Distributed Systems Principles and Paradigms

Transcription

1 Distriuted Systems Principles nd Prdigms Chpter 11 (version April 7, 2008) Mrten vn Steen Vrije Universiteit Amsterdm, Fculty of Science Dept. Mthemtics nd Computer Science Room R4.20. Tel: (020) URL: steen/ 01 Introduction 02 Architectures 03 Processes 04 Communiction 05 Nming 06 Synchroniztion 07 Consistency nd Repliction 08 Fult Tolernce 09 Security 10 Distriuted Oject-Bsed Systems 11 Distriuted File Systems 12 Distriuted We-Bsed Systems 13 Distriuted Coordintion-Bsed Systems 00 1 /

2 Distriuted File Systems Generl gol: Try to mke file system trnsprently ville to remote clients. Requests from client to ccess remote file File stys on server Remote ccess model 1. File moved to client Old file New file 2. Accesses re done on client 3. When client is done, file is returned to server Uplod/downlod model 11 1 Distriuted File Systems/11.1 Architecture

3 Exmple: NFS Architecture NFS is implemented using the Virtul File System strction, which is now used for lots of different operting systems: System cll lyer System cll lyer Virtul file system (VFS) lyer Virtul file system (VFS) lyer Locl file system interfce NFS client NFS server Locl file system interfce RPC client stu RPC server stu Network Essence: VFS provides stndrd file system interfce, nd llows to hide difference etween ccessing locl or remote file system. Question: Is NFS ctully file system? 11 2 Distriuted File Systems/11.1 Architecture

4 NFS File Opertions Oper. v3 v4 Description Crete Yes No Crete regulr file Crete No Yes Crete nonregulr file Link Yes Yes Crete hrd link to file Symlink Yes No Crete symolic link to file Mkdir Yes No Crete sudirectory Mknod Yes No Crete specil file Renme Yes Yes Chnge the nme of file Remove Yes Yes Remove file from file system Rmdir Yes No Remove n empty sudirectory Open No Yes Open file Close No Yes Close file Lookup Yes Yes Look up file y mens of nme Reddir Yes Yes Red the entries in directory Redlink Yes Yes Red the pth nme in symolic link Getttr Yes Yes Get the ttriute vlues for file Setttr Yes Yes Set one or more file-ttriute vlues Red Yes Yes Red the dt contined in file Write Yes Yes Write dt to file Question: Anything unusul etween v3 nd v4? 11 3 Distriuted File Systems/11.1 Architecture

5 Cluster-Bsed File Systems Oservtion: When deling with very lrge dt collections, following simple client-server pproch is not going to work. Solution 1: For speeding up file ccesses, pply striping techniques y which files cn e fetched in prllel: File lock of file File lock of file e c d e c d e c d e While-file distriution c e c d c d d e e File-striped system 11 4 Distriuted File Systems/11.1 Architecture

6 Exmple: Google File System Solution 2: Divide files in lrge 64 MB chunks, nd distriute/replicte chunks cross mny servers. GFS client file nme, chunk index contct ddress Mster Instructions Chunk-server stte Chunk ID, rnge Chunk dt Chunk server Linux file system Chunk server Linux file system Chunk server Linux file system A couple of importnt detils: The mster mintins only (file nme, chunk server) tle in min memory miniml I/O Files re replicted using primry-ckup scheme; the mster is kept out of the loop 11 5 Distriuted File Systems/11.1 Architecture

7 RPCs in File Systems Oservtion: Mny (trditionl) distriuted file systems deploy remote procedure clls to ccess files. When wide-re networks need to e crossed, lterntives need to e exploited: LOOKUP LOOKUP OPEN READ Lookup nme Lookup nme Time READ Red file dt Time Open file Red file dt () () 11 6 Distriuted File Systems/11.3 Communiction

8 Exmple: RPCs in Cod Oservtion: When deling with replicted files, sequentilly sending informtion is not the wy to go: Invlidte Reply Invlidte Reply Invlidte Reply Invlidte Reply Time Time () () Note: In Cod, clients cn cche files, ut will e informed when n updte hs een performed Distriuted File Systems/11.3 Communiction

9 File Shring Semntics (1/2) Prolem: When deling with distriuted file systems, we need to tke into ccount the ordering of concurrent red/write opertions, nd expected semntics (= consistency). mchine #1 Process A c 2. Write "c" 1. Red "" Single mchine Originl file File server Process A c 3. Red gets "" mchine #2 Process B Process B 1. Write "c" 2. Red gets "c" () () 11 8 Distriuted File Systems/11.5 Synchroniztion

10 File Shring Semntics (2/2) UNIX semntics: red opertion returns the effect of the lst write opertion cn only e implemented for remote ccess models in which there is only single copy of the file Trnsction semntics: the file system supports trnsctions on single file issue is how to llow concurrent ccess to physiclly distriuted file Session semntics: the effects of red nd write opertions re seen only to the client tht hs opened ( locl copy) of the file wht hppens when file is closed (only one client my ctully win) 11 9 Distriuted File Systems/11.5 Synchroniztion

11 Exmple: File Shring in Cod Essence: Cod ssumes trnsctionl semntics, ut without the full-fledged cpilities of rel trnsctions. Session S A Open(RD) File f Invlidte Close Open(WR) File f Close Session S B Time Note: Trnsctionl issues repper in the form of this ordering could hve tken plce Distriuted File Systems/11.5 Synchroniztion

12 Consistency nd Repliction Oservtion: In modern distriuted file systems, clientside cching is the preferred technique for ttining performnce; server-side repliction is done for fult tolernce. Oservtion: s re llowed to keep (lrge prts of) file, nd will e notified when control is withdrwn servers re now generlly stteful 1. sks for file 2. delegtes file Old file Locl copy 3. reclls delegtion 4. sends returns file Updted file Distriuted File Systems/11.6 Consistency nd Repliction

13 Exmple: -side Cching in Cod A Session S A Session S A Open(RD) File f Invlidte (cllck rek) Close Open(RD) File f Close File f OK (no file trnsfer) Open(WR) Open(WR) Close Close B Session S B Session S B Time Note: By mking use of trnsctionl semntics, it ecomes possile to further improve performnce Distriuted File Systems/11.6 Consistency nd Repliction

14 Fult Tolernce Oservtion: FT is hndled y simply replicting file servers, generlly using stndrd primry-ckup protocol: Primry server for item x Bckup server W1 W5 R1 R2 W4 W3 W4 W3 Dt store W2 W4 W3 W1. Write request W2. Forwrd request to primry W3. Tell ckups to updte W4. Acknowledge updte W5. Acknowledge write completed R1. Red request R2. Response to red Distriuted File Systems/11.7 Fult Tolernce

15 High Avilility in P2P Systems Prolem: There re mny fully decentrlized file-shring systems, ut ecuse churn is high (i.e., nodes come nd go ll the time), we my fce n vilility prolem. Solution: Replicte files ll over the plce (repliction fctor: r rep ). Alterntive: Apply ersure coding: Prtition file F into m frgments, nd recode into collection F of n > m frgments Property: ny m frgments from F re sufficient to reconstruct F. Repliction fctor: r ec = n/m Distriuted File Systems/11.7 Fult Tolernce

16 Repliction vs. Ersure Coding With n verge node vilility, nd required file unvilility ǫ, we hve for ersure coding: ( ) rec m 1 ǫ = i (1 ) r ec m i i r ec m i=m nd for file repliction: 1 ǫ = 1 (1 ) r rep r req r ec Node vilility Distriuted File Systems/11.7 Fult Tolernce