CS162 Operating Systems and Systems Programming Lecture 24 Distributed File SystemsReview: Virtual File System (VFS)Goals for TodaySimple Distributed File SystemUse of caching to reduce network loadFailuresWorld Wide WebWWW CachingWWW Proxy CachesSchematic View of NFS ArchitectureAdministriviaNetwork File System (NFS)NFS ContinuedNFS Cache consistencySequential Ordering ConstraintsNFS Pros and ConsAndrew File SystemAndrew File System (con’t)SummaryBREAKInternet WormsSapphire (AKA Slammer) WormBefore SapphireAfter SapphireWorm Propagation BehaviorInternet VirusesLoveLetter Virus (May 2000)LoveLetter’s ImpactWorm/Virus SummaryCS162Operating Systems andSystems ProgrammingLecture 24Distributed File SystemsApril 30, 2008Prof. Anthony D. Josephhttp://inst.eecs.berkeley.edu/~cs162Lec 24.24/28/08 Joseph CS162 ©UCB Spring 2008Review: Virtual File System (VFS)•VFS: Virtual abstraction similar to local file system–Instead of “inodes” has “vnodes”–Compatible with a variety of local and remote file systems»provides object-oriented way of implementing file systems•VFS allows the same system call interface (the API) to be used for different types of file systems–The API is to the VFS interface, rather than any specific type of file systemLec 24.34/28/08 Joseph CS162 ©UCB Spring 2008Goals for Today•Examples of Distributed File Systems•Cache Coherence Protocols•Worms and VirusesNote: Some slides and/or pictures in the following areadapted from slides ©2005 Silberschatz, Galvin, and Gagne. Many slides generated from my lecture notes by Kubiatowicz.Lec 24.44/28/08 Joseph CS162 ©UCB Spring 2008Simple Distributed File System•Remote Disk: Reads and writes forwarded to server–Use RPC to translate file system calls–No local caching/can be caching at server-side•Advantage: Server provides completely consistent view of file system to multiple clients•Problems? Performance!–Going over network is slower than going to local memory–Lots of network traffic/not well pipelined–Server can be a bottleneckClientServerRead (RPC)Return (Data)ClientWrite (RPC)ACKcacheLec 24.54/28/08 Joseph CS162 ©UCB Spring 2008ServercacheF1:V1F1:V2Use of caching to reduce network loadRead (RPC)Return (Data)Write (RPC)ACKClientcacheClientcache•Idea: Use caching to reduce network load–In practice: use buffer cache at source and destination•Advantage: if open/read/write/close can be done locally, don’t need to do any network traffic…fast!•Problems: –Failure:»Client caches have data not committed at server–Cache consistency!»Client caches not consistent with server/each otherF1:V1F1:V2read(f1)write(f1)V1read(f1)V1read(f1)V1OKread(f1)V1read(f1)V2Lec 24.64/28/08 Joseph CS162 ©UCB Spring 2008Failures•What if server crashes? Can client wait until server comes back up and continue as before?–Any data in server memory but not on disk can be lost–Shared state across RPC: What if server crashes after seek? Then, when client does “read”, it will fail–Message retries: suppose server crashes after it does UNIX “rm foo”, but before acknowledgment?»Message system will retry: send it again»How does it know not to delete it again? (could solve with two-phase commit protocol, but NFS takes a more ad hoc approach)•Stateless protocol: A protocol in which all information required to process a request is passed with request–Server keeps no state about client, except as hints to help improve performance (e.g. a cache)–Thus, if server crashes and restarted, requests can continue where left off (in many cases)•What if client crashes?–Might lose modified data in client cacheCrash!Lec 24.74/28/08 Joseph CS162 ©UCB Spring 2008World Wide Web•Key idea: graphical front-end to RPC protocol•What happens when a web server fails?–System breaks!–Solution: Transport or network-layer redirection »Invisible to applications»Can also help with scalability (load balancers)»Must handle “sessions” (e.g., banking/e-commerce)•Initial version: no caching–Didn’t scale well – easy to overload serversLec 24.84/28/08 Joseph CS162 ©UCB Spring 2008WWW Caching•Use client-side caching to reduce number of interactions between clients and servers and/or reduce the size of the interactions:–Time-to-Live (TTL) fields – HTTP “Expires” header from server–Client polling – HTTP “If-Modified-Since” request headers from clients–Server refresh – HTML “META Refresh tag” causes periodic client poll•What is the polling frequency for clients and servers? –Could be adaptive based upon a page’s age and its rate of change•Server load is still significant!Lec 24.94/28/08 Joseph CS162 ©UCB Spring 2008WWW Proxy Caches•Place caches in the network to reduce server load–But, increases latency in lightly loaded case–Caches near servers called “reverse proxy caches” »Offloads busy server machines–Caches at the “edges” of the network called “content distribution networks”»Offloads servers and reduce client latency•Challenges:–Caching static traffic easy, but only ~40% of traffic–Dynamic and multimedia is harder»Multimedia is a big win: Megabytes versus Kilobytes–Same cache consistency problems as before•Caching is changing the Internet architecture–Places functionality at higher levels of comm. protocolsLec 24.104/28/08 Joseph CS162 ©UCB Spring 2008Schematic View of NFS ArchitectureLec 24.114/28/08 Joseph CS162 ©UCB Spring 2008Administrivia•Project #4 design deadline is Thu 5/1 at 11:59pm–See newsgroup for design doc details–Code deadline is Wed 5/14•Final Exam –May 21st, 12:30-3:30pm•Final Topics: Any suggestions?–Please send them to me…Lec 24.124/28/08 Joseph CS162 ©UCB Spring 2008Network File System (NFS)•Three Layers for NFS system–UNIX file-system interface: open, read, write, close calls + file descriptors–VFS layer: distinguishes local from remote files»Calls the NFS protocol procedures for remote requests–NFS service layer: bottom layer of the architecture»Implements the NFS protocol•NFS Protocol: RPC for file operations on server–Reading/searching a directory –manipulating links and directories –accessing file attributes/reading and writing files•Write-through caching: Modified data committed to server’s disk before results are returned to the client –lose some of the advantages of caching–time to perform write() can be long–Need some mechanism for
View Full Document
Unlocking...