What does SQL Server's BACKUPIO wait type mean?

Question

I'm using Sql Server 2008 ("R1"), with some maintenance plans that back up my databases to a network share. Some of my backup jobs show long waits of type "BACKUPIO".

Of course it seems like this is an I/O subsystem limitation, but I'm skeptical. Perfmon stats for I/O on the production (source) server are well within normal trends for that server. The destination server shows a sustained 7MB/s write rate, which seems incredibly low, even for a slow disk. The network link is gigabit ethernet and nowhere near saturated.

The few docs I've turned up about BACKUPIO indicate that it's not specifically a wait on I/O, surprisingly enough. This MSFT doc says it's abnormal unless you're using a tape drive, which I'm not. But it doesn't say (or I don't understand) exactly what resource is missing. http://www.docstoc.com/docs/24580659/Performance-Tuning-in-SQL-Server-2005

And this piece says it's not related to I/O performance at all. http://www.informit.com/articles/article.aspx?p=686168&seqNum=5 "Note that BACKUPIO and IO_AUDIT_MUTEX are not related to IO performance."

Anyway, does anyone know what BACKUPIO actually means and/or what I can do to diagnose or eliminate it?

Update: I'm showing 100%+ Disk Time on the destination disk, which sounds to me like a disk bottleneck rather than network. It is a single spindle at 7200RPM, but I'm still skeptical that 7MB/s is the best the disk can do.

Solution

Backup over a network share will show all network latency times as 'BACKUPIO' wait times. Your problem is unlikely in the SQL stack, you'll have to check your network stack, and more specifically the SMB stack. Windows 7, Windows Server 2008 and Wk28 R2 came with significant changes in the network stack, and some are known to cause slow trasnfer rates on certain high-bandwith/high-latency scenarios, specially when the new Receive Window auto-tunning screws up something.

Fore more details, see Differences in network performance between Windows Vista/Windows Server 2008 and Windows XP/Windows Server 2003