David Choffnes, Winter 2006 OceanStore Maintenance-Free Global Data StorageMaintenance-Free Global Data Storage, S. Rhea, C. Wells, P. Eaton, D. Geels, B. Zhao, H. Weatherspoon, J. Kubiatowicz, IEEE Internet Computing, 5(5):40-49,

CS 395/495 Autonomic Computing Systems EECS, Northwestern University 2 Data makes the world go round We’re addicted to persistent storage –Wouldn’t it be great if it would follow us globally? –And automatically make itself resilient to failures? –But that would require 1,000s or millions of PCs! OceanStore –A global data store that manages itself –Scales to billions of users and exabytes of data –Features: Durability Resistance to attack/failures Fault tolerant Churn-resistant Catchy name

CS 395/495 Autonomic Computing Systems EECS, Northwestern University 3 A bit of this, a byte of that Routing messages/data –Self-maintaining (DHT) Durabilitly –M x N erasure encoding Security/Fault tolerance –Byzantine updates, secure hashes, encryption Availability –Introspective replica management

CS 395/495 Autonomic Computing Systems EECS, Northwestern University 4 Don’t call it a comeback Erasure codes –Break object into m chunks of size n; n*m > n –Encode chunks such that any k set of them can reconstruct entire object

CS 395/495 Autonomic Computing Systems EECS, Northwestern University 5 An object by any other GUID would smell as sweet Each object assigned a GUID based on 160- bit SHA-1 hash OceanStore supports versioning, so each object has an active GUID that points to a list of GUIDs from different versions –Each GUID is a B-tree of links to chunks

CS 395/495 Autonomic Computing Systems EECS, Northwestern University 6 Magic Tapestry Ride Tapestry = DHT Allows nodes to join and leave network relatively seamlessly Named objects found in deterministic # of hops OceanStore uses multiple “root nodes” for each object –More redundancy –Lower latency Inner ring made responsible for objects

CS 395/495 Autonomic Computing Systems EECS, Northwestern University 7 He says, she says Possibility for faulty, malicious servers –Byzantine protocol ensures correctness if less than 1/3 of servers is faulty/misbehaving New technique vastly reduces n^2 message complexity Cached data is signed Proactive signature threshold allows same public key despite inner ring membership change

CS 395/495 Autonomic Computing Systems EECS, Northwestern University 8 Know thyself Introspection –Servers measure independence of failure rates and change encoding rate appropriately –Auto repair: root node can check redundancy, regenerate/redistribute blocks –Durability: sweep/repair

CS 395/495 Autonomic Computing Systems EECS, Northwestern University 9 Put it all together

CS 395/495 Autonomic Computing Systems EECS, Northwestern University 10 Some Performance Numbers