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/PassiveActive/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

SQL Server High Availability Solutions (Pros & Cons)

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