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

CPU Cores

Windows

SQL Server

Cache

FC HBA

Fibre Channel Switch

Storage Controller

Storage Controller Cache

Disk

LUN

Republished from http://www.sqlserver-dba.com.