Failover

Question

Hello

Can somebody tell me what happens is there a diffrence in a clustered or in a alwayson AG`s environment. What happens
in the following scenarios....<o:p></o:p>

When a automatic failover occurs..<o:p></o:p>

1. When a transaction is being processed? <o:p></o:p>

2. If a application is writing information into a table? <o:p></o:p>

Is there data lost?

Thank you

<o:p></o:p>

Answer 1

There is a lot of difference between those two...

1) An FCI (clustered instance) writes a transaction to a shared disc. With this write operation it is already highly available.

2) An AG on the other hand writes the data to a local disc and SQL Server transfers the data to the secondary where it is written to another local disc. So there is an overhead on transaction processing in AGs

3) With an AG the SQL Server Service is already running on the secondary. This leads to faster recovery on failover. It also enables you to have Active/Active configurations using multiple AGs.

4) Data in user databases is not lost in either case. One thing is though that master and MSDB data (e.g. Agent Jobs, Logins, etc.) are not in an AG, so if you use AGs those things will not failover automatically.

Answer 2

Thanks For your answer!!

Answer 3

I would just like to expand on Prince's answer a tiny bit and say that you can also have Availability Groups configured to run on Failover Clustered Instances -they don't have to be local standalone installations. However this configuration is slightly more advanced, requires a few additional considerations and configured carefully. However the configuration is possible and is something to remember.

An easy way to think of the two technologies is that FCI provide HA abstraction at the SQL instance level, AGs provide HA abstraction (and read only scalability) at the one to many databases level.

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Regards,
Mark Broadbent.

Contact me through (twitter|blog|SQLCloud)

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