A computer cluster, usually simply cluster (of dt. Kluster - "“which closely and thickly together-sit"”), marks a number of interlaced computers, which in many cases as a computer to be seen from the outside to be able. Usually are the individual elements of a cluster among themselves connected by a fast network. A goal of the "“Clustering"” mostly exists in the increase of the computing speed or the availability opposite an individual computer. The computer in a cluster (also knot or server) server farm are also often called.

Cluster categories

The term cluster actually describes primarily the architecture of the individual components and their Zusammenwirken.Man differentiates between so-called homogeneous and heterogeneous Clustern. Computers homogeneous cluster run under the same operating system and same hardware, with the heterogeneous cluster can different operating systems or hardware be used. Well-known Linux cluster software are e.g.: "“Beowulf"” or "“OpenMosix"”.

Cluster are used for a number of often different intended purposes:

High availability cluster (hectar)

High availability cluster (English High Availability cluster) are used for the increase of the availability and/or for better reliability. Up, are migrated to footstep on a knot of the cluster an error the services running on this cluster on another knot. 2 knots possess most hectar cluster. Cluster exists with those on all knots of services constantly runs, this cluster active-actively and/or symmetrically calls one or hot standby. If not all knots are active, one speaks of active passive or asymmetrically. Both the hardware and the software of a hectar cluster must freely of single POINT OF failures (components, which would bring the entire system due to an error to the loss) its. Apply such hectar cluster in critical environments, in which maximum down-times are permitted by only few minutes in the year. In the context of disaster scenarios critical computer systems must be secured. In addition the cluster knots are often platziert apart several kilometers in different computing centres. In the emergency the knot can transfer the entire load in the computing centre not concerned. One calls this kind of Clustern also "„stretched cluster "“.

Load Balancing cluster

Load Balancing cluster are developed for the purpose of the distribution of load on several machines. The distribution of load is usually made by a redundantly laid out, central instance. Possible operational areas are environments with high requirements at computer performance. The performance need is covered here not by armament of individual computers, but by adding additional computers. Reason for the use is not least the use of low-priced standard computers (COTS components) instead of of expensive special computers.

High performance Computing (HPC) cluster

Serve HPC cluster for the processing of arithmetic problems. These arithmetic problems are divided on several knots. Either the tasks in "„portions will become "“divided and parallel distributed on several knots implemented or the arithmetic problems (jobs called) on the individual knots. HPC cluster often are within the scientific range.

History

The first cluster product available in the trade was ARCnet, which 1977 were developed by DATA POINT. The company DEC had first real success in the year 1983 with the conception of the product VAXCluster for its computer system VAX/VMS. The product did not only support parallel counting on the cluster knots, but also the common use of file systems and devices of all knots involved. These characteristics are not this very day contained with many free and commercial products. VAXCluster is available as VMSCluster also today still from the company HP for the operating system OpenVMS and the processors alpha and Itanium.

Technology

Hectar cluster

The failover function is usually made available by the operating system (Servicefailover, IP assumption). The assumption of services can be achieved e.g. by the automatic migration by IP addresses or using a multicast address.

Generally differentiated between architectures shared emergency-hung and shared all.

Typical representative "„active active " - clusters with shared emergency-hung architecture is DB2 with EEE (speak "“tripolarize i"”). Here each cluster knot accommodates its own data partition. A performance gain is obtained by the Partitionierung of the data and the distributed processing accompanying with it. Reliability is not ensured hereby.

Differently this is with "„shared all " - the cluster. This architecture ensures competitive access to Shared by a STORAGE that all cluster knots can access the entire volume of data. Apart from scaling and performance increase by this architecture also an additional reliability is reached. If a knot precipitates, the other knots take over its task (n). A typical representative shared of all architecture is material Oracle Application cluster (RAC).

Services must be particularly for a cluster programmed. A service is called cluster aware, if it processes these reacted to special events (like e.g. the loss of a cluster knot) and in an appropriate way.

Cluster software can be implemented both in the form of Scripten in addition, be integrated in the Betriebsystemkernel.

HPC cluster

With HPC Clustern becomes the task, which job distributes, which can be settled, often by means of a Decomposition program divided into smaller parts and then on the knots.

Communication between job parts running on different knots usually happens by means of Message Passing interface MPI, since fast communication between individual processes is desired. In addition one couples the knots with a fast network like e.g. InfiniBand.

A usual method for the distribution of jobs on a HPC cluster is a job Scheduling program, which can make a distribution after different categories like e.g. Load sharing Facility (LSF) or network Queueing system (NQS).

Recent time join ever more Linux cluster into the TOP500 of the supercomputers, not least because also for fastidious arithmetic problems cheap COTS hardware be used can.

See also

• Actively/active cluster
• Actively/passive cluster
• Gridcomputing
• High availability

Literature

• Heiko Bauke, Stephan Mertens: Cluster Computing. Springer 2006, ISBN 3-540-42299-4
• Charles Bookman: Linux Clustering. ISBN 1-578-70274-7
• Hartmann Gebauer: Clustering with Windows NT. Addison-Wesley 1999, ISBN 3-827-31403-8
• Andrea hero: Oracle 10g high availability. Addison-Wesley 2004, ISBN 3-827-32163-8
• Michael Soltau: Unix/Linux high availability. MITP 2002, ISBN 3-826-60775-9
• Martin Wieczorek, Uwe Naujoks, Bob Bartlett (Hrsg.): Business Continuity. Springer 2003, ISBN 3-540-44285-5

Related links

• http://linux-ha.org/
• TOP500 list of the highest performance supercomputers
• Cluster from 12 VIA EPIA800-Rechnern

Cluster software

• MPI - The Message Passing interface (MPI) standard
• wackamole - hectar cluster software (shifts IP addresses)
• heartbeat - hectar cluster software
• The Beowulf project - distribution counting
• Open Mosix - distribution counting
• Kimberlite - Hectar-Failover cluster
• HACMP - Hectar cluster software of IBM for AIX (English)
• MC/Service Guard - hectar cluster software of HP for HP-UX and Linux
• VMSCluster - VMSCluster of HP for OpenVMS
• Oracle RAC - Cluster software of Oracle (English)
• SunCluster - cluster software of Sun Microsystems (English)
• Windows 2003 Clustering services hectar cluster software by Microsoft
• Cluster server of VERITAS (Symantec) for AIX, HP-UX, Linux (talk has & Suse), Solaris and Microsoft (W2K & Windows 2003)

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