Distributed File Systems. Distributed Systems IT332

Transcription

1 Distributed File Systems Distributed Systems IT332

2 2 Outline Introduction Network File System (NFS)

3 3 File System Basics A file is a named collection of logically related data A file system Provides a logical view of data and storage functions Provides facility to create, modify, organize, and delete files Provides sharing among users in a controlled manner Provides protection

4 4 Distributed File Systems A distributed file system enables multiple clients to access files and directories that are provided by one or more file servers File service: specification of what the file system offers Client primitives, application programming interface (API) File server: process that implements file service

5 5 File Service Remote access model Work done at the server Consistent sharing (+) Server may be a bottleneck ( ) High communication cost ( ) Upload/download model Work done at the client Low communication cost (+) Consistency is harder to maintain ( )

6 6 Server Types Stateless servers Do not store any session state every client request is treated independently Can easily recover from failure (+) Simple to implement (+) Clients have to provide full file information with each request ( ) Stateful servers Store session state Which clients have opened which files, current read and write pointers for files, which files have been locked by which clients Provide better performance for clients due to shorter request messages (+) Added work and storage on server ( ) Must ensure that the state can be recovered after a crash ( )

7 7 Semantics of File Sharing (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.

8 8 Semantics of File Sharing UNIX semantics: Every operation on a file is instantly visible to all processes Can be achieved if there is only a single file server and no client side caching is used impossible to achieve in practice Session semantics: No changes are visible to other processes until the file is closed Implement the upload/download model What if two clients modify the same file simultaneously? Immutable files: All files are immutable Can replace an existing file with a new file What if two processes try to replace the same file simultaneously? Transactions: a sequence of file operations can be executed indivisibly Expensive to implement

9 9 Network File System (NFS) Network File System (NFS) was introduced by Sun in 1985 Uses remote access model NFS allows a heterogeneous collection of processes, possibly running on different OSs and machines, to share a common file system

10 10 NFS Architecture Uses the virtual file system (VFS) layer to handle local and remote files VFS provides standard file system interface, hides difference between accessing local and remote file system

11 11 RPCs in NFS a) Reading data from a file in v3. b) Reading data using a compound procedure in v4. Both versions use Open Network Computing (ONC) RPC One RPC per operation in v3 Multiple operations can be grouped into a single request in v4

12 Mounting (part of) a remote file system in NFS Naming in NFS A sever can export a directory, i.e., make a directory and its entries available to clients An exported directory can be mounted into a client s local name space Users access remote files using local names

13 13 Naming in NFS A sever can mount directories exported by other servers, but cannot export those directories to its own clients Name resolution is iterative in NFSv3 and recursive in NFSv4 NFSv4 allows lookups to cross mount points at a server Mounting nested directories from multiple servers in NFS 12

14 14 File Sharing in NFS NFS implements session semantics: no changes are visible to other processes until the file is closed Client side caching are used to improve performance NFS supports file locking Applications can use locks to ensure consistency

15 Questions? 15