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I'm looking to set up SQL Server 2012 installation with an Always On Availability Group, where the 'passive' replica will be hosted at another site on the WAN and using synchronous data commit - the idea being that we will have a hot standby with no loss of data in the event of a failure at our primary site.

One potential problem that I foresee is that the secondary site has slower storage than our primary site. I don't care about that in the event of a failure, we can live with slow speeds for a period of time until the primary site is restored. My worry is that, because we are using synchronous commit, that the slower disk speed at the secondary site will affect performance at the primary site during normal operation.

Is this a valid concern, or is it likely that the slower speed will be offset by, for example, the disk not having much read activity in comparison to the primary site?

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The writes on your primary will only be as fast as the writes on your secondary. I would probably at least consider asynchronous commit in that case (and perhaps have a synchronous replica more locally - such that data loss is only really a concern if the whole site goes down instead of a single server). – Aaron Bertrand Jul 29 '13 at 15:07
But the secondary has no read load on the discs - it may well be that the slower discs are in fact faster than the fast discs because they carry a lot less load - depends heavily on the operations done on the server. – TomTom Jul 29 '13 at 15:09
@TomTom in a lot of systems there is very little read load on the disk, because ideally all of the data is in memory, and almost all of the load is write. – Aaron Bertrand Jul 29 '13 at 15:23
I don't dispute that - just saying that this is the case. On other systems you have IO problems. – TomTom Jul 29 '13 at 15:30
Good points. I can't really create a synchronous replica locally though as there just isn't the disk space for it. And zero data loss is important so I don't really want to have just asynchronous replication. I guess it's the usual situation - Speed/Cost/Resilience: choose any two! :-) – paulH Jul 29 '13 at 15:38

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