Transcript NFS - SMU
NFS : Network File System SMU CSE8343 Prof. Khalil September 27, 2003 Group 1 Group members: Payal Patel, Malka Samata, Wael Faheem, Hazem Morsy, Poramate Ongsakorn Outline • • • • • • • • • • • • • Definition NFS OSI v.s. NFS How does NFS work ? File Handles Statelessness Idempotent procedures TCP or UDP How does RPC work ? How does RPC differ from Local procedure call ? Port Mapper/RPCBIND Summary Reference & Appendix Definition • The Network File System (NFS) is a distributed file system that allows users to access files and directories located on remote computers and treat those files and directories as if they were local. • The NFS protocol is designed to be machine, operating system, network architecture, and transport protocol independent. Several workstations with NFS rapid.engr.smu.edu (Ultrix) buz.engr.smu.edu (Tru64) nova.engr.smu.edu (Solaris) Ethernet burn.engr.smu.edu (Solaris) DISK NFS Server Several workstations with NFS (Logical view) rapid:/ etc home buz:/ usr ... etc home nova:/ usr ... etc burn:/ etc home usr NFS Server's disk:/ ... etc pongsak home hmorsy usr wfaheem ... ... home usr ... OSI v.s. NFS Application NFS MOUNT PORT MAPPER NIS(Network Information System) Presentation XDR (eXternal Data Representation) Session RPC (Remote Procedure Call) Transport TCP, UDP Network IP Link Physical OSI Model Ethernet NFS Protocol Layers Outline • • • • • • • • • • • • • Definition NFS OSI v.s. NFS How does NFS work ? File Handles Statelessness Idempotent procedures TCP or UDP How does RPC work ? How does RPC differ from Local procedure call ? Port Mapper/RPCBIND Summary Reference & Appendix How does NFS work ? Client wants to access a file from server user process local file access NFS client client kernel local disk local file access NFS server RPC RPC TCP/UDP IP TCP/UDP IP port 2049 server kernel local disk File Handles • How does a server know which file/directory the client needs to access? – At first, client obtains a file handle for root of the file system – File handle is opaque to the client – Client sends file handle to server when referencing a file/directory – No need to use the full path names • “The file handle can contain whatever information the server needs to distinguish an individual file” Example of File Handles NFS Client NFS Server What is the attribute of current dir(1000) ? These are attributes of 1000 What is FH of "sub2" in 1000 ? FH of sub2 is 1100 What is FH of "myname.txt" in 1100 ? FH of myname.txt is 1200 What is the attribute of 1200 ? Suppose : client needs to cat the file sub2/myname.txt under the current directory (~/pongsak) . . . Statelessness • What is statelessness ? – Server does not need to maintain protocol state about it’s client – Server does not keep previous request information – Client keeps track of all information required to send requests to the server • Advantage : – If server crashes, no state information lost – Client needs only retransmit a request until the server responds Why is idempotent important ? NFS Client NFS Server What is the attribute of 1100 (dir sub2) ? Here is the attr and content of 1100 Remove "myname.txt" from 1100 Remove OK Remove "myname.txt" from 1100 (retransmitted) Error : No such file or dir Suppose : client needs to remove the file sub2/myname.txt Idempotent procedures • Can be executed more than once by the server and still return the same result • Stateless protocol requires idempotent operation • How to makes all NFS requests idempotent: – Server records recently performed operations in cache – Server checks in cache for duplicate requests – Server returns the previous result if it is a duplicate Should NFS use TCP or UDP ? • From the beginning, NFS used UDP – Most NFS systems were on LAN – High overhead if using TCP • Currently, NFS across WAN needs TCP – Reliability and congestion control – Both sides set TCP’s keep alive option – If server crashes, client opens new TCP connection – If client crashes, server will terminate the connection after the next keep alive probe Outline • • • • • • • • • • • • • Definition NFS OSI v.s. NFS How does NFS work ? File Handles Statelessness Idempotent procedures TCP or UDP How does RPC work ? How does RPC differ from Local procedure call ? Port Mapper/RPCBIND Summary Reference & Appendix How does RPC works ? Client Process Client executes Server Process RPC message Server waits Server starts Procedure call Server executes procedure Client waits Client continues RPC return message Procedure return Call terminates How does RPC different from local procedure call ? • Error handling: – failures of the server or network must be handled • Global variables: – arguments cannot be passed as global variables • Performance: – slower than local procedure calls • Authentication: – RPC can be transported over insecure networks Port Mapper : Analogy Airport Entrance Terminal A Terminal F US109 to DFW Terminal D Terminal E Terminal C Terminal B Port Mapper : Analogy Airport Entrance Terminal A Terminal F Flight schedule Flight US109 to DFW DE427 US109 US278 UA0097 Terminal D Terminal C Terminal B Departure time Destination Gate 6:15 AM Cincinnati 7:40 AM Akron DFW TXOH 4:35 PM Detroit MI 6:00 PM LAX CA Philadelphia E8 B5 C9 D12 Terminal E Port Mapper/RPCBIND user process user process Client Process Port Mapper (1) register at start (2) get port# RPC request (3) RPC reply with port# (4) RPC call (request) Client Kernel (5) RPC reply message Server Kernel user process Server Process Outline • • • • • • • • • • • • • Definition NFS OSI v.s. NFS How does NFS work ? File Handles Statelessness Idempotent procedures TCP or UDP How does RPC work ? How does RPC differ from Local procedure call ? Port Mapper/RPCBIND Summary Reference & Appendix Summary • • • • • • • NFS provides transparent file accesses NFS sends messages in XDR format using RPC NFS server is stateless NFS procedures are idempotent RPC executes remote procedure for clients Most RPC programs use ephemeral ports Port Mapper maps RPC programs and version numbers to port numbers Reference & Appendix • • • • • • http://www.faqs.org/rfcs/ http://www.ussg.iu.edu/usail/network/nfs/overview.html http://www.scit.wlv.ac.uk/~jphb/comms/nfs.html http://www.opengroup.org/onlinepubs/9629399/chap1.htm http://msdn.microsoft.com/library/default.asp?url=/library/en -us/rpc/rpc/rpc_start_page.asp http://www.cis.udel.edu/~srisath/NFS/index.html • • Silberschatz/Galvin/Gagne, Operating System Concepts, Wiley William Stallings, Operating Systems, Prentice Hall RFC Documents Request For Comments • • • • • • • • RFC 1014 – XDR (1987) RFC 1057 – RPCv2 (1988) RFC 1094 - NFSv2 (1989) RFC 1813 - NFSv3 (1995) RFC 2054 - WebNFS (1996) RFC 2624 - NFSv4 (1999) RFC 3010 - NFSv4 Protocol (2000) RFC 3530 - NFSv4 Protocol (2003) http://www.faqs.org/rfcs/ NFS v2 • • • • • 18 ops File sizes limited to 32 bit Slow writes Arbitrary transfer limit Lack of cache consistency NFS v3 • • • • • 22 ops File size extended to 64 bit Fast write Increased transfer write Support caching and ACLs WebNFS • Eliminate overhead of PORTMAP and MOUNT • Possible for firewall transit • Reduce number of LOOKUP request NFS v4 • • • • Internet Need for cross platform support Strong security Design for growth NFS procedures NFS Procedures LOOKUP MKDIR RMDIR READDIR RENAME REMOVE CREATE READ WRITE GETATTR SETATTR LINK SYMLINK READLINK STATFS Functions Returns a file handle and attribute corresponding to a file name in a specified directory. Create a directory. Delete a directory. Read a directory. Used by the Unix ls command, for example. Rename a file. Delete a file. Create a file. Read from a file, by specify the file handle, starting offset and max. no. of bytes to read (up to 8192). Write to a file. Returns the attributes of a file: type of file, permissions, size, owner, last-access time, and so on. Set the attributes of a file: permissions, owner, group, size,and last-access and lastmodification time. Create a Unix hard link to a file. Create a symbolic link to a file. Returns the name of the file to whidh the symbolic link points. Returns the status of a file system. Used by the Unix df command, for example. Format of RPC call IP header common for all Sun RPC procedure call depends on specific procedure being called 20 bytes UDP header 8 Transaction ID (XID) Send direction (0) RPC version (2) Program number Version number Procedure number 4 4 4 4 4 4 Credentials up to 408 bytes Verifier up to 408 bytes Procedure call parameters N Format of RPC reply IP header common for all Sun RPC procedure depends on specific procedure 20 bytes UDP header 8 Transaction ID (XID) Send direction (1) Status (0=accepted) 4 4 4 Verifier up to 400 bytes Accept status (0=success) 4 Procedure results N Questions and Comments Thank You!