Home » Beginner »sqlserverpedia-syndication »SSC »Tech and Learning » Currently Reading:

Clustering Concepts for the Common Man

January 29, 2013 Beginner, sqlserverpedia-syndication, SSC, Tech and Learning 2 Comments

SQL Server clusters are widely used, and widely misunderstood, which makes people like our good friend (and clustering MVP) Allan Hirt cry bitter tears. So here are a few notes for you and yours.

Cluster IS NOT Disaster Recovery

A cluster does not protect against disaster. Unless it’s a very, very, very limited scope disaster (that scope being, the hardware of the one server running SQL, not including the disks). A cluster is considered a High Availability solution.

Why: A typical cluster lives in a single physical area (typically, the server room). And, a cluster shares disks. Anything that goes wrong with the SAN, the power, the building, or the network, goes wrong for the ENTIRE cluster.

Failover IS NOT Ultimate Solution

Failing over the cluster should not be your first reaction to any given problem. Letting your DBA troubleshoot the issue should be the first reaction.

Why:  If there’s a problem, you need to know the source of the problem, so that it doesn’t happen again. Not to mention, restarting the service and/or failing over the cluster can exacerbate the problem AND wipe out important troubleshooting data. It’s lose-lose!

Failover IS NOT A Freebie

A failover is (potentially) quicker than a restart of a single server, sure. But you still lose all your connections, all open transactions are rolled back (and not automatically restarted), and you still have to wait for the SQL Server service to spin up.

Why: SQL Server itself doesn’t know the difference between failing over, and simply being restarted on a single instance. It’s the same thing, in SQL’s eyes.

Second Node IS NOT Magical

This one is a two-fer.  First, understand that if your secondary node isn’t functional for whatever reason, you don’t actually have a failover cluster. Plenty of shops ignore issues on their secondary node for years, just assuming that they’ll never need it.

Second, moving the services and disks over to the second node rarely gets you anything that a service restart wouldn’t.  That second node isn’t magically immune to deadlocks, long transactions, nor memory and disk pressure. Even in cases where SQL Server isn’t responding, or is deeply broken in some way, a service or server restart is at least as good as a failover.

Why Do We Even Have This Cluster?

I don’t know. Maybe someone high up fell in love with the word “cluster”, the way they’re falling in love with “The Cloud” right now. Maybe you have a legitimate need for, say, rolling upgrades with minimal downtime.

Regardless, you have it. Understand the concepts, and understand the few cases in which failing over is actually beneficial. Here, have an Introduction to Clusters, with diagrams. Need more? Check out Allan’s 4-day training classes.

Happy days,
Jen McCown

P.S. I’m on a boat this week, and so I’ll be out of touch for a while. The out-of-touchness includes our awesome DBAs@Midnight webshow, which next airs on  February 8 at 11pm Central Time. Catch up on back episodes at http://www.MidnightDBA.com

P.P.S  More resources, from SQL Server BOL:

Comment on this Article:

Minion Reindex by MidnightDBA is here!


Excellent Index Maintenance

Download Minion Reindex, log feature requests, read documentation, and sign up for the newsletter at MidnightSQL.com/Minion!


MidnightSQL Consulting

Need help? Got an emergency? Write us at Support@MidnightDBA.com!

We can schedule time to help with your backup/restore issues, high availability and disaster recovery setup, performance problems, and a great deal more. Very often, we're even available on the moment for downtime issues and emergencies.

For more information about MidnightSQL consulting, email us or check out www.MidnightSQL.com. Happy days!

Where are We?

November 3-7: PASS Summit, Seattle, WA

PASS Summit: Jen is presenting How to Interview a DBA: A Panel Debate on Thursday 11/6 1:30pm, room 401 (along with Adam Machanic, Sean McCown, Bob Pusateri, and Michelle Ufford).

PASS Summit: Sean is presenting Performance Tuning Your Backups on Wednesday 11/5 3:00pm, room 602-604.

December 11: Presenting "Powershell Cmdlets.." at Alaska SQL Server User Group

January 30: "Become an Enterprise DBA" precon at Austin SQL Saturday

Blog Posts by Category


How to Eat Pop-tarts
Watch DBAs@Midnight live on Fridays,m 11pm Central time

The best database career advice you’ve never heard!


The DBA Roadmap Seminar is 7 MP3 tracks (over 5 hours!) of insider guidance on your database career. We'll teach you how and what to study as a DBA, weigh in on controversial resume debates, teach you to recognize a worthy recruiter, and discuss the new professionalism of interviews. Also some bonus materials, PDF companion guides, and really spiffy intro music!

Once your $99 PayPal payment is submitted, you'll get the download link in e-mail! (Download is a 370Mb ZIP file.)

Become a DBA. Become a BETTER DBA. Use the Roadmap.

Visit www.DBARoadmap.com for info, forums, and more!

Add to Cart View Cart

Cunningham’s Law

"The best way to get the right answer on the Internet is not to ask a question, it's to post the wrong answer."
Relevant: http://xkcd.com/386/