File System Implementation

1
File System Implementation

• Chapter 11

2
Objectives

• To describe the details of implementing local
  file systems and directory structures
• To describe the implementation of remote file
  systems
• To discuss block allocation and free-block
  algorithms and trade-offs

3
File-System Structure

• A file system poses two distinct design problems
• Defining how the file system should look to the
  user
• Creating algorithms and data structures to map
  the logical file system onto the physical device
• File system resides on secondary storage (disks)
• File system organized into layers.
• Each level uses the lower level to create new
  features for use by higher levels
• File control block storage structure consisting
  of information about a file

4
A Typical File Control Block
5
File-System Implementation

• Several on-disk and in-memory structures are used
  to implement a file system
• On-disk structures
• A boot control block
• A partition control block (superblock)
• A directory structure
• file control blocks
• In-memory structures
• An in-memory partition table
• An in-memory directory structure
• The system-wide open-file table
• The per-process open-file table

6
Creating a File

• To create a new file the application program
  calls the logical file system (which knows the
  format of the directory structures)
• Allocates a new FCB
• Reads the appropriate directory into memory
• Updates the directory with new file name and FCB
• Writes back to disk
• Some operating systems (UNIX) treat a directory
  exactly as a file, other operating systems
  (Windows), implement separate system calls for
  files and directories and treat directories
  separate from files.

7
Opening a File

• Before a file can be used for I/O operations it
  must first be opened
• Open call passes the file name to the file system
• The directory structure (usually cached) is
  searched for the given file name
• Once the file is found, the FCB is copied into
  the system-wide open-file table in memory
• An entry is made in the per-process open-file
  table, with a pointer to the system-wide
  open-file table
• The open call returns a pointer to the
  appropriate entry in the per-process open-file
  table, all file operations are performed via this
  pointer (file descriptor in Unix, file handle in
  Windows)

8
In-Memory File System Structures
(a) refers to opening a file. (b) refers to
reading a file.
9
Closing a File

• After all I/O operations are complete a file
  should be closed
• The per-process table entry is removed and the
  system-wide entrys open count is decremented
• When all users that have opened the file close
  it, the updated file information is copied back
  to the disk-based directory structure and the
  system-wide open-file table entry is removed
• Some systems Use a caching a scheme. All
  information about an open file, except for its
  actual data blocks, is in memory

10
Disk Partition and Mounting

• The layout of a disk can have many variations,
  depending on the operating system
• A disk can be divided into multiple partitions,
  or a partition can span multiple disks
• Raw containing no file system
• Cooked containing a file system
• Boot information can be stored in a separate
  partition
• The root partition which contains the
  operating-system kernel is mounted at boot time
  (other partitions can be mounted later)
• The operating system notes in its mount table
  that a file system is mounted and the type of
  file system
• Windows mount each partition in a separate drive
  letter
• UNIX, file systems can be mounted at any
  directory

11
Directory Implementation

• Linear List
• Uses a linear list of file names with pointers to
  data blocks, requires a linear search to find a
  particular entry
• Simple to program but time-consuming to execute
• Hash Table
• Uses a linear list to stores directory entries
  but uses hashing to find the entry
• Hashing can greatly decrease the directory search
  time
• Handle collisions situations where two file
  names hash to the same location
• Major difficulties with a hash table are its
  fixed size and dependence on the hash function

12
Allocation Methods

• An allocation method refers to how disk blocks
  are allocated for files
• Three major methods of allocating disk space are
  in wide use
• Contiguous allocation
• Linked allocation
• Indexed allocation
• Each method has its advantages and disadvantages
• Some systems support all three but more commonly
  a system will use one particular method

13
Contiguous-Allocation

• Requires each file to occupy a set of contiguous
  blocks on the disk
• Disk addresses define a linear ordering on the
  disk
• Simple only starting location (block ) and
  length (number of blocks) are required
• For a file n blocks long and starts at location
  b, then it occupies blocks b, b1, b2, , bn-1
• The directory entry for each block represents
  indicates the starting address of each block and
  the length allocated for this file
• Both sequential and direct access is supported

14
Contiguous Allocation of Disk Space
15
Contiguous Allocation (Cont.)

• Contiguous allocation has some problems
• Dynamic storage-allocation
• How to satisfy a request of size n from a list of
  free blocks
• External fragmentation
• Free space is broken into chunks and the largest
  chunk is insufficient for a request
• Determining how much space is needed for a file
• Allocate too little and the file may not be
  extended
• Allocate too much and space is wasted
• File cannot grow

16
Extent Based Systems

• To minimize the drawbacks of contiguous file
  allocation some file systems (I.e. Veritas File
  System) use a modified scheme
• A contiguous chunk of space is allocated
  initially and when the amount is not large
  enough, another chunk of contiguous space
  (extent) is added to the initial allocation
• Extent-based file systems allocate disk blocks in
  extents
• Internal fragmentation can still be a problem if
  the extents are too large
• External fragmentation can be a problem as
  extents of various sizes are allocated and
  de-allocated

17
Linked Allocation

• Solves all the problems of contiguous allocation
• Each file is a linked list of disk blocks blocks
  may be scattered anywhere on the disk
• The directory contains a pointer to the first and
  last blocks of a file

18
Linked Allocation
19
Linked Allocation (Cont.)

• No external fragmentation
• Any free block on the free-space list can be used
  to satisfy a request
• A file can grow as long as free blocks are
  available, never need to compact disk space
• Linked allocation does have disadvantages
• Only effective for sequential-access files
• Space required for the list pointers. Use
  clusters to improve disk usage and access time.
• Reliability
• The File Allocation Table (FAT) is a variation to
  the linked allocation method used to support
  direct access

20
File-Allocation Table
21
Indexed Allocation

• Solves the external-fragmentation and
  size-declaration problems of contiguous
  allocation
• Supports direct access by bringing all the
  pointers together into the index block
• Each file has its own index block, which is an
  array of disk-block addresses

22
Example of Indexed Allocation
23
Indexed Allocation (Cont)

• Indexed allocation does suffer from wasted space
• Every file must have an index block. So the block
  needs to be as small as possible.
• A File may require more than one index blocks.
  Why?
• Linked scheme
• Multilevel scheme
• Combined scheme

24
Linked Index Scheme

• An index block is normally one disk block
• Can be read and written directly by itself
• To allow for large files, link together several
  index blocks (no limit on size)

25
Multilevel Index

• Use index of index blocks
• Use a first-level index block to point to a set
  of second-level index blocks, which in turn point
  to the file blocks
• With 4KB blocks and index size of 4 bytes, what
  is the maximum file size using 2-level index?
• Could be extended to a third or fourth level,
  depending on the maximum file size

26
Multi-level Index mapping
27
Combined Scheme UNIX (4K bytes per block)

• keep the first n pointers of the index block in
  the files inode
• Indexed-allocation suffers from some of the same
  performance problems as does linked allocation
• The index blocks can be cached in memory, but the
  data blocks may be spread all over a volume

The Unix inode
28
Free-Space Management

• Need to reuse the space from deleted files for
  new files
• To keep track of free disk space, the system
  maintains a free-space list
• Stores all free blocks those not allocated to a
  file or directory
• To create a file the free-space list is searched
  and that space is allocated to the new file, this
  space is then removed form the list
• When a file is deleted its disk space is added to
  the free space list

29
Bit Vector

• Frequently, the free-space list is implemented as
  a bit-map or bit vector
• Each block is represented by 1 bit
• If the block is free the bit is 1 if the block
  is allocated the bit is 0

30
Linked List

• Link together all the free disk blocks
• The first block contains a pointer to the next
  free disk block,
• Grouping
• Stores the addresses of n free blocks in the
  first free block
• Large numbers of free blocks can be found quickly
• Counting
• Stores the address of the first free block and
  the number n of free contiguous blocks
• The overall list will be shorter

31
Linked Free Space List on Disk
32
Efficiency and Performance

• Efficiency dependent on
• disk allocation and directory algorithms, e.g.
  pointer size.
• types of data kept in files directory entry
• Performance
• disk buffer cache separate section of main
  memory for frequently used blocks
• free-behind and read-ahead techniques to
  optimize sequential access
• improve PC performance by dedicating section of
  memory as virtual disk, or RAM disk
  (memory-mapped IO)

33
Page Cache

• A page cache caches pages rather than disk blocks
  using virtual memory techniques
• Memory-mapped I/O uses a page cache
• Routine I/O through the file system uses the
  buffer (disk) cache

34
Unified Buffer Cache

• A unified buffer cache uses the same page cache
  to cache both memory-mapped pages and ordinary
  file system I/O

35
Recovery

• Care must be taken to ensure that system failure
  does not result in loss of data or in data
  inconsistency
• Consistency checking
• Compares data in directory structure with data
  blocks on disk, and tries to fix inconsistencies
• The allocation and free-space-management
  algorithms dictate what types of problems the
  checker can find
• Backup and Restore
• Use system programs to back up data from disk to
  another storage device (floppy disk, magnetic
  tape).
• Recover lost file or disk by restoring data from
  backup

36
Log Structured File Systems

• Log structured (or journaling) file systems
  record each update to the file system as a
  transaction.
• All transactions are written to a log. A
  transaction is considered committed once it is
  written to the log. However, the file system may
  not yet be updated.
• The transactions in the log are asynchronously
  written to the file system. When the file system
  is modified, the transaction is removed from the
  log.
• If the file system crashes, all remaining
  transactions in the log must still be performed.