# ASYNC_IO_COMPLETION

SQL Server has encountered 1 occurrence(s) of I/O requests taking longer than 15 seconds to complete on file [D:\xxx\yyy.ndf] in database [database_name] (8).  The OS file handle is 0x00000000000010E4.  The offset of the latest long I/O is: 0x00000482f60000

1. 同时有其他Application在进行大量的IO操作，导致用来存储备份数据的Disk特别忙，备份操作必须等待Disk IO完成。
2. 如果备份数据存储到remote server上，那么可能是network被其他application占用，导致用于传输备份数据的带宽变小。

Appendix：使用以下脚本查看备份操作的等待类型，已经完成的百分比，对于预估的剩余时间，只是个预估值，不可信。

select  r.session_id,
r.blocking_session_id as blocking,
r.wait_type as Current_Wait_Type,
r.wait_time/1000/60/60 as wait_h,
r.status,
r.command,
r.total_elapsed_time/1000/60/60 as total_h,
r.percent_complete,
r.estimated_completion_time/1000/60/60 as estimated_h
from sys.dm_exec_requests r
outer APPLY sys.dm_exec_sql_text(r.sql_handle) as st
where (r.wait_type<>'MISCELLANEOUS' or r.wait_type is null)
and r.session_id>50
and r.session_id<>@@spid

Normally this wait type can be seen in backup and restore transactions, and whenever you will see this wait type your backup/restore process will be in suspended state most of the time because the process is waiting to get IO resource to proceed its operation and it will wait till certain time period then moved in suspended state. In that case your process will take more than its usual time to complete or most of the time it will hung or will showing in executing state for unknown time duration.
This wait type occurs when a task is waiting for asynchronous I/Os to finish. This wait type is normally seen with few other wait types like BACKUPBUFFER,BUCKIO etc. This is clear indication of DISK IO issue.You can also get the Average disk queue length or current disk queue length value at the same time when you are getting this wait type.Compare both counters and if these counters have high value then you should look into your storage subsystem. Identify disk bottlenecks, by using Perfmon Counters, Profiler, sys.dm_io_virtual_file_stats and SHOWPLAN.
Any of the following reduces these waits: