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Truly Distributed File Systems

Truly Distributed File Systems. Paul Timmins CS 535. Centralized Network File Systems. NFS, AFS, CIFS provide distributed access to a centralized file system. Primary storage resides on one server. Data may be redundant and possibly replicated. Metadata is maintained by server.

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Truly Distributed File Systems

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  1. Truly Distributed File Systems Paul Timmins CS 535

  2. Centralized Network File Systems • NFS, AFS, CIFS provide distributed access to a centralized file system. • Primary storage resides on one server. • Data may be redundant and possibly replicated. • Metadata is maintained by server

  3. Problems in Centralized Systems • Scalability: Each client adds a fixed overhead • Performance: Server becomes the bottleneck • Reliability: Data resides with a single server

  4. Centralized Network File Systems Server Client Client Client Client

  5. Distributed File Systems • Physical data resides on the disks of multiple servers • Metadata is maintained by multiple servers, although one may be elected as master • Cooperative Caching • Disconnected Operation • Cache Coherency

  6. xFS • Clusters of clients • Log-structured File System, striped with parity • Block level exclusive locks for writes • Server sends cache invalidations • Cooperative Caching, clients serve clients • Ownership based

  7. JetFile • Versioning file system, allows simultaneous writes • Clients can serve data to other clients • Modified data can be cached at the client • Invalidation notices are sent from client to multiple clients • Coherency is dependant on multicast reliability

  8. Coda • Replicated (mirrored) servers, with one elected as lock manager. • Servers replicate on file access • Versioning file system • Disconnected operation allowed • Simultaneous or divergent writes which conflict and cannot be resolved automatically must be manually resolved

  9. Cache Invalidation • Invalidations are transmitted to clients upon changes, as opposed to NFS-style validate upon request • Coda and xFS track clients caches, which consumes some server resources • JetFile uses multicast from one client to all other clients to invalidate their cache

  10. Cooperative Caching Server 2. Client B requests Foo, but is referred to Client A 1. Client A requests and reads Foo Client A Client B 3. Client B reads Foo from Client A

  11. Cache Coherency Most: xFS (Ownership Based) AFS (Stateful server invalidation) JetFile (Multicast client invalidation) NFS (Stateless) Least: Coda (Disconnected)

  12. Applications • WWW Replication • FTP Sites • Network Computers • Corporate file system

  13. Current Status • xFS and Berkeley NOW project is dead, unclear whether any continued xFS work will be done • JetFile seems to be under continued research, no significant new work • Coda is in use by developers at CMU. No significant new research, focus is on stabilizing. Recent work includes a Solaris port and bug fixing.

  14. Summary • Naming: location independent • Migration: Moving the location of data is transparent in all cases • Directories: Handled the same as in local file • Sharing Semantics: JetFile and Coda are unix-style, xFS is session-based • Caching: Clients cache, and can serve from their cache • Locking: JetFile and Coda don’t allow locking, xFS locks

  15. Summary • Replication/Reliability: All provide server replication • Scalability: Should scale to thousands and tens of thousands • Homogeneity: Not required • File system interface: Unix style semantics • Security: xFS provides encryption, but marginal authentication, Jetfile provides none, Coda provides authentication and encryption • State/Stateless: xFS and Coda are statefull, JetFile is stateless but still sends cache invalidation

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