11 distributed file systems

8/19/2019 11 Distributed File Systems

1/19

CS-550: Distributed File Systems [SiS] 1

Resource Management in Distributed Systems:

Distributed File Systems


2/19


Distributed File Systems

Deinition:

! "m#lement a common ile system t$at can be s$ared by allautonomous com#uters in a distributed system

%oals:

! &et'or( trans#arency

! )ig$ a*ailability

+rc$itectural o#tions:

! Fully distributed: iles distributed to all sites

, "ssues: #erormance im#lementation com#le.ity! Client-ser*er Model:

, Fileser*er: dedicated sites storing iles #erorm storage and retrie*al

o#erations

, Client: rest o t$e sites use ser*ers to access iles


3/19

CS-550: Distributed File Systems [SiS] /

Distributed File Systems: Client-Ser*er +rc$itecture


4/19

CS-550: Distributed File Systems [SiS]

Distributed File Systems Ser*ices

! Ser*ices #ro*ided by t$e distributed ile system:

1 &ame Ser*er : 3ro*ides ma##ing name resolution t$e namessu##lied by clients into ob4ects iles and directories

! a(es #lace '$en #rocess attem#ts to access ile or directory t$e irst

time6

2 Cac$e manager: "m#ro*es #erormance t$roug$ ile cac$ing! Cac$ing at t$e client - 7$en client reerences ile at ser*er:

, Co#y o data broug$t rom ser*er to client mac$ine

, Subse8uent accesses done locally at t$e client

! Cac$ing at t$e ser*er:

, File sa*ed in memory to reduce subse8uent access time

9 "ssue: dierent cac$ed co#ies can become inconsistent6 Cac$e

managers at ser*er and clients $a*e to #ro*ide coordination6


5/19


y#ical Data +ccess in a ClientFile Ser*er +rc$itecture


6/19

CS-550: Distributed File Systems [SiS] ;

1 Mounting

! $e mount mec$anism binds toget$er se*eral ilename s#aces collection

o iles and directories into a single $ierarc$ically structured name s#ace


7/19

CS-550: Distributed File Systems [SiS] B

Mec$anisms used in distributed ile systems cont6

1 Mounting cont6

! ocation o mount inormation

a6 Mount inormation maintained at clients ,


8/19

CS-550: Distributed File Systems [SiS]

Mec$anisms used in distributed ile systems cont6

2 Cac$ing , "m#ro*es ile system #erormance by e.#loiting t$e locality o reerence

, 7$en client reerences a remote ile t$e ile is cac$ed in t$e mainmemory o t$e ser*er ser*er cac$e and at t$e client client cac$e

, 7$en multi#le clients modiy s$ared cac$ed data cac$e consistency becomes a #roblem

, "t is *ery diicult to im#lement a solution t$at guarantees consistency

/ )ints , reat t$e cac$ed data as $ints i6e6 cac$ed data may not be com#letely

accurate

, Can be used by a##lications t$at can disco*er t$at t$e cac$ed data isin*alid and can reco*er

!


9/19


10/19


Design "ssues

16 &aming and name resolution , erminology

! &ame: eac$ ob4ect in a ile system ile directory $as a uni8ue name! &ame resolution: ma##ing a name to an ob4ect or multi#le ob4ects re#lication

! &ame s#ace: collection o names 'it$ or 'it$out same resolution mec$anism

, +##roac$es to naming iles in a distributed system

a Concatenate name o $ost to names o iles on t$at $ost

, +d*antage: uni8ue ilenames sim#le resolution , Disad*antages:

G Conlicts 'it$ net'or( trans#arency

G Mo*ing ile to anot$er $ost re8uires c$anging its name and t$e a##lications using it

b Mount remote directories onto local directories , Re8uires t$at $ost o remote directory is (no'n

, +ter mounting iles reerenced location-trans#arent "6e6 ile name does not re*eal itslocation

c )a*e a single global directory , +ll iles belong to a single name s#ace

, imitation: $a*ing uni8ue system 'ide ilenames re8uire a single com#uting acility orcoo#erating acilities


11/19


Design "ssues cont6

16 &aming and &ame Resolution cont6 , Conte.ts

! Sol*e t$e #roblem o system-'ide uni8ue names by #artitioning a name s#aceinto conte.ts geogra#$ical organiHational etc6

! &ame resolution is done 'it$in t$at conte.t

! "nter#retation may lead to anot$er conte.t

! File &ame I Conte.t J &ame local to conte.t

, &ameser*er! 3rocess t$at ma#s ile names to ob4ects iles directories! "m#lementation o#tions

, Single name Ser*er

G Sim#le im#lementation reliability and #erormance issues

, Se*eral &ame Ser*ers on dierent $osts

G


12/19


Design "ssues Cont6

26 Cac$ing

, Cac$ing at t$e client: Main memory *s6 Dis(

! Main memory: J Fast J 7or(s or dis(less clients -


13/19


Design "ssues Cont6

/6 7riting #olicy , Luestion: once a client 'rites into a ile and t$e local cac$e '$en s$ould

t$e modiied cac$e be sent to t$e ser*er , #tions:

! 7rite-t$roug$: all 'rites at t$e clients immediately transerred to t$eser*ers

, +d*antage: reliability

, Disad*antage: #erormance it does not ta(e ad*antage o t$e cac$e! Delayed 'riting: delay transer to ser*ers

, +d*antages:

G Many 'rites ta(e #lace including intermediate results beore atranser

G Some data may be deleted , Disad*antage: reliability

! Delayed 'riting until ile is closed at client

, For s$ort o#en inter*als same as delayed 'riting

, For long inter*als reliability #roblems


14/19


Design "ssues Cont6

6 +*ailability

, "ssue: '$at is t$e le*el o a*ailability o iles in a distributed ile system

, Resolution: use re#lication to increase a*ailability i6e6 many co#iesre#licas o iles are maintained at dierent sitesser*ers

, Re#lication issues:

! )o' to (ee# re#licas consistent

! )o' to detect inconsistency among re#licas

, =nit o re#lication

! File

! %rou# o iles

a Kolume: grou# o all iles o a user or grou# or all iles in a ser*er

G +d*antage: ease o im#lementation

G Disad*antage: 'asteul user may need only a subset re#licated

b 3rimary #ac( *s6 #ac(

G 3rimary #ac(:all iles o a user

G 3ac(: subset o #rimary #ac(6 Can recei*e a dierent degree o re#lication or

eac$ #ac(


15/19


Design "ssues Cont6

56 Scalability

, "ssue: can t$e design su##ort a gro'ing system

,


16/19


Case Studies:

$e Sun &et'or( File System &SF

! De*elo#ed by Sun Microsystems to #ro*ide a distributed ile

system inde#endent o t$e $ard'are and o#erating system

! +rc$itecture

, Kirtual File System KFS:

File system interace t$at allo's &SF to su##ort dierent ile systems

, Re8uests or o#eration on remote iles are routed by KFS to &FS , Re8uests are sent to t$e KFS on t$e remote using

! $e remote #rocedure call R3C and

! $e e.ternal data re#resentation >DR

, KFS on t$e remote ser*er initiates iles system o#eration locally

, Vnode Kirtual &ode:

! $ere is a net'or(-'ide vnode or e*ery ob4ect in t$e ile system ile or

directory- e8ui*alent o =&"> inode

! vnode $as a mount table allo'ing any node to be a mount node


17/19


Case Studies: &FS +rc$itecture


18/19


&FS Cont6 ! &aming and location:

, 7or(stations are designated as clients or ile ser*ers

, + client deines its o'n #ri*ate ile system by mounting a subdirectory o

a remote ile system on its local ile system

,


19/19


&FS Cont6 ! Cac$ing:

, Cac$ing done in main memory o clients

, Cac$ing done or: ile bloc(s translation o ilenames to vnodes and attributeso iles and directories

1 Cac$ing o ile bloc(s! Cac$ed on demand 'it$ time stam# o t$e ile '$en last modiied on t$e ser*er

!

11 distributed file systems

Documents