Project: File System Textbook: pages 501506

1
Project File System(Textbook pages 501-506)

• ICS 145B
• L. Bic

2
Assignment

• Design and implement a simple file system using
  the disk driver from previous project
• Overall organization

File System
I/O System (your driver)
user
3
I/O System

• I/O system presents disk as a linear sequence of
  blocks
• we will refer to logical disk as ldiskLB
• L of logical blocks on disk
• B block length (bytes)
• I/O system interface provided by your driver
• read_block(int i, char p)
• write_block(int i, char p)
• FS can access the emulated disk using only these
  functions

4
File System -- User Interface

• create(symbolic_file_name) return ok/error
• destroy(symbolic_file_name) return ok/error
• open(symbolic_file_name) return index
• close(index) return ok/error
• read(index, mem_area, count) return bytes read
• write(index, mem_area, count) return bytes
  written
• lseek(index, pos) return ok/error
• directory return ok/error

5
Review of concepts

• directory structure
• tree, DAG, graph, symbolic links, path names
• this project single flat list of all files
• organization of entries within directory
• array of slots, linked list, hash table, B-tree
• this project unsorted array of fixed-size slots
• each directory entry contains
• all descriptive info, parts of info, name only
• this project symbolic name plus index of
  descriptor

6
Review of concepts

• file descriptor contents
• owner, file type, protection, length, disk map,
  access times
• this project length (bytes), list of logical
  disk blocks
• location of file descriptors
• dedicated portion of disk, special files, in
  directories
• this project first k disk blocks
  (ldisk0..k-1)

7
Review of concepts

• disk blocks (physical organization)
• contiguous, linked list, indexed, multi-level
• this project
• flat index (fixed list of disk blocks)
• 1-level incremental (for groups of 3)
• free storage management
• linked lists, bit map
• this project bit map

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Organization of the file system

• groups of 3 addl task 2, 1-level incremental
  index

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The Directory

• only one directory (root)
• regular file
• described by the first descriptor
• contains array of entries
• symbolic file name
• index of the descriptor

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Create a file

• find a free file descriptor
• find a free directory entry
• fill both entries

i
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Destroy a file

• find file descriptor by searching directory
• remove directory entry
• update bit map
• free file descriptor
• return status

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Open a file
OFT
fixed length array
current position
index

• search directory to find index of file descriptor
• allocate a free OFT entry
• fill in current position (0) and file descriptor
  index
• read in first file block into the buffer
  (read-ahead)
• return OFT index (or error)

13
Close a file

• write buffer to disk
• update file length in descriptor
• free OFT entry
• return status

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Read a file

• compute position in the r/w buffer
• copy from buffer to memory until
• desired count or end of file is reached
• update current pos, return status
• end of buffer is reached
• write the buffer to disk (if modified)
• read the next block
• continue copying

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Write a file

• write into buffer
• when full, write buffer to disk
• if block does not exist (file is expanding)
• allocate new block
• update file descriptor
• update bit map
• update file length in descriptor

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Seek in a file

• if the new position is not within the current
  block
• write the buffer to disk
• read the new block
• set the current position to the new position
• return status

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List the directory

• read directory file
• for each entry
• find file descriptor
• print file name and file length

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Presentation shell

• develop presentation shell
• repeatedly accept command (e.g. cr abc)
• invoke corresponding FS function (e.g.
  create(abc))
• display status/data on screen (e.g. file created
  or Error file exists)
• project will be tested using an input file and an
  output file (similar to process/resource manager)

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Shell commands

• cr ltnamegt
• de ltnamegt
• op ltnamegt
• cl ltindexgt
• rd ltindexgt ltcountgt
• wr ltindexgt ltchargt ltcountgt
• sk ltindexgt ltposgt
• dr
• in ltno_cylgt ltno_surfgt ltno_sectgt ltsect_lengt
  ltdisk_contgt
• sv ltdisk_contgt
• q

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Summary of tasks

• design and implement FS using I/O system from
  previous project
• develop test/presentation shell
• submit code to Masterhit for testing turn in
  design document to Distribution Center
• groups of 2
• error checks on all commands
• groups of 3
• error checks on all commands
• additional task 2 (pg 506) 1-level expanding
  index