File Management in Representative Operating Systems

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File Managementin Representative Operating
Systems

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UNIX File Management

• File
• Stream of bytes
• Internal logical structure is application
  specific
• Types of files
• Ordinary
• Contains information entered by user, application
  program, or system utility program
• Directory
• Ordinary file with special protection privileges
  (file system can write, user programs can read)
• Contains a list of file names and pointers to the
  associated index or information nodes (inodes)
• Special
• Used for I/O devices, each device is associated
  with a special file
• Named
• Named pipe

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UNIX File Management (cont.)

• Inodes
• Control structure that contains information
  needed by the OS for a particular file
• Contents
• File mode File type, execution flags, access
  permissions (RWE for OGO)
• Link count Number of directory references to
  this node
• Owner ID Owner of file
• Group ID Group owner
• File size Number of bytes in file
• File address 39 bytes of address information
• Last accessed Time of last file access
• Last modified Time of last file modification
• Inode Modified Time of last inode modification

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UNIX File Management (cont.)

• File allocation on secondary storage
• Dynamic allocation, on a block basis,
  non-contiguous
• Indexed method of addressing, with part of the
  index stored in files inode
• Inode has 39 bytes of address information,
  structured as thirteen 3-byte pointers
  (addresses)
• First ten addresses point to the first 10 data
  blocks
• Eleventh address points to a block on disk that
  contains the next stage of the index (single
  indirect block)
• Twelfth address points to a double indirect block
• Thirteenth address points to a triple indirect
  block
• Total number of blocks depends on the capacity of
  the fixed-size blocks in the system (e.g., in
  UNIX System V each block has 1Kbyte and can hold
  256 block addresses. Total size of a file is
  16Gbytes.)
• Advantages
• The inode has a fixed size, is small, and can be
  kept be in main memory
• Small files can be accessed with no indirection
• The maximum size can accommodate any application

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UNIX File Management (cont.)
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Windows 2000 File Management

• File systems supported
• File Allocation Table (FAT) from Windows 95,
  MS-DOS, and OS/2
• W2K File System (NTFS)
• NTFS key features
• Recoverability
• Uses transaction processing model to reconstruct
  disk volumes after failures
• Uses redundant storage for critical system data
• Security
• Security descriptor for each file object defines
  security attributes
• Large disks and large files
• Supports very large disks and files
• Multiple data streams
• Multiple data streams can be defined for a file
• General indexing facility
• A file can be indexed by any of the attributes
  associated with the file

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Windows 2000 File Management (cont.)

• NTFS volume and file structure
• Disk storage units
• Sector Smallest physical storage unit on disk
  power of 2 in bytes, usually 512 bytes
• Cluster One or more contiguous sectors size in
  sectors is power of 2
• Volume
• Logical partition on disk used by file system to
  allocate storage, one or more clusters
• Consists of file system information, a collection
  of files, and any remaining unallocated space on
  the volume
• Can be a portion or an entire single disk, or can
  extend across multiple disks
• Maximum size 2e64
• Cluster is the fundamental unit of allocation
• System is configured initially with a certain
  number of sectors per cluster
• Use of clusters allows the use of non-standard
  disks, with sectors different from 512 bytes
• Maximum file size is 2e32, which represents 2e48
  bytes

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Windows 2000 File Management (cont.)

• NTFS volume layout
• Every element on a volume is a file and every
  file consists of a collection of attributes
• Regions on the NTFS volume
• Partition boot sector
• Information about the volume layout and the file
  system structures
• Boot startup information and code
• Master File Table (MFT)
• Information about all the files and folders
  (directories) on this volume
• Information about available unallocated space
• Organized as a table of variable-length rows,
  called records
• Each row describes a file or folder on this MFT
  and contains
• Attributes for the file or folder (e.g., file
  name, access rights, time stamps)
• Pointers to clusters containing file
• System files region
• MFT2 duplicate of the first three rows of MFT
• Log file list of transaction steps for file
  recoverability
• Cluster bit map volume representation, showing
  which clusters are in use
• Attribute definition table defines the attribute
  types supported on this volume

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Windows 2000 File Management (cont.)

• Recoverability
• The file system can be recovered to a consistent
  state following a system crash or disk failure
• Elements that support recoverability
• I/O manager
• NTFS driver provides the basic open, close,
  read, write functions
• Fault-tolerant driver (FTDISK) provides the
  software RAID functionality
• Log file service
• Maintains a log of disk writes
• Used to recover volume after a system failure
• Cache manager
• Caches file reads and writes for improved
  performance (lazy write and lazy commit)
• Virtual memory manager
• Cached file are accessed by mapping file
  references to virtual memory references and
  reading and writing virtual memory
• Logging is the main technique used to implement
  recoverability
• Each operation that changes the file system is
  treated as a transaction
• Transactions are recorded in a log file
• Partially completed transactions can be redone or
  undone after recovery

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Windows 2000 File Management (cont.)