My Windows 7 desktop is set up to back up to a Drobo B800i (over iSCSI) every night at 04:00, using Windows Backup. Even though it only uses about 700 GB of its 2 TB mirror, and only backs up a small fraction of that, backup jobs routinely took 15 hours or more. It could have copied the entire disk in half that time!
I set about hunting for a solution. One suggestion that turned up repeatedly in Google searches was to turn off the Background Intelligent Transfer Service. BITS is basically a download manager designed to only run when there is little or no other network traffic; among other thing, it is used by Windows Update to download patches. I couldn’t understand how this could help, but I had no better ideas and nothing to lose, so I stopped BITS. The next backup job completed in 45 minutes.
Patch Tuesday came along, and I rebooted the computer. Since I had only stopped BITS and not disabled it, it started again when the machine booted. Backup jobs slowed down again. This time, I disabled BITS, and I was back to sub-hour backups.
This makes absolutely no sense. BITS wasn’t even downloading anything; as far as I know, the only program or service I have running that actually uses it is Windows Update. BITS was slowing down backups just by being there. I don’t remember having this issue when I ran backups to an eSATA drive, so there must be some network-related interaction between BITS and iSCSI, but I have no idea what.