Project 3 File System and IO

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Project 3 File System and I/O

• Assignment
• Design and implement a driver for a (simulated)
  magnetic disk
• Design and implement a simple file system using
  the driver
• Overall Organization

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Organization of the disk

• A disk consist of surfaces (num of surfaces)
• Surfaces consist of tracks
• Tracks consist of sectors (num of sectors)
• Sectors consists of bytes (len of a sector)
• Tracks with the same diameter form a cylinder
  (num of cylinders)

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Disk Operations

• SEEK move the read/write arm to a desired
  cylinder
• READ transfer one block of data from disk to
  memory
• WRITE transfer one block of data from memory to
  disk
• REPORT indicate completion status

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Disk Controller (interface 3) See Using the
disk controller

• Registers
• OP_REG SEEK, READ, WRITE, REPORT
• CYL_REG cylinder
• SURF_REG surface
• SECT_REG sector
• MEM_REG MM address
• BUSY true/false
• STAT completion status

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Disk Controller functions

• Initialize a disk init_disk(surf, cylinder,
  sector, block, block_len)
• disk retains its content indefinitely
• Set registers write_reg(reg, content)
• Can not set status register
• Read registers read_reg(reg)
• Reg STAT or BUSY
• Start disk operation start()
• May be ignored if BUSYtrue
• Request interrupt request_int()
• May block the current process

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An example of a disk operation

• Set necessary registers OP, SURF, CYL, SECT, MEM
• Start disk operation
• Wait until BUSYfalse by requesting interrupt
• Or polling the completion status
• Note STAT register is not set automatically it
  must be requested
• Place REPORT into OP_REG
• Start()
• Busy wait until BUSYFALSE
• Read STAT register
• Seek, read, and write operation may fail
  unpredictably repeat if necessary

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Disk Driver (interface 2)

• Disk is viewed as a sequential file of data
  blocks
• Length of a block length of a sector
• Blocks are mapped onto the disk starting with
  cylinder 0, surface 0, sector 0
• Data access
• FS requests a block by specifying a block
• Driver transforms it to cylinder/surface/sector
  address and commands disk controller

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Disk Driver Functions

• Read a block
• int read_block( int b, char p)
• Copy the logical block b from disk to memory
  starting at p
• Return 0 if successful -1 otherwise
• Write a block
• int write_block( int b, constant char p)
• Copy the logical block b from memory starting at
  p to disk
• Return 0 if successful -1 otherwise

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Block Numbering Scheme

• Start with cylinder 0, surface 0, sector 0 ?block
  0
• Fill all sectors within a track before move to
  the next track
• Fill tracks within a cylinder before move to the
  next cylinder
• Formula
• Block(CylNUM_OF_SURFSurf)
• NUM_OF_SECTORSect
• Sect Block mod NUM_OF_SECT
• Surf Block / NUM_OF_SECT mod NUM_OF_SURF
• Cyl Block / (NUM_OF_SECTNUM_OF_SURF)

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File System

• create(file_name) return ok/error
• destroy(file_name) return ok/error
• open(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

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Organization of the file system
Track 0 on Cylinder 0
File descriptors
Track 1/Cylinder 0
Data blocks
bit map

Descriptor for FS directory
Each descriptor
Block numbers
length
File length in bytes
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FS Directory

• Only one root directory
• A regular file
• Described by the first descriptor
• Contains array of entries
• Each entry record info of a file
• 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
• Remove file descriptor
• Update bit map

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Open a file
OFT
Fixed length array
Current position
index

• Search directory to find the index of file
  descriptor
• Allocate a free OFT entry
• Fill in current position (0) and file descriptor
  index
• Read in first block into the buffer ( read-ahead
  )
• Return OFT index

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Close a file

• Write buffer to disk
• Delete OFT entry

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Read a file

• Compute position in the r/w buffer
• Copy from buffer to memory until
• Desired count is reached update current pos, or
• End of file is reached update current pos, or
• End of buffer is reached
• Write the buffer into disk
• Read the next block
• Continue copying

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Write a file

• Write into buffer
• When full, write buffer to disk
• Allocate new block if necessary
• Change file descriptor
• Change bit map
• Update file length in descriptor if necessary

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Seek a file

• If the new position is not within the current
  block
• Write the buffer into disk
• Load the new block
• Set the current position to be the new position

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List the directory

• Read directory file
• For each entry do
• Print file name
• Find file descriptor print file length