1. Disconnected Operation in the Coda File System
James J. Kistler
M. Satyanarayanan

2. History Descendent of AFS, also a CMU project
In development since 1987
Support is included in the official Linux kernel

3. Motivation Distributed file systems are not immune to failures
Laptop computers are popular

4. Key Points Caching file data can be used to provide high availability
High availability allows disconnected operation
The Coda file system provides these features

5. Design Goals Scalability Voluntary vs. Involuntary Disconnections
First vs. Second Class Replication
Optimistic vs. Pessimistic Replica Control

6. Scalability Experience with AFS Whole-file caching
Functionality at clients, rather than servers

7. Voluntary vs. Involuntary Disconnections
Laptops
Types of disconnection are not functionally different

8. First vs. Second Class Replication
Why have server replicas at all?
Server replicas (first class)
High quality
Additional cost
Client replicas (second class)
Generally inferior
Very little cost

9. Optimistic vs. Pessimistic Replica Control
Pessimistic control
Avoids conflicting operations
Clients acquire shared or exclusive control of objects
Optimistic control
Reads and writes everywhere!
Detects and resolves conflicts after they occur

10. Venus States

11. Venus States: Hoarding
Client is connected to server
Caches information in anticipation of disconnection
Factors complicate implementation
Prioritized cache management
Hoard walking

12. Hoarding: Prioritized Cache Management
Hoard database
Explicit prioritization, generated by user
Contains pathnames and priorities corresponding to file system objects
Recent reference history
Implicit prioritization
Updated continuously as objects are referenced

13. Hoarding: Hoard Walking
Equilibrium
Two phases:
Reevaluate HDB entries to reflect activity by other clients
Reevaluate priorities and restore equilibrium

14. Performance Duration of reintegration Cache size
Likelihood of conflicts

15. Cache Size

16. Likelihood of Conflicts
Data based on AFS server usage
Tracked who made modifications
Measured time between modifications
Promising results: 99% of all modifications were made by the previous writer

17. Likelihood of Conflicts

18. Recap
Coda provides disconnected operation by aggressively caching file data

19. My Evaluation
Resource exhaustion
Likelihood of conflicts

20. More Info