Things have not been going well in my quest for Time Machine backup perfection.
When the 7.3 AirPort firmware came out, I was really excited to hear that the ability to use an Airport Extreme Base Station (AEBS) plus an external hard drive as a Time Machine backup volume had returned. In my case, I was using a Western Digital MyBook as the external drive.
For a while it seemed to be working fine, but periodically the Time Machine volume would disappear from the network, causing all of its Time Machine clients to fail. Curiously, when this happened, the AEBS was still perfectly functional as a base station -- the drive had just vanished. Rebooting the AEBS would resurrect the Time Machine volume... until eventually it disappeared again.
When I read yesterday that this setup is officially unsupported, I was annoyed. This feature was available, pulled, available again, and now unsupported.
Backup is not something I can afford to screw around with. If there's any possible point of failure, it's unacceptable. So, I decided to drop this "unsupported" configuration and picked up a 1 TB Time Capsule. The situation has not improved.
Because there are so many variables involved with WiFi performance and reception, I'll spare you the details of all the wireless attempts and skip straight to the part where I have the Time Capsule directly connected via ethernet cable to the MacBook Air's USB-ethernet adapter, with wireless completely turned off on both devices.
The most I have been able to backup in this configuration is about 2 GB out of 40 GB before the backup volume in the Time Capsule simply stops responding. The Time Machine progress bar stops updating. At this point, the Time Capsule also has vanished from Airport Utility and Rescan does not bring it back. It is like it has simply ceased to exist. If you try to access the Time Machine volume from the Finder now, it beachballs the Finder so badly that you can't even restart the Mac without pressing and holding the power button. The Time Capsule also requires a restart at this point.
If, on the other hand, you just let it sit for a few hours, it eventually times out with either an error message that Time Machine could not write to the backup volume, or a "Server Volume Connection Interrupted" dialog. How a WIRED connection can be interrupted, I'm not sure. I did try a different ethernet cable, to no avail. I tried this maybe 10 times and was unable to get a single successful initial backup.
So, as far as I can tell, this particular Time Capsule is completely defective. Or is there a deeper problem here?
I started a support forum thread and so far have one (albeit just one) person claiming to have the exact same problem.
But the distinct similarity in failures between the AEBS+HD configuration and the Time Capsule has me nervous that maybe there is a systemic problem in remote Time Machine backup. Does it just simply not work?
I'll probably take this Time Capsule in to get it exchanged, and see if anything gets better. If the replacement has the same problem, then I'll be really worried.
I got the Time Capsule exchanged and experienced the same problem, so it was clearly time to get creative.
Checking the console log of the MacBook Air gave me a clue which lead me down a path to what I believe is the culprit: a third-party process called "nmnetmgrd"
This daemon got installed alongside a program called "SierraWatcher", which is used to drive AT&T's USB Cellular Modems. nmnetmgrd appears to be part of a product called launch2net by NovaMedia.
Anyway, long story short, I saw this in the console log:
Followed by:
Not more than 2-3 seconds after this, there is cascading failure -- the kernel loses its connection the AFP volume on the Time Capsule. backupd thinks the backup volume has been ejected -- basically everything implodes.
It's hard to say for sure, but it sure looks like this daemon literally causes the network interface to go down and back up, and backupd sure doesn't care for that.
I tracked down the LaunchDaemon responsible for starting nmnetmgrd and deleted it. (I don't even have the modem plugged in, so who knows why it's running in the first place.)
And now I'm 15 GB into my 40 GB backup; much, much further than I've ever gotten before. I really think it will complete normally this time.
This was not an easy one to track down. I've reported it to both Apple and NovaMedia for further debugging, but if you're having Time Capsule trouble, and you have a cellular modem, see if you have this daemon running, or anything else that might fiddle with your network interfaces behind the scenes.