The Hidden Danger of Readable Secondaries in SQL Server AlwaysOn Availability Groups

A long time ago in a galaxy far, far away, I had to troubleshoot interesting performance issue in SQL Server. Suddenly, the CPU load on the server started to climb up. Nothing changed in terms of workload. The system still processed the same amount of requests. The execution plans of the critical queries stayed the same. Nevertheless, the CPU usage grew up slowly and steadily by a few percent per hour.

Eventually, we nailed it down. The problem occured in very busy OLTP system with very volatile data. We noticed that system performed much more I/O (logical and physical) than before. It was very strange, because nothing should have changed that day. Finally, we found that we have large number of deleted rows in the database that had not been cleaned up by ghost cleanup task.

It was the most impactful for the few “queue” tables in the system that handled hundreds of inserts and deletes per second. The tables had millions of data pages despite that they stored just a handful of rows. The tables supposed to be small and there were bunch of queries that performed Clustered Index Scan over them burning a lot of CPU resources to perform mullions of logical and physical reads.

The output of sys.dm_db_index_physical_stats looked similar to Figure 1 below. Very little record_count with very large number of page_count and version_ghost_record_count. The latter one indicates how many ghosted rows cannot be cleaned up due to row-versioning transactions in the system.

01. Output of sys.dm_db_index_physical_stats

We looked at the active transaction and discovered, that one of our readable secondaries ran terrible SELECT that cross-join several hundred-million row tables. Unfortunately, SQL Server allowed it to execute and this select was running for more than 12 hours deferring ghost and version store clean-up on primary node. The bottom line – non-optimized workload on the readable secondary node can affect your primary. Heh.

You can read why it happens in my recent blog post at Apress.com and in my upcoming book on locking, blocking and concurrency (should be published in October). However, I also wanted to mention it here and provide you the demo scripts and video that demonstrates it.

Remember about this overhead. Secondary nodes are great to scale your read workload but they are by no means “set it and forget about it” kind of solution.

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