(Republishing, or using this info in a commercial product/website, is prohibited without permission. All other uses are permitted. If in doubt, please ask.)
This wait type is when a thread is waiting for access to the list of dirty pages during parallel redo or a read query on an Availability Group readable secondary replica.
(Books Online description: N/A)
Questions/comments on this wait type? Click here to send Paul an email, especially if you have any information to add to this topic.
Added in SQL Server version:
Removed in SQL Server version:
Extended Events wait_type value:
The map_key value in sys.dm_xe_map_values is 1128 in 2016 RTM. After 2016 RTM, you must check the DMV to get the latest value as some map_key values have changed in later builds.
This wait type can become prevalent on a replica where there is a lot of read activity on the readable secondary and there is a heavy update workload on the primary replica, and the reads and updates are for the same table(s). The redo and read threads are contending for access to the same page(s).
This wait type is very similar to the DIRTY_PAGE_TABLE_LOCK wait, but for that wait type the read and update activity does not have to be on the same page(s).
Anecdotal evidence is that using indirect checkpoints might help to alleviate this.
On some servers running builds before SQL Server 2016 SP1 CU2, this wait type is the symptom of a crippling bottleneck during redo of transaction log records on an AG replica.
If you are experiencing this wait type as one of the highest waits on your AG secondaries, you can disable parallel redo by turning on trace flag 3459 using DBCC TRACEON (3459, -1), or adding it as a startup trace flag using the Configuration Manager. The trace flag does not come into effect until the instance is restarted.
You can read more about parallel redo in this blog post.
Known occurrences in SQL Server (list number matches call stack list):
Abbreviated call stacks (list number matches known occurrences list):