COMP3891

EXT3 File System

Overview

Design goals:

• Add journalling capability to the EXT2 FS.
• Backward and forward compatibility with EXT2.

Journal

Disk Block Updates

Example:

• Start transaction.
• Update block #n1 (contains directory entry).
• Update block #n2 (inode allocation bitmap).
• Update block #n2 (data block allocation bitmap).
• End transaction.

Advantages:

• Multiple update to the same block can be aggregated into a single update.
• Journalling layer is FS-independent.

Disadvantage:

• Even a small update adds a whole block to the journal.

Journal Block Device

Transaction Lifecycle

|Stage|Description| |-|-| |In progress|Updates are buffered in RAM.| |Completed|Updates are buffered in RAM; no additional updates are allowed in the same transaction.| |Committed|Updates are written to the journal and marked as committed. Transaction can be replayed after an unclean unmount.| |Checkpointed|Updates are written to the file system; the transcation is removed from the journal.|

Journaling Modes

EXT3 supports two journalling modes:

• Metadata + data:
  • Enforces atomicity of all FS operations.
• Metadata journalling:
  • Metadata is journalled.
  • Data blocks are written directly to the disk.
  • Improves performance.
  • Enforces file system integrity.
  • Does not enforce atomicity of write.
    • New file content can be stale blocks.