FileSystem compl

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FileSystem (compl)
2
Checking the file-system

• If system crashes in the middle of an update?
• Recall, for a data-block update, need to
• Update file directory info
• Update file-control block
• Update data-block itself

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Checking the file-system (2)

• On rebooting, file system check is performed
• Checks inconsistencies among blocks and
  reference-info
• Fsck operation (unix)
• Scandisk (windows)

4
Journaling file-system

• An alternative approach
• a basic journal file system is made up of
• Journal Data Structure (log-type data structure)
• Meta-Data for the actual data
• The actual data

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Functionality of JFS

• Before updating data on the disk
• Add entry into the journal about what is going to
  happen
• When fsys operation is completed
• Commit the transaction by adding info on
  meta-data.

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Functionality of JFS (2)

• logical file system interface
• JFS
• Intermediate layer fsys (FCBs, block org.)
• Physical Layer IO

Transaction-based interface
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JFS does

• transaction management
• All operations of a transaction succeed or none
• Maintenance of log-info
• Ordering
• Can help recovery of operations/data after a
  crash
• Undo/redo

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Without JFS

• IF UNMOUTED NICELY THEN
• ASSUME data is Okay
• Mount the file system
• ELSE IF UNMOUNTED DIRTY THEN
• ASSUME potential inconsistency
• FIX problem, scan whole disk
• // above takes a long time
• // more so as the partition is
  larger
• Mount the file system
• END IF

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With JFS

• IF UNMOUTED NICELY THEN
• ASSUME data is Okay
• Mount the file system
• ELSE IF UNMOUNTED DIRTY THEN
• // no assumptions made
• READ LAST FEW JOURNAL Entries
• // takes seconds, not much data to look
  at
• FIX inconsistencies
• Mount file system
• ENDIF

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On JFS implementation

• JFS builds on data-base technology
• Uses intermediate fsys of the system it is
  implemented on (files, fcbs, ) to store info

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JFS systems

• Unix/linux
• Ext3
• FeiserFS (suitable for small files)
• XFS (SGI)
• Veritas (Sun)
• Windows
• NTFS