|
SQL Server Ver |
Secondaries |
Synchronous |
Asynchronous |
|
2012 |
4 |
2 |
1 |
|
2014 |
8 |
2 |
6 |
|
2016 |
8 |
2 |
6 |
|
2017 |
8 |
2 |
6 |
|
2019 |
8 |
5 |
3 |
|
2022 |
8 |
5 |
3 |
Always on Availability group is one of the high availability and disaster recovery solution in SQL Server. Always on Feature have introduced in SQL Server 2012 Version.
SQL Server 2012
2012 Limits: 1 primary replica, 4 secondary replicas, 2 of
the replicas can be synchronous.
Only two automatic
failovers will take place in
In SQL Server 2012.
Three synchronous
mode replicas available in SQL server
2012.
SQL Server 2014
2014 Limits: 1 primary replica, 8 secondary replicas, 2
of the replicas can be synchronous.
Two automatic
failovers will take place in
SQL server 2014.
Three synchronous
replicas are there in SQL server 2014.
Read intent was
introduced in SQL Server 2014.
Read intent routing
command
ALTER AVAILABILITY GROUP [AG2]
MODIFY REPLICA ON
N’WIN2012R2-VM1\GeoPITS_PR’
WITH
(PRIMARY_ROLE(READ_ONLY_ROUTING_LIST=(‘WIN2012R2-VM2\GeoPITS_SR1’,’WIN2012R2-VM3\GeoPITS_SR2',’WIN2012R2-VM1\GeoPITS_PR’)));
In this read intent
routing the read request will direct based on the order, which you have given
in above command. Based on the above routing list all read request will direct
to the ‘WIN2012R2-VM2\GeoPITS_SR1’ which is give in first in above routing list
in case if these secondary replica is next available the read requests will
direst to next replica which is second(WIN2012R2-VM3\GeoPITS_SR2) in routing
list. In both first and second secondary replicas not available read requests
will go next secondary replica based on routing list.
SQL server 2016
2016 Limits: 1 primary replica, 8 secondary replicas,
3 of the replicas can be synchronous.
The Enhancements of
ALWAYS ON in SQL server 2016 given below
-> Round-robin
Load-balanced readable secondary’s.
-> Direct
seeding of new DB replicas.
-> supports
Azure integration.
-> Always on
Supports TDE.
->Support for
Distributed Transactions (DTC).
-> SQL Server
Standard Edition support always on Availability group(Basic Availability
Group).
->‘ALWAYSON’
renamed as ‘ALWAYS ON’.
Round-robin load
balancing in readable secondary’s:
In SQL server 2016
round-robin load balancing concept was introduced. Before SQL server 2016 there
is no round-robin load balancing. Main use of the round-robin read requests
will distributes equality to all secondary replicas as show in below figure.
Prior of SQL server 2016 the read request will connect to the secondary replica which will responds quickly for listener.
Automatic Seeding:
Automatic seeding
is new enhancement of alwayson in SQL server 2016.
SQL Server performs
a full database backup using Microsoft SQL Server Virtual Device Interface
(VDI) full database.
This VDI database
backup is streamed through the network to all available secondary replicas.
Secondary replica
restores this streamed backup.
Once the database
restoration is complete, it is added into the availability group.
GRANT CREATE ANY
DATABASE permission needed.
Supports Azure:
In SQL Server 2016
Always on support AZURE for Secondary replica.
Recommended is
secondary replica in Asynchronous mode.
Always on Supports
TDE:
Now always on
supports for Encrypted Databases.
However
authentication requires when adding encrypted database to Availability Group.
Support for Distributed
Transactions (DTC)
Always on supports
the Distributed Transaction in SQL server 2016, now we register a resource
manager per availability database. The resource manager works with DTC services
to track distributed Transactions. Because of this now we can guarantee for
integrity of a distributed Transaction.
To use DTC with
Always on we require Windows Server 2012 with KB3090973.
We need to create
Availability Group with the WITH DTC_SUPPORT =PER_DB.
Database
level health detection failover:
In SQL server 2016
always on one of the new enhancement is DATABSE level Health detection
failover was introduced. in prior SQL server 2016 only Availability group
failover takes place when the instance fails. From SQL server 2016 availability
group failover takes place even on database fails or not available for long
time.
We can enable this
feature using below commands.
ALTER AVAILABILITY
GROUP [AG1] SET (DB_FAILOVER = ON);
ALTER AVAILABILITY
GROUP [AG2] SET (DB_FAILOVER = OFF);
We can see how to
enable Database level Health detection for Availability group in below pictures
for SQL server 2016 when creating new group.
SQL Server 2017
Read-scale Availability Group in SQL server 2017 and later we can create the availability group without windows failover cluster.
But it will not
provides the high availability.
Basic Availability
Group
Basic Availability
group feature was introduced in SQL server 2016, which was supports SQL Server
2016 standard edition. Basic availability behaves like mirroring feature.
In below picture
shows how to enable Basic availability in SQL Server 2016 and higher SQL server
Standard edition.
Limitations
of Basic Availability Group:
Secondary replica
not allows read operations
No backup in
secondary replica
Basic availability
group supports only in standard editions in 2016 and later
Basic availability
group supports failover environment for a single database.
In Basic
availability group we have only on secondary replica.
·
Cross database transactions are
now supported among all databases that are part of an Always On
Availability Group, including databases that are part of same instance.
· New Availability Groups functionality includes read-scale support without a cluster, Minimum Replica Commit Availability Groups setting, and Windows-Linux cross-OS migrations and testing. (CTP 1.3)
SQL Server 2019
2019 Limits: 1 primary replica, 8 secondary
replicas, 5 of the replicas can be synchronous (one primary and up to four secondary
replicas with automatic failover between these replicas).
- Up to 5 synchronous commits to replicas in a SQL server 2019
availability group. This means that you can have a 4 + 1 setup, where
there are 1 primary server and 4 synchronous commit replicas. Since all
databases are in synchronous availability group that forms a quorum, you
can safely setup automatic failover with zero-data-loss.
- Availability Groups now supports Kubernetes as an orchestration
layer in the 2019 edition
- You can now connect to any available replicas and would be automatically redirected to available replicas based on a read or write request. This would obviously depend on the configuration that you need to set up at the time of creation.
SQL
Server 2016 is making some significant improvements to the Always On
Availability Groups set of features. There are a number of features, such as:
- Round-robin
load balancing in readable secondaries
- Increased
number of auto-failover targets
- Enhanced
log replication throughput and redo speed
- Support
for group-managed service accounts
- Support
for Distributed Transactions (DTC)
- Basic
HA in Standard edition
- Direct
seeding of new database replicas
|
SQL
Server 2017 – Availability Groups Features |
SQL
Server 2019 Availability Groups Features |
|
|
Standard edition support here.
With limited
2019 Limits: 1 primary replica, 8 secondary
replicas, 5 of the replicas can be synchronous (one primary and up to four
secondary replicas with automatic failover between these replicas).10 AG Groups can be
setup
SQL Server 2022 Availability
|
New feature or update |
Details |
|
Link to Azure SQL Managed Instance |
Connect your SQL Server instance to Azure
SQL Managed Instance. See Link
feature for Azure SQL Managed Instance. |
|
Contained availability group |
Create an Always On availability group that: |
|
Distributed availability group |
- Now using multiple TCP connections for
better network bandwidth utilization across a remote link with long tcp
latencies. |
|
Improved backup metadata |
|
No comments:
Post a Comment