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Description:
Just as the Books Online description below. There is one offline checkpoint thread in 2014, and multiple threads in 2016 onward, that process operations around checkpoint file pairs (a data and delta file pair). In this case, one of those threads is waiting for a log read operation to complete. You can read more about CFP processing in Books Online here.
(Books Online description: “Occurs when offline checkpoint is waiting for a log read IO to complete.”)
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Added in SQL Server version:
2014
Removed in SQL Server version:
N/A
Extended Events wait_type value:
Maps to the XTP_OFFLINE_CKPT_LOG_IO map_value in sys.dm_xe_map_values.
The map_key value in sys.dm_xe_map_values is 925 in 2014 RTM. After 2014 RTM, you must check the DMV to get the latest value as some map_key values have changed in later builds.
Other information:
I have not seen this wait to be a noticeable contention point.
Known occurrences in SQL Server (list number matches call stack list):
- Waiting for the next log block to be read from disk.
Abbreviated call stacks (list number matches known occurrences list):
- SOS_Task::PostWait+9e
EventInternal<SuspendQueueSLock>::Wait+1fb
OfflineCheckpointWorker::GetNextLogBlock+1d1
OfflineCheckpointWorker::DoWorkInternal+f7
OfflineCheckpointWorker::DoWork+3aa
OfflineCheckpointWorker::WorkLoop+40f
SOS_Task::Param::Execute+21e
SOS_Scheduler::RunTask+a8