January 24, 2013 at 4:12 am
Hi,
I hope I can find the answer here because I red tons of websites and documents and I just can't figure out WHY should I configure the Always On feature when the SQL-server is in a Failover Cluster already? Then what is the advantage? The only advantage I can think of is that Always On provides less downtime. Well in my situation it is not a problem if the server is a few seconds or max a few minutes down. And I can achieve that with Failover Cluster.
Hopefully someone can help me with this! Thanks!
January 24, 2013 at 8:25 am
There are several reasons why Availability Groups might be preferred to a Cluster. First, and most importantly, you're not sharing any resources at all. In a cluster if the disk goes, you're done. Because of that shared resource, it represents a single point of failure that some people can't put up with. Availability Groups allow you to have the functionality of a cluster without the shared resource. Additionally, you can just set up Availability Groups for select databases instead of an entire instance. That gives people a lot more flexibility. Also, you can use commodity and mismatched hardware if you so choose. You don't have to use approved vendors or hardware which is a cost savings. Because of the shared-nothing approach you can also place the servers in different physical locations so that you now have effectively a cluster with one machine in Paris and one in Taipei, thus improving your availability pretty radically. Last one that comes to mind immediately is the ability to have a read copy of your databases available so that you can run reports from a source other than your production server, but against the same data.
It's not for everyone, but there are lots and lots of reasons why it may serve someone better than a cluster.
"The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood"
- Theodore Roosevelt
Author of:
SQL Server Execution Plans
SQL Server Query Performance Tuning
January 24, 2013 at 8:47 am
I think Grant's last item (readable secondary) is one of the MOST commonly used reasons why people are hungry for SQL 2012 and it's "advanced mirroring" capabilities. The other one in my opinion is that you can have up to FOUR copies of the same data being sent to mirrors (2 sync and 2 async), EACH of which can be used for scale-out read-only activities if you desire.
The shared-nothing architecture is also very important. have had 3 separate incidents with various clients over the years where their disk subsystem completely went offline (TOTALLY NON-FUNCTIONAL) leaving their "high-availability windows/sql server cluster" FUBAR.
Best,
Kevin G. Boles
SQL Server Consultant
SQL MVP 2007-2012
TheSQLGuru on googles mail service
January 24, 2013 at 8:56 am
Oh, that's right Kevin. I completely forget that you get four mirrors of the active server. That one is big too. Nice catch.
I'm just a little surprised that this information isn't all over the place in pretty explicit terms.
"The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood"
- Theodore Roosevelt
Author of:
SQL Server Execution Plans
SQL Server Query Performance Tuning
January 24, 2013 at 9:05 am
Thanks!! Very nice forum 😀
I really appreciate the help and it's all clear now.
Till next time!
Edit: and specs are all over the place but really, I red many documents but I couldn't understand why it was better or different from the Failover Cluster. Sorry for that and thanks for the help again!
January 25, 2013 at 2:15 am
Gijs Laandrecht (1/24/2013)
I just can't figure out WHY should I configure the Always On feature when the SQL-server is in a Failover Cluster already?
The AO nodes must be part of a Windows Server Failover Cluster, this itself doesn't require any shared storage to be present. The AO feature utilises the cluster service to controol the failover of the AO groups between the group replicas.
See more on AO groups here[/url].
You can combine AO group nodes with Failover Cluster Instances but they get awfully complicated and require extra hardware, see my guide at this link[/url] for more info.
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