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. In short

3
Long-term Information Storage

• Files Good!
• No Files Bad!

4
File Naming

• Typical file extensions.

5
File Structure

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

6
File Types Text and Binary

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

7
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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Example
Assume all counters are currently 0. Consider
the case when pages 0,2,4, and 5 are referenced
between last interrupt.
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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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• -rwxr-xr-x 1 dickens spcprj 20580000 Nov 16
  2003 BM.Contention.Big
• -rw-r--r-- 1 dickens spcprj 5832 Nov 14
  2003 LR.Contention.Low
• -rwxr-xr-x 1 dickens spcprj 5180000 Nov 14
  2003 bitmap.contention.1
• -rwxr-xr-x 1 dickens spcprj 12434 Nov 14
  2003 Companion.4
• -rw-r--r-- 1 dickens spcprj 2767 Nov 14
  2003 temp.Swap
• -rw-r--r-- 1 dickens spcp 2767 Nov 14
  2003 temp
• -rwxr-xr-x 1 dickens spc 16969 Nov 14
  2003 ind_calc
• -rw-r--r-- 1 dickens spcprj 1217 Nov 14
  2003 Temp_File.contention

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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

• Use system calls like
• map (filename, starting address, size)
• unmap (filename, starting address, size)
• Implemented through paging mechanism.
• Advantages of this ?

15
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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Path Names

• To Open dict path is /usr/jim/dict.

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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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• Entry 4 bytes. Blocks 1K. 20 Million Entries
  80 MB for table.

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Implementing Files (4)

• An example i-node

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Double Indirection
Triple Indirection
Disk block containing addresses of disk blocks
containing addresses

• 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 (1)

• Cyclic Family Tree

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Links

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

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Symbolic Links

• Provide the path name of the target file in the
  linked 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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• Quotas for keeping track of each users disk use

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Consistency Issues

• File system can become inconsistent if there is a
  system crash and recent changes have not all been
  written to disk.
• Consider
• inode cached
• New block allocated to file
• Block filled, written to disk
• Free list updated and written to disk
• System crash
• Now block not on either free list or listed as
  part of a file

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• Consider
• inode cached
• Free list cached
• Block 10 deleted from file A. Cached inode
  updated.
• Block 10 now on free list and allocated to file
  B.
• File B closed, inode written to disk.
• System crash.
• Now block 10 listed as belonging to two files.

39
File System Consistency

• Build two tables, each of which counts number of
  blocks.
• Table 1 Number of times block is in a file.
• Table 2 Number of times block is in the free
  list.
• Read all inodes.
• Increment tables.

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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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• Figure b- missing block.
• Adds it back to the free list.
• Figure c- block listed twice in free list
• Rebuilds the free list.
• Figure d- Same block belongs to two or more
  files.
• Make copy and insert in one of the files.

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File System Performance (1)

• The block cache data structures

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Problem with LRU

• If frequently used block is inode or directory
  block, may want to write it out.
• Modify LRU to account for importance of block.
• Unix synch()
• MS-DOS Write-through cache

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• Most systems write critical blocks to disk
  immediately (but not necessarily non-critical
  blocks).
• If system crashes, file system likely intact (but
  users may become unglued).
• Unix System call sync that causes all modified
  blocks to be written to disk.
• An update daemon is running in the background and
  alternates between sleeping and calling synch.
  Generally every 30 seconds.
• MS-DOS Write-through cache. Every change is
  written through to disk.

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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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CP/M

• 8080 Chip
• Max 64K RAM
• 720K Floppy.
• Size of entire OS 3584 bytes.
• Shell 2K bytes.

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The CP/M File System (1)

• Memory layout of CP/M

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Directory

• One directory
• Entries 32 bytes (fixed).
• After booting, reads in directory and computes
  free list.
• Does not save free list.
• Provides 38 systems calls, 17 File system
  related.
• File blocks are 1K.

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The CP/M File System (2)

• The CP/M directory entry format

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MS-DOS

• Added hierarchical directory structure.
• Use fixed 32-byte directory entry
• Added attributes read-only, archived, hidden,
  system.
• Time field seconds (5 bits), minutes (6 bits),
  hours (5 bits).
• Date day (5 bits), month (4 bits), year (7 bits
  only good to 2107).

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The MS-DOS File System (1)

• The MS-DOS directory entry

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File Allocation Table (FAT)

• Entry 4 bytes. Blocks 1K. 20 Million Entries
  80 MB for table.

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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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Windows 98

• Used 32-bit FAT.
• Used the additional 10 bits from DOS entry.
• Allowed arbitrarily long file names
• Backward compatible to MS-DOS.

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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 (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