
  • comparison to SJF CPU scheduling
  • possible starvation problem
  • optimality?
  • example

• SCAN, C-SCAN, LOOK, C-LOOK
  • minimize back and forth movement
  • avoid starvation
  • C-SCAN: one directional with reset
  • LOOK: not going all the way if not necessary
  • C-LOOK: similar modifications
  • examples
  • more commonly used: why?

Mass-storage management: Other Issues

• other issues with disk management
  • formatting
  • boot blocks and boot disks
  • bad blocks
  • disk reliability through redundant arrays, etc.
  • swap space management
  • details in textbook

• tertiary storage structure
  • removable disks: floppy disks, CD-ROM etc.
  • tapes: mostly for backup and archiving
  • OS: virtual memory and virtual machines
  • management of all the extensions