1. Mobile File Systems

2. Puzzle You have a deck of 52 cards
You draw out 5 cards randomly and look at the cards
You can now show 4 of the cards to a friend, and the friend should identify the 5th card
How do you do this?

3. Puzzle Let cards be numbered 1-52
Use permutation of the 4 cards to index into
the 5th card (how many possible?)
Choose cards to avoid having to index a number
greater than 24
Eg1: => show appropriately
Eg2: => show appropriately
You have a deck of 52 cards
You draw out 5 cards randomly and look at the cards
You can now show 4 of the cards to a friend, and the friend should identify the 5th card
How do you do this?

4. Mobility and Distributed File-systems
Examples of distributed file-systems – NFS (network file system), AFS (andrew file system), etc.
Key problems in a wireless/mobile environment?
Disconnections
Low bandwidths

5. File-systems … CODA – supports disconnected operation
PFS – targets partially connected environments
Bayou – support for data-sharing among mobile users
others …

6. CODA Three key components Hoarding Emulating Reintegrating Hoarding

7. Hoarding Cache maintained locally at clients
Hoarding – fills in cache when mobile client is connected
Two types of hoarding
Usage based
User specified
Cache management
User driven
LRU

8. Emulating
When mobile client is disconnected, coda enters the emulation mode
All requests are served from the local cache
change modify log (CML) maintained based on user-writes
CML used later in conflict detection and resolution
CML optimized periodically to reduce the size of the CML – delete unnecessary entries

9. Reintegrating Transparent conflict resolution for directories
Except under impossible scenarios – directory permission changes, add/deletion of same directories …
File conflict resolution
ASR – application specific resolvers
Possess information about nature of files (say calendars) – resolve based on application semantics
Manual repair
User provided a view-graph of conflicts and asked to resolve

10. Optimizations Rapid cache validations
Original mechanism in CODA for cache coherence based on callbacks
Callbacks cannot be used when clients are disconnected
Client validates cached copies explicitly upon reconnection
Potentially time-consuming
CODA uses cache coherence checks at multiple levels of granularity

11. Rapid cache validations - Illustration
Vol 1
Vol 2
Vol 3
Vol 4
Version x
Version y
Version z
Check volume version stamp
If version stamps different, check individual object
version stamps

12. Trickle Re-integration
When network is weakly connected, propagate updates to server asynchronously
Trade-offs
lower bandwidth efficiency as fewer CML optimizations are possible
More up to date copies at server – fewer conflicts possible
CODA allows users to dynamically set the period of the asynchronous updates based on requirements

13. User assisted cache miss handling
When a cache miss occurs, what should be done?
Option 1 – fetch from server
Option 2 – convey error message to user
Trade-offs?
Latency vs. availability
Patience vs. importance
CODA uses a “patience threshold” to decide whether a file can be retrieved within this threshold

14. MIMIC

15. Recap Wireline networking Wireless Ad-hoc networks Sensor networks
MAC
Scheduling
Routing
Transport
Ad-hoc networks
Sensor networks
Miscellaneous topics
Application acceleration, mobile file systems
P2P (skype, bittorrent), Programmable/evolvable networks

16. Puzzle Doors numbered 1-100 All doors initially open
Toggle switch outside every door
If switch is pressed, door will close if it is currently open, and open if it is currently closed
For i=1 to 100, you press switches of doors that are multiples of i
Which doors are closed at the end of the process?