A FAST FILE SYSTEM FOR UNIX

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A FAST FILE SYSTEM FOR UNIX

• Marshall K. Mckusick William N. JoySamuel J.
  LefflerRobert S. Fabry
• CSRG, UC Berkeley

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PAPER HIGHLIGHTS

• Main objective of FFS was to improve file system
  bandwidth
• Key ideas were
• Subdividing disk partitions into cylinder groups,
  each having both i-nodes and data blocks
• Using larger blocks but managing block
  fragments
• Replicating the superblock

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THE OLD UNIX FILE SYSTEM

• Each disk partition contains
• a superblock containing the parameters of the
  file system disk partition
• an i-list with one i-node for each file or
  directory in the disk partition and a free list.
• the data blocks (512 bytes)

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More details

• File systems cannot span multiple partitions
• Must use mount() to merge several file systems
  into a single tree
• Superblock contains
• The number of data blocks in the file system
• A count of the maximum number of files
• A pointer to the free list

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File types

• Three types of files
• ordinary filesuninterpreted sequences of bytes
• directoriesaccessed through special system
  calls
• special files allow access to hardware devices
  but are not really files

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Ordinary files (I)

• Five basic file operations are implemented
• open() returns a file descriptor
• read() reads so many bytes
• write() writes so many bytes
• lseek() changes position of current byte
• close() destroys the file descriptor

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Ordinary files (II)

• All reading and writing are sequential.
• The effect of direct access is achieved by
  manipulating the offset through lseek()
• Files are stored into fixed-size blocks
• Block boundaries are hidden from the usersSame
  as in FAT and NTFS file systems

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The file metadata

• Include file size, file owner, access rights,
  last time the file was modified, but not the
  file name
• Stored in the file i-node
• Accessed through special system callschmod(),
  chown(), ...

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I/O buffering

• UNIX caches in main memory
• I-nodes of opened files
• Recently accessed file blocks
• Delayed write policy
• Increases the I/O throughput
• Will result in lost writes whenever a process or
  the system crashes
• Terminal I/O are buffered one line at a time

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Directories (I)

• Map file names with i-node addresses
• Do not contain any other information!

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Directories (II)

• Two or more directory entries can point to the
  same i-node
• A file can have several names
• Directory subtrees cannot cross file system
  boundaries
• To avoid loops in directory structure, directory
  files cannot have more than one pathname

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Mounting a file system
Root partition
/
Other partition
usr
mount
bin
After mount, root of second partition can be
accessed as /usr
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Special files

• Map file names with system devices
• /dev/tty your terminal screen
• /dev/kmem the kernel memory
• /dev/fd0 the floppy drive
• Main motivation is to allow accessing these
  devices as if they were files
• no separate I/O constructs for devices

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A file system
Superblock
I-nodes
Data Blocks
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The i-node (I)

• Each i-node contains
• The user-id and the group-id of the file owner
• The file protection bits
• The file size
• The times of file creation, last usage and last
  modification

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The i-node (II)

• The number of directory entries pointing to the
  file, and
• A flag indicating if the file is a directory, an
  ordinary file, or a special file.
• Thirteen block addresses
• The file name(s) can be found in the directory
  entries pointing to the i-node.

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Storing block addresses
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Addressing file contents

• I-node has ten direct block addresses
• First 5,120 bytes of a file are directly
  accessible from the i-node
• Next block address contains address of a block
  containing 512/4 128 blockaddresses
• Next 64K of a file require one level of
  indirection

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Addressing file contents

• Next block address allows to access a total of
  (512/4)2 16K data blocks
• Next 8 MB of a file require two levels of
  indirection
• Last block address allows to access a total of
  (512/4)3 2M blocks
• Next GB of a file requires one level of
  indirection

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Explanation

• File sizes can vary from a few hundred bytes to a
  few gigabytes with a hard limit of 4 gigabytes
• The designers of UNIX selected an i-node
  organization that
• Wasted little space for small files
• Allowed very large files

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Discussion

• What is the true cost of accessing large files?
• UNIX caches i-nodes and data blocks
• When we access sequentially a very large file we
  fetch only once each block of pointers
• Very small overhead
• Random access will result in more overhead if we
  cannot cache all blocks of pointers

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First Berkeley modifications

• Staging modifications to critical file system
  information so that they could either be
  completed or repaired cleanly after a crash
• Increasing the block size to 1,024 bytes
• Improved performance by a factor of more than two
• Did not let file system use more than four
  percent of the disk bandwidth

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What is disk bandwidth?

• Maximum throughput of a file system if disk drive
  was continuously transferring data
• Actual bandwidths are much lower because of
• Disk seeks
• Disk rotational latency

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Major issue

• As files were created and deleted, free list
  became entirely random
• Files were allocated random blocks that could be
  anywhere on the disk
• Caused a very significant degradation of file
  system performance (factor of 5!)
• Problem is not unique to old UNIX file system
• Still present in FAT and NTFS file systems

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THE FAST FILE SYSTEM

• BSD 4.2 introduced the fast file system
• Superblock is replicated on different cylinders
  of disk
• Have one i-node table per group of cylinders
• It minimizes disk arm motions
• I-node has now 15 block addresses
• Minimum block size is 4K
• 15th block address is never used

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Cylinder groups

• Each disk partition is subdivided into groups of
  consecutive cylinders
• Each cylinder group contains a bit map of all
  available blocks in the cylinder group
• Better than linked list
• The file system will attempt to keep consecutive
  blocks of the same file on the same cylinder group

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Larger block sizes

• FFS uses larger blocks
• At least 4 KB
• Blocks can be subdivided into 2, 4, or 8
  fragments that can be used to store
• Small files
• The tails of larger files

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Replicating the superblock

• Each cylinder group has
• Ensures that a single head crash would never
  delete all copies of the superblock

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Explanations (I)

• Increasing the block size to 4K eliminates the
  third level of indirection
• Keeping consecutive blocks of the same file on
  the same cylinder group reduces disk arm motions

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Internal fragmentation issues
Since UNIX file systems typically store many very
small files, increasing the block size results
in an unacceptablyhigh level of internal
fragmentation
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The solution

• Using 4K blocks without allowing fragments
  would have wasted 45.6 of the disk space
• This would be less true today
• FFS solution is to allocate block fragments to
  small files and tail end or large files
• Allows efficient sequential access to large files
• Minimizes disk fragmentation

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Layout policies (I)

• FFS tries to place all data blocks for a file in
  the same cylinder group, preferably
• At rotationally optimal positions
• In the same cylinder.
• Large files could quickly use up all available
  space in the cylinder group

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Layout policies (II)

• FFS redirects block allocation to a different
  cylinder group
• a file exceeds 48 kilobytes
• at every megabyte thereafter

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PERFORMANCE IMPROVEMENTS

• Read rates improved by a factor of seven
• Write rates improved by a factor of almost three
• Transfer rates for FFS do not deteriorate over
  time
• No need to defragment the file system from time
  to time
• Must keep a reasonable amount of free space
• Ten percent would be ideal

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Limitations of approach (I)

• Even FFS does not utilize full disk bandwidth
• Log-structured file systems do most writes in
  sequential fashion
• Crashes may leave the file system in an
  inconsistent state
• Must check the consistency of the file system at
  boot time

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Limitations of approach (II)

• Most of the good performance of FFS is due to its
  extensive use of I/O buffering
• Physical writes are totally asynchronous
• Metadata updates must follow a strict order
• Cannot create new directory entry before
  newi-node it points to
• Cannot delete old i-node before deleting last
  directory entry pointing to it

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Example Creating a file (I)
i-node-1
abc
ghi
i-node-3

Assume we want to create new file tuv
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Example Creating a file (II)
i-node-1
abc
ghi
i-node-3
tuv
?
Cannot write directory entry tuv before i-node
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Limitations of approach (III)

• Out-of-order metadata updates can leave the file
  system in temporary inconsistent state
• Not a problem as long as the system does not
  crash between the two updates
• Systems are known to crash
• FFS performs synchronous updates of directories
  and i-nodes
• Solution is safe but costly

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OTHER ENHANCEMENTS

• Longer file names
• 256 characters
• File locking
• Symbolic links
• Disk quotas

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File locking

• Allows to control shared access to a file
• We want a one writer/multiple readers policy
• Older versions of UNIX did not allow file locking
• System V allows file and record locking at a
  byte-level granularity through fcntl()
• Berkeley UNIX has purely advisory file
  lockslike asking people to knock before entering

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Symbolic links

• With Berkeley UNIX, symbolic links you can write
• ln -s /usr/bin/programs /bin/programs
• even tough /usr/bin/programs and /bin/programs
  are in two different partitions
• Symbolic links point to another directory entry
  instead of the i-node.