Clusters What is a distributed system (again) § “True” Distributed

u Loosely-coupled hardware § Distributed system def. Review • No , but provides the “feel” of a single memory § Cluster definition u Tightly-coupled software § Clusters vs. distributed systems • One single OS, or at least the feel of one § Cluster example 1 – reliable file service u Machines are somewhat, but not completely, autonomous § Cluster example 2 – fast web service § Classification of clusters Disk1 M1 P1 M2 P2 M3 P3

Network

M4 P4 M5 P5

1 Printer4 Disk5 2

Clusters (C) vs. Distributed systems (D) Clusters § structure u [C] - homogeneous - purchased to perform a certain task

u [D] - heterogeneous - put together from the available hardware § A subclass of distributed systems § scale § a small scale (mostly) homogeneous (the same hardware and OS) u [C] - small scale - don’t have to make sure that the setup scales array of computers (located usually in one site) dedicated to small u [D] - medium/large - have to span (potentially) large number of machines number of well defined tasks in solving of which the cluster acts as one single whole. § task § typical tasks for “classic” distributed systems: u [C] - specialized - made to perform a small set of well-defined tasks u [D] - general - usually have to be general-user computing environments u file services from/to distributed machines over (college) campus § price u distributing workload of all machines on campus u [C] - (relatively) cheap § typical tasks for a cluster: u [D] - free(?)/expensive u high-availability web-service/file service, other high-availability applications § reliability u [C] - as good as it needs to be u computing “farms”. u [D] - high/low? § security

3 u [C] - nodes trust each-other 4

u [D] - nodes do not trust each other

Cluster Cluster examples (cont.) examples

§ active machine - serves files to the network of computers § standby machine -listens to network and updates it’s own copy of pictures taken from “In Search of Clusters”, G.F. Pfister, 1998 files § branches get access to shared information even if one of § in case of machine failure - standby machine takes over file service 5 6 the links or computers fails transparent to users Cluster examples (cont.) Classification of clusters

§ Dispatcher (sprayer) machine - sends the web requests to server § By architecture: machines and makes sure that the servers are evenly loaded u with hardware additions - OpenVMS, Tandem Himalaya, § web service continues even if a server fails Parallel Syspex

u pure software - Beowulf, … § By task. There is no dividing line between clusters and true distributed systems - as we add features the clusters start to resemble D.S.

u availability

u batch processing

u database

u generic (scientific) computation

u full clusters (distributed systems) - single system image

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