Other File Systems: AFS, Napster

2 Recap NFS: –Server exposes one or more directories Client accesses them by mounting the directories –Stateless server Has problems with cache consistency, locking protocol –Mounting protocol Automounting P2P File Systems: –PAST, CFS –Relies on DHTs for routing

3 Andrew File System (AFS) Named after Andrew Carnegie and Andrew Mellon –Transarc Corp. and then IBM took development of AFS –In 2000 IBM made OpenAFS available as open source Features: –Uniform name space –Location independent file sharing –Client side caching with cache consistency –Secure authentication via Kerberos –Server-side caching in form of replicas –High availability through automatic switchover of replicas –Scalability to span 5000 workstations

4 AFS Overview Based on the upload/download model –Clients download and cache files –Server keeps track of clients that cache the file –Clients upload files at end of session Whole file caching is central idea behind AFS –Later amended to block operations –Simple, effective AFS servers are stateful –Keep track of clients that have cached files –Recall files that have been modified

5 AFS Details Has dedicated server machines Clients have partitioned name space: –Local name space and shared name space –Cluster of dedicated servers (Vice) present shared name space –Clients run Virtue protocol to communicate with Vice Clients and servers are grouped into clusters –Clusters connected through the WAN Other issues: –Scalability, client mobility, security, protection, heterogeneity

6 AFS: Shared Name Space AFS’s storage is arranged in volumes –Usually associated with files of a particular client AFS dir entry maps vice files/dirs to a 96-bit fid –Volume number –Vnode number: index into i-node array of a volume –Uniquifier: allows reuse of vnode numbers Fids are location transparent –File movements do not invalidate fids Location information kept in volume-location database –Volumes migrated to balance available disk space, utilization –Volume movement is atomic; operation aborted on server crash

7 AFS: Operations and Consistency AFS caches entire files from servers –Client interacts with servers only during open and close OS on client intercepts calls, and passes it to Venus –Venus is a client process that caches files from servers –Venus contacts Vice only on open and close Does not contact if file is already in the cache, and not invalidated –Reads and writes bypass Venus Works due to callback: –Server updates state to record caching –Server notifies client before allowing another client to modify –Clients lose their callback when someone writes the file Venus caches dirs and symbolic links for path translation

8 AFS Implementation Client cache is a local directory on UNIX FS –Venus and server processes access file directly by UNIX i-node Venus has 2 caches, one for status & one for data –Uses LRU to keep them bounded in size

9 Napster Flat FS: single-level FS with no hierarchy –Multiple files can have the same name All storage done at edges: –Hosts export set of files stored locally –Host is registered with centralized directory Uses keepalive messages to check for connectivity –Centralized directory notified of file names exported by the host File lookup: client sends request to central directory –Directory server sends 100 files matching the request to client –Client pings each host, computes RTT and displays results –Client transfers files from the closest host File transfers are peer-to-peer; central directory not part

10 Napster Architecture Network Firewall IP Sprayer/ Redirector Napster Directory Server 1 Napster Directory Server 2 Napster Directory Server 3 Napster.com H1 H2 H3

11 Napster Protocol Network Firewall IP Sprayer/ Redirector Napster Directory Server 1 Napster Directory Server 2 Napster Directory Server 3 Napster.com H1 H2 H3 I have “metallica / enter sandman”

12 Napster Protocol Network Firewall IP Sprayer/ Redirector Napster Directory Server 1 Napster Directory Server 2 Napster Directory Server 3 Napster.com “who has metallica ?” “check H1, H2” H1 H2 H3 I have “metallica / enter sandman”

13 Napster Protocol Network Firewall IP Sprayer/ Redirector Napster Directory Server 1 Napster Directory Server 2 Napster Directory Server 3 Napster.com “who has metallica ?” “check H1, H2” ping H1 H2 H3 I have “metallica / enter sandman”

14 Napster Protocol Network Firewall IP Sprayer/ Redirector Napster Directory Server 1 Napster Directory Server 2 Napster Directory Server 3 Napster.com “who has metallica ?” “check H1, H2” ping H1 H2 H3 transfer I have “metallica / enter sandman”

15 Napster Discussion Issues: –Centralized file location directory –Load balancing –Relies on keepalive messages –Scalability an issue! Success: ability to create and foster an online community –Built in ethics –Built in faults –Communication medium Had around users in November 2000!

16 Other P2P File Systems Napster has a central database! –Removing it will make regulating file transfers harder Freenet, gnutella, kazaa … all are decentralized Freenet: anonymous, files encrypted –So not know which files stored locally, which file searched Kazaa: allows parallel downloads Torrents for faster download