sql server high availability solutions (pros & cons)

SQL Server High Availability Solutions

Copyrights © 2016 Fard Solutions Sdn Bhd, All Rights Reserved.

Applied To: SQL Server 2005, SQL Server 2008, SQL Server 2008 R2, SQL Server 2012, SQL Server 2014 and SQL Server 2016.

Witten By: Hamid J. Fard – Microsoft Certified Master: SQL Server 2008

It is quiet tough to decide what kind of high availability solution is available on the table and which

one can fit all your production server requirements in terms of licenses, hardware cost, maintenance

and professional services. In this blog post, I would like to explain every single Sql server high

availability solution that is currently feasible with the related pros and cos.

Log Shipping SQL Server Log shipping allows you to automatically send transaction log backups from a primary

database on a primary server instance to one or more secondary databases on separate secondary

server instances. The transaction log backups are applied to each of the secondary databases

individually. An optional third server instance, known as the monitor server, records the history and

status of backup and restore operations and, optionally, raises alerts if these operations fail to occur

as scheduled.

Log shipping consists of three operations:

1. Back up the transaction log at the primary server instance.

2. Copy the transaction log file to the secondary server instance.

3. Restore the log backup on the secondary server instance.

The log can be shipped to multiple secondary server instances. In such cases, operations 2 and 3 are

duplicated for each secondary server instance.

A log shipping configuration does not automatically fail over from the primary server to the secondary

server. If the primary database becomes unavailable, any of the secondary databases can be brought

online manually.

Copy the Log Backup Restore Log Backup

Primary Server Secondary Server

Backup Shared Location

Monitoring Server



Database Mirroring Database mirroring is a primarily software solution for increasing database availability. Mirroring is

implemented on a per-database basis and works only with databases that use the full recovery model.

The simple and bulk-logged recovery models do not support database mirroring. Therefore, all bulk

operations are always fully logged. Database mirroring works with any supported database

compatibility level.

Database mirroring maintains two copies of a single database that must reside on different server

instances of SQL Server Database Engine. Typically, these server instances reside on computers in

different locations. One server instance serves the database to clients (the principal server). The other

instance acts as a hot or warm standby server (the mirror server), depending on the configuration and

state of the mirroring session. When a database mirroring session is synchronized, database mirroring

provides a hot standby server that supports rapid failover without a loss of data from committed

transactions. When the session is not synchronized, the mirror server is typically available as a warm

standby server (with possible data loss).

Database mirroring is a simple strategy that offers the following benefits:

1. Increases data protection.

2. Increases availability of a database.

3. Improves the availability of the production database during upgrades.

Principal Database

Witness Server

Mirror Database

Data Flow



Failover Cluster SQL Server failover clustering provides high-availability support for an entire SQL Server instance. SQL

Server failover clusters are built on top of Windows Server failover clusters. To create a SQL Server

failover cluster, you need to first create the underlying Windows Server failover cluster.

A SQL Server failover cluster, also known as a failover cluster instance, consists of the following:

One or more Windows Server failover cluster nodes

A cluster resource group dedicated for the SQL Server failover cluster which contains the

following:

o Network name to access the SQL Server failover cluster

IP addresses

Shared disks used for the SQL Server database and log storage

Resource DLLs that control the SQL Server failover behavior

o Check-pointed registry keys that are automatically kept in sync across the failover

cluster nodes

A SQL Server failover cluster appears on the network as a single SQL Server instance on a single

computer. Internally, only one of the nodes owns the cluster resource group at a time, serving all the

client requests for that failover cluster instance. In case of a failure (hardware failures, operating

system failures, application or service failures), or a planned upgrade, the group ownership is moved

to another node in the failover cluster. This process is called failover. By leveraging the Windows

Server failover cluster functionality, SQL Server failover cluster provides high availability through

redundancy at the instance level.

SAN System

Domain Controller

Active/Passive Active/Passive



AlwaysOn AG The AlwaysOn Availability Groups feature is a high-availability and disaster-recovery solution that

provides an enterprise-level alternative to database mirroring. Introduced in SQL Server 2012,

AlwaysOn Availability Groups maximizes the availability of a set of user databases for an enterprise.

An availability group supports a failover environment for a discrete set of user databases, known as

availability databases that fail over together. An availability group supports a set of read-write primary

databases and one to eight sets of corresponding secondary databases. Optionally, secondary

databases can be made available for read-only access and/or some backup operations.

An availability group fails over at the level of an availability replica. Failovers are not caused by

database issues such as a database becoming suspect due to a loss of a data file, deletion of a

database, or corruption of a transaction log.

AlwaysOn Availability Groups provides a rich set of options that improve database availability and that

enable improved resource use. The key components are as follows:

Supports up to nine availability replicas. An availability replica is an instantiation of an

availability group that is hosted by a specific instance of SQL Server and maintains a local copy

of each availability database that belongs to the availability group. Each availability group

supports one primary replica and up to eight secondary replicas.

Supports an availability group listener for each availability group. An availability group listener

is a server name to which clients can connect in order to access a database in a primary or

secondary replica of an AlwaysOn availability group. Availability group listeners direct

incoming connections to the primary replica or to a read-only secondary replica. The listener

provides fast application failover after an availability group fails over.

Supports a flexible failover policy for greater control over availability-group failover.

Supports automatic page repair for protection against page corruption.

Supports encryption and compression, which provide a secure, high performing transport.

Active/Passive Active/Passive

Domain Controller

File Server



Pros and Cons Table Solution Hardware Cost Maintenance Real-Time License Cost Auto Failover

Log Shipping Low High Yes High No

Mirroring Low Low Yes High Yes

Failover Cluster (Active-Passive)

with SAN

High High Yes Low Yes

Failover Cluster (Active-Active)

with SAN

High High Yes High Yes

Failover Cluster (Active-Passive) with File Server

Medium High Yes Low Yes

Failover Cluster (Active-Active)

with File Server

Medium High Yes High Yes

AlwaysOn Availability Group

Low Low Yes High Yes

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Primary Replica

Secondary Replica – Read Only

Secondary Replica

Secondary Replica

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sql server high availability solutions (pros & cons)

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