Don’t Use Time Machine for Wireless Backups

Time Machine completed a verification of your backups on “<your server name>”. To improve reliability, Time Machine must create a new backup for you.

Time Machine Sparse Bundle Volume CorruptionThe error message above can be a common sight for people who uses Time Machine over Wi-Fi to safeguard their Mac’s data. If you’d ever saw that message, you’ll know how disappointing it felt when you had to lose months’ worth of Time Machine backup. However hard this may seem, it looks like there isn’t a way around it – you’ll need to switch to local drives for your Time Machine backups from now on.

On the other hand if you’re thinking to buy a Time Capsule or some other 3rd party NAS (Network Attached Storage) devices like Synology DiskStation or WD MyBook Live and use them primarily for using it with Time Machine, think again. You’re likely won’t be happy with your decision, especially if your home network is primarily on Wi-Fi. If the network ever gets interrupted while Time Machine is doing its thing, there’s a good chance of you corrupting the entire backup volume.

There have been many complaints about this in the interwebs and even in Apple’s own support forums and there isn’t any viable solution so far – except for not using Time Machine over Wi-Fi to backup laptops and stick to local USB drives instead. The problem is so deeply rooted inside Time Machine’s architecture that there isn’t much that Apple can do except to re-engineer their entire approach of network backups.

You see, Time Machine works by simulating a virtual disk over the network. It uses a Mac feature called sparse bundle disk image so that any filesystem can be made to look like it’s a native HFS+ file system. In turn, the NAS (or server) can use any filesystem that it sees fit as long as it can speak the AFP language to the client Mac. Time Machine needs this because it uses a special HFS+ feature called directory hard links at the heart of its operation.

Using a virtual disk through a network significantly increases the potential mishaps that can occur. First there’s the usual problems of your file’s contents not getting through and written to the remote disk’s platter. Then there are the problems of the virtual disk’s book-keeping information (filesystem data structure) that may not get written correctly in addition to the data itself. What’s worse is that if the file system information gets corrupted during a write operation, the entire disk’s integrity may be compromised.

This is made worse by wireless networks being typically unreliable.  Connections get dropped due to radio interference, cellular phones, or even nearby microwave ovens operating. So using a virtual disk through a wireless network significantly increases the ways that things can go wrong – and indeed they do which is reflected by that error message you’ve read at the top of this article.

So Time Machine over Wi-Fi is pretty similar to using a backup drive that often develops bad sectors or randomly shuts down and restarts itself at times. Your data could get lost mid-write – even worse, critical file system data structures could get damaged and render your entire backup volume useless.

I’ve found these suggestions to “fix” this problem and tried them but didn’t found them effective – even after I tried these “tips”, my backup volume still gets corrupted.

However if you already have a broken Time Machine disk image, you could try Garth Gillespie’s guide to fix your broken backup bundle. Just make sure that after you’ve re-gain access to your network backup volumes, you take out all the files that you need then discard that backup volume. If you keep using it, a few weeks down the line you’ll see that dreaded “… Time Machine must create a new backup for you…” error.

Then what options available for keeping your files safe?

  • Local disk. A dedicated hard drive connected through USB, FireWire, or Thunderbolt is still the best way to use Time Machine.
  • Arq backups – save your backups to the cloud, even use Amazon’s Glacier where you can get gigabytes of cloud storage for cheap. Caveat though that a Glacier restore can take a very long time.
  • Dropbox  – $10/month will get you 100GB of online storage.

Keep in mind that cloud-based backups may not be much help if you lose your Mac. You can’t just buy a new Mac then have all of your applications and data restored in one night like the way you can use Migration Assistant to restore data from a Time Machine drive. Chances are you’ll need to install your apps manually and then re-download all your files from your cloud storage provider of choice. But cloud storage can be useful to keep your important files in major disasters like  the floods caused by Hurricane Sandy (which could ruin your Time Machine drive along with your Mac).

That’s all for now, take care and keep backed up!

Update on 27 December 2013: I’ve formally report this to Apple as rdar://15278629.


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