Using The AlwaysOn Feature of SQL Server 2012
Prior to SQL Server 2012, SQL Server had several high availability and disaster recovery solutions for an enterprise’s mission critical databases such as failover
clustering, database mirroring, log shipping or combinations of these. Each solution typically has a major limitation, in the case of
failover clustering for example, its configuration is very tedious and
complex and you arguably have single shared storage or single point of failure. Database mirroring is relatively easy to configure in
comparison with failover clustering, but you can have only one database in a
single mirroring setup and you cannot read from the mirrored database. Log shipping
does not provide automatic failover (higher availability) though it be used
for disaster recovery with some expected data loss.
SQL Server 2012 (Code name SQL Server Denali) introduces a new feature called
AlwaysOn which combines the best of failover clustering and database mirroring and
overcomes major of the limitations imposed in failover clustering or a database
note, in this article instance, server, replica and node refer to same thing
and will be used interchangeably.
AlwaysOn is a new High Availability (HA) and Disaster Recovery (DR) solution in SQL
Server 2012 which improves high availability and protects data of your mission
critical applications. AlwaysOn lets you utilize your current hardware
and provides a flexible and simplified configuration, deployment and
management experience. AlwaysOn is common name for two high availability and
disaster recovery solutions:
Failover Cluster Instance (FCI)
This is an enhancement to the existing SQL
Server failover clustering (which is based on Windows Server Failover Cluster
(WSFC)) which provides higher availability of SQL Server instance after
failover. Some of the enhancements in AlwaysOn Failover Cluster Instance over the existing
SQL Server failover clustering are:
improved site protection you can now set up multisite failover clustering
can now define flexible failover policy to better control instance failover
now have better and improved diagnostics capabilities out of the box
Availability Group (AG)
This is a completely new HA/DR feature and combines
best of failover clustering and database mirroring. It allows you to create a
group of databases which failover together as a unit from one replica/instance
of SQL Server to another replica/instance of SQL Server in the same
availability group. Each availability group that we create, allows you to
create one (and only) availability group listener which is nothing but a Virtual
Network Name (VNN) to be used by clients to connect to the availability group.
availability group is based on Windows Server Failover Cluster (WSFC) and hence
you need to install the failover clustering feature on each server/replica and
create a failover cluster adding all these server/replicas before you can
start enabling/creating the availability group.
You can have numerous
availability groups on a single instance but a database from that instance can only
be part of one availability group thus you cannot also create a database
mirroring session for databases already participating in an availability group.
Availability Groups Compared To Traditional SQL Server Failover Clustering
a typical SQL Server failover cluster (at the instance level), you will have two
nodes/instances (Active-Passive or Active-Active) connected to shared storage
drives. Though SQL Server failover clustering has been good and is used in many
deployments for higher availability and disaster recovery, it has several
limitations and pain points, such as:
- The process of setting up SQL Server failover clustering is
tedious and complex – there are some 30-40 steps that you have to
perform missing any of those steps can result in hours of additional work. This is why setting up SQL Server failover clustering is only recommended to be performed by highly
the nodes are connected to a shared storage drive; though these drives might
have their own failover mechanisms we still can have a
single point of failure.
of the nodes is idle all the time in case of Active-Passive cluster
(recommended) and hence resources are underutilized. Though you have an
Active-Active failover cluster this is not recommended as after failover
one node will have double the load from both the cluster setup/applications.
infrastructure and configuration of each node should be exactly same as other
nodes and mimic each other.
cannot distribute or load balance your read-write load from read only load on
AlwaysOn availability group is superior to SQL Server failover clustering because the
configuration, deployment and management is relatively simple and all the
nodes/replicas will a copy of the databases and hence there is no shared
storage or a single point of failure. You can have readable secondary and hence
you can route your read-only load to a secondary replica and the read-write load to
primary replica and hence have better
utilization of your hardware resources.
How Availability Group
differs from database mirroring
mirroring (at database level) can be set up in either synchronous mode or
asynchronous mode but not both in a single mirroring setup.
Synchronous Commit mode (high-safety) : The transaction logs are hardened at
both the principal server as well as at the mirror server before commit
acknowledgement is returned to the client; it may introduce some latency but
ensures no data loss after failover. In this mode you can also set automatic
failover and for that you need another instance which will work as a witness and
peforms the job of role switching.
Asynchronous Commit mode (high performance) : The principal server hardens the
transaction log at the principal server and returns the commit acknowledgement to
client without waiting for transaction log hardening acknowledgement to be received
from the mirror server. Transaction log hardening at the mirror server happens in a
all sound good solutions but like SQL Server failover clustering, it has also several limitations:
can have only one database in a single mirroring session/setup, though you
can define multiple mirroring sessions/setups (one for each database) but it is not possible to have a group of databases failover together.
on mirror server are always in recovery mode and hence you cannot read from a
mirrored database (though you can create a database snapshot and read from it
but but would only reflect data till the particular point in time when it was
cannot load balance your read-write requests on one server and read-only on
can have only one mirror server; you cannot have one for higher availability (synchronous
commit mode) and one for disaster recovery (asynchronous commit mode) in one single
mirroring session, although you can combine it with log shipping for disaster
availability group is recommended over database mirroring as
this overcomes several limitations imposed in database mirroring, for example
with an AlwaysOn availability group:
can have multiple mirrored instance/nodes/replicas (up to four secondaries
apart from one primary replica) with a combination of synchronous commit mode
and asynchronous commit mode both at the same time. The replica set up in synchronous
commit mode can be used for higher availability (or for automatic failover) and
the replica set up in asynchronous commit mode can be used for disaster
can combine two or more database together and failover them as a unit, you
don’t need to do it for each database separately as you were doing in case of
can offload the read-only load from the primary replica to the secondary by configuring
the secondary as readable. In this way you can have better utilization of
secondary replica’s hardware resources.
can also offload backup operations from the primary replica to the
secondary replica and hence have less workload/IO on the primary replica and better
utilization of the secondary replica’s hardware.
Preparing the Environments for an AlwaysOn Availability Group
An AlwaysOn availability group is based on a Windows Server Failover
Cluster (Windows Server 2008 and later versions) and hence before you can start
creating an AlwaysOn Availability Group you need to first setup/install failover
clustering on each node/replica and then create a failover cluster with
all the nodes/replicas joined to it. These are steps that you need to perform:
SQL Server 2012 on each node/replica/server
the failover clustering feature on each node/replica/server
a failover cluster and let all nodes/replicas/servers join to the cluster
AlwaysOn availability group can have one primary replica/node/server and up to
four secondary replicas. Out of these four, two of them can be set to have
synchronous commit mode and one of it can have setting for automatic failover.