Recently at SQL Saturday Philadelphia, we started discussing failover as it relates to mirroring and Always On Availability Groups. Specifically, we were wondering what would happen if you had a relatively busy readable secondary replica (which would have a lot of pages in the buffer pool on the secondary instance) and if those pages would be flushed from cache or anything like that. So I reached out to the product group and Kevin Farlee from Microsoft was extremely helpful:
“Pages in the buffer pool are still valid, as they are updated by the redo thread. By the time the secondary has transitioned to primary, all pending updates will have been applied, so pages in the bufferpool will all have correct contents. Note that the set of pages in bufferpool may be different between primary and secondary so you may have some cache warm up to do.”
So in a nutshell, if you have a busy readable secondary your cache won’t take much time to warmup, since the hot pages are already there.
