SQL Server Books online defines Async_io_completion as “Occurs when a task is waiting for I/Os to finish “
Mainly a symptom of an IO bottleneck. Cross check by identifying other wait stat indicators such as: IO_COMPLETION, WRITELOG,PAGEIOLATCH_X. The appearance of at least one of these along with ASYNC_IO_COMPLETION – near the top of the wait stats indicates a serious IO bottleneck.
Any SQL Server task waits for IO completion – this is completely normal part of SQL Server processing. But when the wait for IO completion is slow there is a problem.
A few issues to consider and approached to minimizing Async_io_completion.
1) Maintain Random access \ Sequential access files on different drives .
2) Are you experiencing IO delay messages in the error logs. This is a strong indicator, as it reports on IO delays – outside of the SQL Server process space.
the article: IO Requests taking longer than 15 seconds to complete gives an in depth explanation
3) Analyse the SQL Server Performance Stack
Fibre Channel Switch
Storage Controller Cache
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