Mobile File Systems

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?

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: 1 2 3 4 10 => show 1 2 3 4 appropriately Eg2: 1 2 3 4 30 => show 30 1 2 3 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?

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

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

CODA Three key components Hoarding Emulating Reintegrating Hoarding

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

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

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

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

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

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

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

MIMIC

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

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?