Hi, ever since last week I have a doubt about a type of clustering
implementation. Is it possible to cluster more than one SQL Server
instance. I mean i know it is possible to cluster one instance... so
that when that instance fails the other one enter into context be it
either manually or automatically... but what happens when there are 3
instances? if one instance fails the other two instances would be
forced to pass their execution to the other server even when maybe
everthing was ok with them

Thanks in advance for the response
Alan

I think you misunderstand the relationship between an instance and a server
in a cluster.

A cluster is a group of machines that share a common storage unit and are
configured for failover. The cluster software arbitrates the shared storage
unit so that only one host computer (node) can control a logical disk (LUN)
at a time. Note that the storage logical disk appears to a node as a
physical disk.

A node is a particular host computer within a cluster.

An instance is a virtualized SQL server. It has one or more LUNs from the
shared unit, its own network name and IP address. It is hosted on a single
node at a tim, but can move to another node either in response to a failure
event or by administrator command. It is important to understand the
separation of nodes and instances. Instances are part of the overall
cluster. Nodes are either current or potential hosts. This type of
clustering is called "shared nothing" since no node requires any resource
from any other node in order to operate. When an instance moves to another
host node, the client sees the SQL Server stop and a restart. It still
connects exactly as before, since the IP address and network name moved to
the new host node along with the rest of the instance resources.

Just as a stand-alone system can support multiple instances, o can a
cluster. Each instance is completely independent of any other instance
(more of the "shared nothing" design). Each new instance must have its own
disk(s), IP address, and network name. You can move instances around on
various cluster nodes as you want, completely independent of each other. Of
course, each host node has a finite amount of processor and memory, which
must be shared between any instances it currently hosts.

So yes, you can have many instances in a cluster. Note that with shared
storage systems other than SCSI, you can have more than two nodes in a
cluster. Clusters with X nodes and X- instances are classified as N-1
Clusters.

--
Geoff N. Hiten
Senior SQL Infrastructure Consultant
Microsoft SQL Server MVP





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