Today I started with upgrades to Service Pack 3 for Exchange 2010. The download is around 549MB.
Because all my sites run DAG’s I started with the smallest one. Starting with the Passive server I ran the upgrade, the install runs through a list of checks. A new feature that needs to be installed is the RPC over HTTP Proxy. If this is not installed the checks will fail.
The upgrade takes approximately 45 min, time might be different on another set of hardware. SP2 for Exchange is removed and the SP3 installed. So far no issues experienced after the upgrade.
Once the Passive server was completed you need to move your database to the upgraded server and then run the upgrade on the Active one. You should also lockdown your DAG so that the DB does not automatically failover.
Do take note that you still cannot run in coexistence with Exchange 2013 until CU1 is released for Exchange 2010 SP3.
Here is the URL to download SP3:
http://www.microsoft.com/en-us/download/details.aspx?id=36768
Also make sure you read the System Requirements prior to upgrading:
http://technet.microsoft.com/en-us/library/aa996719(EXCHG.141).aspx
Hope it helps.
