Distributed File Systems

1. Distributed File Systems Chapter 10

2. NFS Architecture (1) • The remote access model. • The upload/download model • Advantages or disadvantages?

3. NFS Architecture (2) • The basic NFS architecture for UNIX systems. • VFS: hides differences between various (distributed) file systems • Advantages of this architecture?

4. File System Model • File Name • File Handle • File Name  file handle  all operations on the file

5. File System Model • An incomplete list of file system operations supported by NFS.

6. Communication • Reading data from a file in NFS version 3. • Reading data using a compound procedure in version 4. No transactional semantics • Why compound procedures?

7. NFS Server:Stateless or Stateful • V2, V3: stateless • Simple, but if we need to implement lock mechanism? • V4: stateful • Performance issue in wide-area NFS systems • Caches at client-side: Cache consistency • Lease?

8. Naming (1) • Mounting (part of) a remote file system in NFS.Drawback?

9. Naming (2) • Mounting nested directories from multiple servers in NFS. • Why the client need to import the directory from Sever B explicitly?

10. Naming Resolution • V3: iterative – look up a single file name each step • V4: Support recursive – look up the full name • Example: /bin/draw/install • V3: 3 separate name lookups • V4: one

11. File Handles • V4: 128bytes • Unique • Persistent in the file’s lifetime • Could not be reused after file deletion • All file operations need the file handle • Q1:Why persistent file handle? • Q2: Why could not be reuse after deletion?

12. Automounting • When should a remote file system be mounted? • During user login time • Mount other uses on-demand • On-demand mounting of a remote file system is handled in NFS by an automounter, that runs as a separate process on the client’s machine (user-level NFS server)

13. Automounting (1) • A simple automounter for NFS.

14. Automounting (2) • Using symbolic links with automounting.

15. File Attributes (1) • Some general mandatory file attributes in NFS.

16. File Attributes (2) • Some general recommended file attributes.

17. Semantics of File Sharing (1) • On a single processor, when a read follows a write, the value returned by the read is the value just written. • In a distributed system with caching, obsolete values may be returned.

18. Semantics of File Sharing (2) • Four ways of dealing with the shared files in a distributed system.

19. File Locking in NFS (1) • NFS version 4 operations related to file locking.

20. File Locking in NFS (2) Requestaccess Currentaccessstate • The result of an open operation with share reservations in NFS. • When the client requests shared access given the current denial state. • When the client requests a denial state given the current file access state.

21. Client Caching (1) • Client-side caching in NFS. • Session Semantics

22. Open Delegation • Using the NFS version 4 callback mechanism to recall file delegation. • With open Delegation, the client handles open/close operations from other clients on the same machine. •  Stateful server?

23. RPC Failures • Three situations for handling retransmissions. • The request is still in progress • The reply has just been returned • The reply has been some time ago, but was lost.

24. File Locking in face of Failures • Why need add lease to a lock? • If a server crashes, recovery needs a grace period • It allows clients to reclaim locks granted previously • It denies new request for locks • Lease problem: synchronized clocks

25. Security • The NFS security architecture.

26. Secure RPCs • Secure RPC in NFS version 4. • User authentication/server authentication

27. Access Control • The classification of operations recognized by NFS with respect to access control.

28. The Coda File System • The various kinds of users and processes distinguished by NFS with respect to access control.

29. Overview of Coda (1) • The overall organization of AFS.

30. Overview of Coda (2) • The internal organization of a Virtue workstation.

31. Communication (1) • Side effects in Coda's RPC2 system.

32. Communication (2) • Sending an invalidation message one at a time. • Sending invalidation messages in parallel.

33. Naming • Clients in Coda have access to a single shared name space.

34. File Identifiers • The implementation and resolution of a Coda file identifier.

35. Sharing Files in Coda • The transactional behavior in sharing files in Coda.

36. Transactional Semantics • The metadata read and modified for a store session type in Coda.

37. Client Caching • The use of local copies when opening a session in Coda.

38. Server Replication • Two clients with different AVSG for the same replicated file.

39. Disconnected Operation • The state-transition diagram of a Coda client with respect to a volume.

40. Secure Channels (1) • Mutual authentication in RPC2.

41. Secure Channels (2) • Setting up a secure channel between a (Venus) client and a Vice server in Coda.

42. Access Control • Classification of file and directory operations recognized by Coda with respect to access control.

43. Plan 9: Resources Unified to Files • General organization of Plan 9

44. Communication • Files associated with a single TCP connection in Plan 9.

45. Processes • The Plan 9 file server.

46. Naming • A union directory in Plan 9.

47. Overview of xFS. • A typical distribution of xFS processes across multiple machines.

48. Processes (1) • The principle of log-based striping in xFS.

49. Processes (2) • Reading a block of data in xFS.

50. Naming • Main data structures used in xFS.