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Other File Systems: AFS, Napster

Other File Systems: AFS, Napster. 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

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Other File Systems: AFS, Napster

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

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

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

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

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

  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 640000 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

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