Discovery and Hot Replacement
of Replicated Read-Only File Systems,
with Application to Mobile Computing

Erez Zadok


Computer Science Department
Columbia University


MS Thesis


©1994, Erez Zadok
All Rights Reserved

Thesis Advisor: Daniel Duchamp

CUCS-036-94

October 19, 1994

Abstract:

We describe a mechanism for replacing files, including open files, of a read-only file system while the file system remains mounted; the act of replacement is transparent to the user. Such a ``hot replacement'' mechanism can improve fault-tolerance, performance, or both. Our mechanism monitors, from the client side, the latency of operations directed at each file system. When latency degrades, the client automatically seeks a replacement file system that is equivalent to but hopefully faster than the current file system. The files in the replacement file system then take the place of those in the current file system. This work has particular relevance to mobile computers, which in some cases might move over a wide area. Wide area movement can be expected to lead to highly variable response time, and give rise to three sorts of problems: increased latency, increased failures, and decreased scalability. If a mobile client moves through regions having partial replicas of common file systems, then the mobile client can depend on our mechanism to provide increased fault tolerance and more uniform performance.

 

• Contents
• List of Figures
• List of Tables
• 1. Introduction
• 2. Background
  • 2.1 NFS
    • 2.1.1 struct vfs
    • 2.1.2 struct vnode
    • 2.1.3 struct file
  • 2.2 RLP
  • 2.3 Amd
    • 2.3.1 How Our Work Goes Beyond Amd
• 3. Design
  • 3.1 Demonstrating the Need
  • 3.2 When to Switch
  • 3.3 Locating a Replacement
  • 3.4 Using the Replacement
    • 3.4.1 Relevant Changes to Kernel Data Structures
      • 3.4.1.1 struct vfs
      • 3.4.1.2 struct vnode
      • 3.4.1.3 struct file
    • 3.4.2 After Replacement: Handling New Opens
    • 3.4.3 After Replacement: Handling Files Already Open
  • 3.5 Security
  • 3.6 Code Size
• 4. Implementation
  • 4.1 RLP
  • 4.2 Management and Control Facilities
  • 4.3 Debugging Facilities
• 5. Evaluation
  • 5.1 Experience
    • 5.1.1 What is Read-Only
    • 5.1.2 Suitability of Software Base
      • 5.1.2.1 Kernel
      • 5.1.2.2 RLP
      • 5.1.2.3 NFS
• 6. Experiences
  • 6.1 Experiences in Kernel Development
    • 6.1.1 Debugging
      • 6.1.1.1 printf()s
      • 6.1.1.2 Kernel Debuggers
      • 6.1.1.3 Source Browsing
    • 6.1.2 Coding Practices
  • 6.2 Vendor Bugs
• 7. Related Work
• 8. Conclusion
  • 8.1 Future Work
• Acknowledgments
• Bibliography
• About this document ...