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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)
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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 1127 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. The read queries do not have to be on the same tables that are being updated, as the redo and read threads just need access to the list of dirty pages itself.
This wait type is very similar to the DPT_ENTRY_LOCK wait, but that wait is when the redo and reads are for the same page or set of pages.
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.
There is also a fix planned for a future build of SQL Server 2016 and SQL Server 2017 that will fix this issue.
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):