Background: File System Implementation, Disk Access Costs and Free Space Management

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Background File System Implementation, Disk
Access Costs and Free Space Management
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File Names

• Unix
• long file names (255 bytes)
• include special characters
• case sensitive (myshell.c ! Myshell.c)
• MS-DOS
• file name 8 chars, ., 3-char extension
• case insensitive (Grades.txt, grades.txt,
  GRADES.txt, all the same file)

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File Attributes
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File Operations(system calls related to files)

• Create
• Delete
• Open
• Close
• Read
• Write

• Append
• Seek
• Get attributes
• Set Attributes
• Rename
• Lock

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Directories

• Each directory is a collection of entries, one
  entry per file. Among attributes the address on
  disk where the file starts.
• Opening a file
• search directory for the corresponding file entry
• Read attributes and disk addresses into the main
  memory

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Directory Structure
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A Unix Directory Tree
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Directory Operations

• Create
• Delete
• Opendir
• Closedir

• Readdir
• Rename
• Link
• Unlink

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Moving-head Disk Mechanism

• Disk access costs Transfer time access time
• Access time
• Seek time time to move the disk heads to the
  desired cylinder
• Rotational latency time for rotating to desired
  sector

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Files Contiguous allocation

• (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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Files Linked List
The links are on the disk (using block numbers)
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Linked List File Allocation Table

• Keep the list links in a separate table in memory

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i-nodes
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A UNIX i-node
Max file size d direct pointers, n
indirect pointers p/block Blocksize (d n
n2 n3)
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A UNIX V7 directory entry
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Reminder Disk organization in UNIX
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The steps in looking up /usr/ast/mbox
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Disk Space Management

• (a) Storing the free list on a linked list
• (b) A bit map