Distributed System by Pratik Tambekar
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This document discusses several distributed file systems including NFS, Coda, Plan 9, xFS, and SFS. It provides details on each system's architecture, communication methods, naming, caching, replication, fault tolerance, security and access control approaches. The key aspects covered are remote access models, file operations, attributes, sharing semantics, client caching, server organization, naming schemes, and consistency models used by the different systems.
Distributed System by Pratik Tambekar
- 2. NFS Architecture (1) a) The remote access model. b) The upload/download model
- 3. NFS Architecture (2) The basic NFS architecture for UNIX systems.
- 4. File System Model An incomplete list of file system operations supported by NFS. Operation v3 v4 Description Create Yes No Create a regular file Create No Yes Create a nonregular file Link Yes Yes Create a hard link to a file Symlink Yes No Create a symbolic link to a file Mkdir Yes No Create a subdirectory in a given directory Mknod Yes No Create a special file Rename Yes Yes Change the name of a file Rmdir Yes No Remove an empty subdirectory from a directory Open No Yes Open a file Close No Yes Close a file Lookup Yes Yes Look up a file by means of a file name Readdir Yes Yes Read the entries in a directory Readlink Yes Yes Read the path name stored in a symbolic link Getattr Yes Yes Read the attribute values for a file Setattr Yes Yes Set one or more attribute values for a file Read Yes Yes Read the data contained in a file Write Yes Yes Write data to a file
- 5. Communication a) Reading data from a file in NFS version 3. b) Reading data using a compound procedure in version 4.
- 6. Naming (1) Mounting (part of) a remote file system in NFS.
- 7. Naming (2) Mounting nested directories from multiple servers in NFS.
- 8. Automounting (1) A simple automounter for NFS.
- 9. Automounting (2) Using symbolic links with automounting.
- 10. File Attributes (1) Some general mandatory file attributes in NFS. Attribute Description TYPE The type of the file (regular, directory, symbolic link) SIZE The length of the file in bytes CHANGE Indicator for a client to see if and/or when the file has changed FSID Server-unique identifier of the file's file system
- 11. File Attributes (2) Some general recommended file attributes. Attribute Description ACL an access control list associated with the file FILEHANDLE The server-provided file handle of this file FILEID A file-system unique identifier for this file FS_LOCATIONS Locations in the network where this file system may be found OWNER The character-string name of the file's owner TIME_ACCESS Time when the file data were last accessed TIME_MODIFY Time when the file data were last modified TIME_CREATE Time when the file was created
- 12. Semantics of File Sharing (1) a) On a single processor, when a read follows a write, the value returned by the read is the value just written. b) In a distributed system with caching, obsolete values may be returned.
- 13. Semantics of File Sharing (2) Four ways of dealing with the shared files in a distributed system. Method Comment UNIX semantics Every operation on a file is instantly visible to all processes Session semantics No changes are visible to other processes until the file is closed Immutable files No updates are possible; simplifies sharing and replication Transaction All changes occur atomically
- 14. File Locking in NFS (1) NFS version 4 operations related to file locking. Operation Description Lock Creates a lock for a range of bytes Lockt Test whether a conflicting lock has been granted Locku Remove a lock from a range of bytes Renew Renew the leas on a specified lock
- 15. File Locking in NFS (2) The result of an open operation with share reservations in NFS. a) When the client requests shared access given the current denial state. b) When the client requests a denial state given the current file access state. Current file denial state NONE READ WRITE BOTH READ Succeed Fail Succeed Succeed WRITE Succeed Succeed Fail Succeed BOTH Succeed Succeed Succeed Fail (a) Requested file denial state NONE READ WRITE BOTH READ Succeed Fail Succeed Succeed WRITE Succeed Succeed Fail Succeed BOTH Succeed Succeed Succeed Fail (b) Request access Current access state
- 16. Client Caching (1) Client-side caching in NFS.
- 17. Client Caching (2) Using the NFS version 4 callback mechanism to recall file delegation.
- 18. RPC Failures Three situations for handling retransmissions. a) The request is still in progress b) The reply has just been returned c) The reply has been some time ago, but was lost.
- 19. Security The NFS security architecture.
- 20. Secure RPCs Secure RPC in NFS version 4.
- 21. Access Control The classification of operations recognized by NFS with respect to access control. Operation Description Read_data Permission to read the data contained in a file Write_data Permission to to modify a file's data Append_data Permission to to append data to a file Execute Permission to to execute a file List_directory Permission to to list the contents of a directory Add_file Permission to to add a new file t5o a directory Add_subdirectory Permission to to create a subdirectory to a directory Delete Permission to to delete a file Delete_child Permission to to delete a file or directory within a directory Read_acl Permission to to read the ACL Write_acl Permission to to write the ACL Read_attributes The ability to read the other basic attributes of a file Write_attributes Permission to to change the other basic attributes of a file Read_named_attrs Permission to to read the named attributes of a file Write_named_attrs Permission to to write the named attributes of a file Write_owner Permission to to change the owner Synchronize Permission to to access a file locally at the server with synchronous reads and writes
- 22. The Coda File System The various kinds of users and processes distinguished by NFS with respect to access control. Type of user Description Owner The owner of a file Group The group of users associated with a file Everyone Any user of a process Interactive Any process accessing the file from an interactive terminal Network Any process accessing the file via the network Dialup Any process accessing the file through a dialup connection to the server Batch Any process accessing the file as part of a batch job Anonymous Anyone accessing the file without authentication Authenticated Any authenticated user of a process Service Any system-defined service process
- 23. Overview of Coda (1) The overall organization of AFS.
- 24. Overview of Coda (2) The internal organization of a Virtue workstation.
- 25. Communication (1) Side effects in Coda's RPC2 system.
- 26. Communication (2) a) Sending an invalidation message one at a time. b) Sending invalidation messages in parallel.
- 27. Naming Clients in Coda have access to a single shared name space.
- 28. File Identifiers The implementation and resolution of a Coda file identifier.
- 29. Sharing Files in Coda The transactional behavior in sharing files in Coda.
- 30. Transactional Semantics The metadata read and modified for a store session type in Coda. File-associated data Read? Modified? File identifier Yes No Access rights Yes No Last modification time Yes Yes File length Yes Yes File contents Yes Yes
- 31. Client Caching The use of local copies when opening a session in Coda.
- 32. Server Replication Two clients with different AVSG for the same replicated file.
- 33. Disconnected Operation The state-transition diagram of a Coda client with respect to a volume.
- 34. Secure Channels (1) Mutual authentication in RPC2.
- 35. Secure Channels (2) Setting up a secure channel between a (Venus) client and a Vice server in Coda.
- 36. Access Control Classification of file and directory operations recognized by Coda with respect to access control. Operation Description Read Read any file in the directory Write Modify any file in the directory Lookup Look up the status of any file Insert Add a new file to the directory Delete Delete an existing file Administer Modify the ACL of the directory
- 37. Plan 9: Resources Unified to Files General organization of Plan 9
- 38. Communication Files associated with a single TCP connection in Plan 9. File Description ctl Used to write protocol-specific control commands data Used to read and write data listen Used to accept incoming connection setup requests local Provides information on the caller's side of the connection remote Provides information on the other side of the connection status Provides diagnostic information on the current status of the connection
- 39. Processes The Plan 9 file server.
- 40. Naming A union directory in Plan 9.
- 41. Overview of xFS. A typical distribution of xFS processes across multiple machines.
- 42. Processes (1) The principle of log-based striping in xFS.
- 43. Processes (2) Reading a block of data in xFS.
- 44. Naming Main data structures used in xFS. Data structure Description Manager map Maps file ID to manager Imap Maps file ID to log address of file's inode Inode Maps block number (i.e., offset) to log address of block File identifier Reference used to index into manager map File directory Maps a file name to a file identifier Log addresses Triplet of stripe group, ID, segment ID, and segment offset Stripe group map Maps stripe group ID to list of storage servers
- 45. Overview of SFS The organization of SFS.
- 46. Naming A self-certifying pathname in SFS. /sfs LOC HID Pathname /sfs/sfs.vu.sc.nl:ag62hty4wior450hdh63u623i4f0kqere/home/steen/mbox
- 47. Summary A comparison between NFS, Coda, Plan 9, xFS. N/S indicates that nothing has been specified. Issue NFS Coda Plan 9 xFS SFS Design goals Access transparency High availability Uniformity Serverless system Scalable security Access model Remote Up/Download Remote Log-based Remote Communication RPC RPC Special Active msgs RPC Client process Thin/Fat Fat Thin Fat Medium Server groups No Yes No Yes No Mount granularity Directory File system File system File system Directory Name space Per client Global Per process Global Global File ID scope File server Global Server Global File system Sharing sem. Session Transactional UNIX UNIX N/S Cache consist. write-back write-back write-through write-back write-back Replication Minimal ROWA None Striping None Fault tolerance Reliable comm. Replication and caching Reliable comm. Striping Reliable comm. Recovery Client-based Reintegration N/S Checkpoint & write logs N/S Secure channels Existing mechanisms Needham-Schroeder Needham-Schroeder No pathnames Self-cert. Access control Many operations Directory operations UNIX based UNIX based NFS BASED