Clustering can be best described as a
technology that automatically allows one physical server to take over the tasks
and responsibilities of another physical server that has failed. The obvious
goal behind this, given that all computer hardware and software will eventually
fail, is to ensure that users running mission-critical applications will have
little or no downtime when such a failure occurs. Downtime can be very
expensive, and our goal as DBA is to help reduce it as much as possible.
One very important aspect of clustering that often gets overlooked is that it is not a complete backup system for your applications. It is only one part of a multi-part strategy required to ensure minimum downtime and 100% recoverability.
An Active/Passive SQL Server cluster refers to a SQL Server cluster where only one instance of SQL Server is running on one of the physical servers in the cluster, and the other physical server does nothing, other that waiting to takeover should the primary node should fail.
More specifically, clustering refers to a group of two
or more servers (generally called nodes) that work together and represent
themselves as a single virtual server to a network. In other words, when a
client connects to clustered SQL Servers, it thinks there is only a single SQL
Server, not more than one. When one of the nodes fails, its responsibilities
are taken over by another server in the cluster, and the end-user notices
little, if any differences before, during, and after the failover
One very important aspect of clustering that often gets overlooked is that it is not a complete backup system for your applications. It is only one part of a multi-part strategy required to ensure minimum downtime and 100% recoverability.
The main benefits that clustering
provides is the ability to recover from failed server hardware (excluding the
shared disk) and failed software, such as failed services or a server lockup.
It is not designed to protect data, to protect against a shared disk array from
failing, to prevent hack attacks, to protect against network failure, or to
prevent SQL Server from other potential disasters,
Clustering is just one part of an entire strategy needed to help reduce application downtime. You will also need to purchase a shared disk array (more on this later) that offers redundancy, make tape backups, put the server behind a firewall, make sure your network connections have redundancy, use battery backup, and locate the server in a secure facility, among many other steps you can take. So don’t think that clustering is all you need for creating a highly available SQL Server. It is just one part.
Clustering is just one part of an entire strategy needed to help reduce application downtime. You will also need to purchase a shared disk array (more on this later) that offers redundancy, make tape backups, put the server behind a firewall, make sure your network connections have redundancy, use battery backup, and locate the server in a secure facility, among many other steps you can take. So don’t think that clustering is all you need for creating a highly available SQL Server. It is just one part.
We have different types of Clusters such as
- Active/active cluster
- Active/passive cluster
An Active/Active SQL Server cluster means that SQL
Server is running on both nodes of a two-way cluster. Each copy of SQL Server
acts independently, and users see two different SQL Servers. If one of the SQL
Servers in the cluster should fail, then the failed instance of SQL Server will
failover to the remaining server. This means that then both instances of SQL
Server will be running on one physical server, instead of two.
An Active/Passive SQL Server cluster refers to a SQL Server cluster where only one instance of SQL Server is running on one of the physical servers in the cluster, and the other physical server does nothing, other that waiting to takeover should the primary node should fail.
Advantages of Clustering
- Reduces downtime to a bare minimum.
- Permits an automatic response to a failed server or software. No human intervention is required.
- It allows you to perform upgrades without forcing users off the system for extended periods of time.
- It allows you to reduce downtime due to routine server, network, or database maintenance.
- Clustering doesn't require any servers to be renamed. So when failover occurs, it is relatively transparent to end-users.
- Failing back is quick, and can be done whenever the primary server if fixed and put back on-line.
- In some cases, clustering can be used to increase the scalability of an application. For example, if a current cluster is getting too busy, another server could be added to the cluster to expand the resources and help boost the performance of the application.
- More expensive than other failover alternatives, such as log shipping or stand-by servers.
- Requires more set up time than other alternatives.
- Requires more on-going maintenance than other alternatives.
- Requires more experienced DBAs and network administrators.
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