Ivan Stankovic

What is a SQL Server disaster recovery?

March 14, 2014 by
A SQL Server disaster is an event that causes data loss or any serious SQL Server disruption. Causes of a disaster can be numerous: power failure, hardware failure, virus attack, natural disaster, human error, etc. Some SQL Server disasters cannot be prevented. That’s why a good disaster recovery plan is important

A SQL Server disaster recovery plan (DRP) is a process to have SQL Server up and running, and to overcome data loss after a disaster. A disaster recovery plan must be planned and documented in order to prevent catastrophic data loss and incidents

High-Availability is the term that shows the percentage of time a system needs to be available. In some environments, it goes as high as 99.999% or just 5.26 minutes downtime per year

A good SQL Server disaster recovery plan must take into account numerous factors: sensitivity of data, data loss tolerance, required availability, etc. The plan can be based on few a solutions:

  • Failover clustering
  • Database mirroring
  • Replication
  • Log shipping
  • Backup and restore

Each solution has its own advantages and cost of implementing. Based on the needs, a SQL Server disaster recovery plan should include on one or more available solutions

Failover clustering is a concept where a SQL Server instance is installed on the shared storage. It provides the infrastructure that supports high-availability and disaster recovery scenarios of hosted server applications. If a cluster node fails, the services that were hosted on that node can be automatically or manually transferred to another available node in a process known as failover. There is a short period of downtime while SQL Server is failing over

Dialog showing basic concept of the failover clustering

Database mirroring is a solution for increasing availability of a SQL Server database. It maintains two exact copies of a single database. These copies must be on different SQL Server instances. Two databases form a relationship known as a database mirroring session. One instance acts as the principal server, while the other is in the standby mode and acts as the mirror server. Two SQL Server instances that act in mirroring environment are known as partners, the principal server is sending the active portion of a transaction log to the mirror server where all transactions are redone

There can be two types of mirror servers: hot and warm. A hot mirror server has synchronized sessions with quick failover time without data loss. A warm mirror server doesn’t have synchronized sessions and there is a possibility of data loss

This solution will be removed in future versions of SQL Server

Illustration of the database mirroring concept

Replication can be used as a technology for coping and distributing data from one SQL Server database to another. Consistency is achieved by synchronizing. Replication of a SQL Server database can result in benefits like: load balancing, redundancy, and offline processing. Load balancing allows spreading data to a number of SQL Servers and distributing the query load among those SQL Servers. A replication consists of two components:

  • Publishers – databases that provide data. Any replication may have one or more publishers
  • Subscribers – databases that receive data from publishers via replication. Data in subscribers is updated whenever data the publisher is modified

SQL Server supports three types of replication: