SQL Server Books online defines IO_COMPLETION as “Occurs while waiting for I/O operations to complete. This wait type generally represents non-data page I/Os. Data page I/O completion waits appear as PAGEIOLATCH_* waits”

As with  ASYNC_IO_COMPLETION  it is a symptom of an  IO bottleneck  . Cross check by identifying other wait stat indicators such as : ASYNC_IO_COMPLETION  ,WRITELOG,PAGEIOLATCH_X. The appearance of at least one of these along with IO_COMPLETION – near the top of the wait stats indicates a serious IO bottleneck

It’s useful to differentiate between WRITELOG – which occurs when waiting for a log flush to occur – and IO_COMPLETION. IO_COMPLETION reports on non-data pages and essentially is a SQL Server thread awaiting  the kernel to return the IO request status.

The complexity in diagnosing the source of the problem , is to understand the OS , SQL server , IO configurations which could include drive profile, controllers etc.Tools such as SQL Profiler & Perfmon assist

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 minimising IO_COMPLETION.

1)Maintain Random access \ Sequential access files  on different drives . Create an io profile of the database server and balance drive appropriately .

2)Commit some analysis of poorly running queries. Is excessive table and index scanning occurring ? This could be causing many round-trips to  the disk

 3)End to end analysis – for example , is IO being throttled ?