COW File System and Journaling

1. Copy-on-Write (CoW) File Systems

• Definition: CoW is a resource management technique allowing parent and child processes to share the same memory pages initially. Only when a modification occurs, a copy of the page is made.
• Usage: Efficiently copies data resources. If the copied data is unmodified, it remains as a reference to the original.
• Implementation: Commonly used in the fork() system call, creating a child process that shares the parent's memory pages.
• Operating Systems: Utilized by Linux, Solaris, Windows XP.
• Advantages: Only modified pages are copied, reducing overhead. Free pages are allocated from a pool of zeroed-out pages.

2. CoW Technique Details

• Process Creation: Parent and child processes share memory pages marked as copy-on-write. Modifications trigger the creation of a new page copy.
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• Example: Process A and Process B share the same memory pages. If Process A modifies a page, a copy of that page is created for Process A.
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3. Journaling File Systems

• Purpose: Addresses file system inconsistency by logging actions before committing them to the disk.
• Examples: Linux ext3/ext4, Windows NTFS.
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• Components of a Journal:
  • TxB (Transaction Begin Block): Contains transaction ID (TID).
  • Metadata Blocks: Copies of blocks to be updated.
  • TxE (Transaction End Block): Marks the end of a transaction.
• Process:
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  1. Write actions to the journal.
  2. Commit the transaction by writing TxE.
  3. Write data to the disk (checkpoint).

4. Benefits of Journaling

• Improved Recovery Time: Quick recovery after crashes by referring to the journal.
• Enhanced Data Integrity: Ensures updates are complete and recoverable.
• Reduced Disk Scans: Minimizes the need for full disk scans to fix inconsistencies.

5. Types of Journaling File Systems

• Physical Journaling: Records an advance copy of every block to be written, offering high fault protection but with a performance penalty.
• Logical Journaling: Stores changes to metadata, offering better performance but less fault tolerance.

6. Alternatives to Journaling

• Log-Structured File Systems: Occupy the entire storage device, avoiding the write-twice penalty.
• Copy-on-Write File Systems: Full CoW systems like ZFS and Btrfs avoid in-place changes by writing new data blocks and updating metadata.
• Soft Updates: Used in some Unix file systems to maintain consistency without journaling by ordering writes to prevent inconsistency.