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21. I-Nodes & File Systems

02/12/22

I-nodes​

  • Two approaches to share a file
    • Hard Links: Maintain two (or multiple) references to the same i-node in B and C
    • Symbolic/soft links: Owner maintains a reference to the i-node. The 'referencer' maintains a small file (has its own i-node) that contains the location and name of the shared file in directory

Fastest way of linking files!

  • Disadvantages:
    • If the i-node is deleted, any hard link will point to an invalid i-node
    • If the i-node gets deleted and recycled to point to an other file, the hard links will point to the wrong file!
  • Only solution is to delete the file, and leave the i-node intact if the 'reference count' is larger than 0
  • Disadvantages:
    • They result in an extra file lookup (once the link file has been found, the original file needs to be found as well)
    • They require an extra i-node for the link file
  • Advantages:
    • No problems with deleting the original file -> the file simply does not exist any more
    • They can cross the boundaries of machines. Linked file can be located on a different machine

Log Structured File System​

Context​

  • Consider the creation of a new file

    1. Allocate, initialise and write the i-node for the file
    2. Update and writ the directory entry for the file
    3. Write the data to the disk

  • The corresponding blocks are not necessarily in adjacent locations!

  • Also in linked lists/FAT file systems blocks can be distributed all over the disk

  • Due to seek and rotational delays, hard disks are slow compared to other components in a computer

  • A log structured file system aims to improve the speed of a file system on a traditional hard disk by minimising head movements and rotational delays using the entire disk as a great big log

  • A log is a data structure that is written only at the end

Concept​

  • Log structured file systems buffer read and write operations (i-nodes) in memory, enabling us to write 'larger volumes' in one go

  • Once the buffer is full it is 'flushed' to the disk and written as one contiguous segment and the end of a log

    • I-nodes and data are all written to the same segment
    • Finding -nodes (normally at the start of the partition) becomes more difficuly

  • An i-node map is maintained in memory to quickly find the address of i-nodes on the disk

  • A cleaner thread is running in the background and spends its time scanning the log circularly and compacting it.

  • Hard drive is treated as a circular buffer

  • It removes deleted files and files being used right now are marked as free segments as they will be later written at the end

Advantages​

  • Greatly increases disk performance on writes, file creates, deletes
  • Writes are more robust as they are done as a single operation(multiple small writes are more likely to expose the file system to serious inconsistency)

Disadvantages​

  • Not been widely used because it is highly incompatible with existing file systems
  • In addition, the cleaner thread takes additional CPU time

File System Implementations​

Deleting a file consists of:

  1. Remove the files directory entry
  2. Add the files i-node to the pool of free i-nodes
  3. Add the files disk blocks to the free list
  • Where does it go wrong:
    • Directory entry has been deleted and a crash occurs -> i-nodes and disk blocks become inaccessible
    • The directory entry and i-nodes have been released and a crash occurs -> disk blocks become inaccessible
  • Changing the order of the events does not necessarily resolve the issues
  • Journaling file systems aim at increasing the resilience of file systems against crashes by recording each update to the file system as a transaction

Concept​

  • Key idea behind a journaling file system is to log all events (transactions) before they take place
    • Write the actions that should be undertaken to a log file
    • Carry them out
    • Remove/commit the entries once completed
  • If a crash happens in the middle of an action the entry in the log file will remain present after the crash
  • The log can be examined after the crash and used to restore the consistency of the file system
  • NTFS and EXT3-4 are examples of journaling file systems

Virtual File Systems​

  • Multiple file systems usually coexists on the same computer

  • These file systems can be seamlessly integrated by the operating system

  • Usually achieved by using virtual file systems (VFS)

  • VFS relies on standard object oriented principles (polymorphism)

  • Can define a generic interface public DataObject readData(); This would be the POSIX interface containing reads, writes, closes etc

  • Unix and LInux unify different file systems and present them as a single hierarchy and hides away/abstracts the implementation specific details for the user

  • The VFS presents a unified interface to the 'outside'

  • File system specific code is dealt with in an implementation layer that is clearly seperated from the interface

  • Each VFS interface commonly contains the POSIX system calls
  • Each file system that meets the VFS requirements provides an implementation for the system calls contained in the interface
  • Implementations can be for remote file systems

Summary​

  • Logs - store everything as close as possible
  • Journaling - apply the transaction principle
  • VFS - apply good software design