My take on the “unsupported network volumes” Time Machine hack

UPDATE2: Snow Lep really broke TM backups for me – I think the extra complication I had over everyone else is that my server was a ways back on 10.3.9. I’m finally today (Oct 16) writing to a backup again – thanks to switching to SMB (but sharing anything other than your home folder is a further trick). So I think we need a bit of a rewrite here! In the meantime, this would be the best, shortest article I’ve found on doing this with snolep: macosxhints.com – 10.6: Set up Time Machine on networked AFP volume.

UPDATE: Yep, Snow Leopard has broken my backups, which worked perfectly for 6 months. Initial reports are you just need start from scratch. I’ll get back to you on that. The short version is these unsupported backups are a little prone to corruption, but as we say “soooo much better than no backup”.

There are a few posts about on the interweb regarding using the “TMShowUnsupportedNetworkVolumes” flag to use any old network share as your Time Machine destination (I found this one the most useful). I think a lot date from about 10.5.2., and several have little retractions at the end, some then state that later 10.5 revisions made it work better. The recent ones say “just turn it on and go”, which certainly wont work if your server is a 1999-built G4 running 10.3 like mine.

So, does it work or not? How hard is it, what do you do? I just had a stab, it appears to work and I think I have streamlined the process somewhat over any of the posts I’ve read.

In super-short, you only have to do two things:

  1. Turn on the showing of any network volumes as possible TM disks (because you can’t back up to them if they’re not listed)
  2. Create a special “sparse bundle” disk file for the backups to go on (because TM’s automatic one will fail on non-10.5 AFP shares)

Before going any further, some sober words:

  • It says pretty clearly “unsupported”. Do not, under any circumstances bet the farm on this working as your backup. It is a little troubling that you cannot create a sparsebundle on a “normal” AFP share – what else might fail?
  • I don’t know if anyone doing this has actually restored a file “in anger” – i.e. actually had to seriously test this under real circumstances
  • So, why even try this? I had no backup at all prior to Leopard, and only one a fortnight or so with wired Time Machine, so really I’m ahead if even a small number of the files I created or changed in the last fortnight can be saved.

Here’s what I did (I have condensed fairly heavily, you need to know how to use a Mac, but this is still the simplest method I’ve seen):

  1. Turn on unsupported volumes by typing this in terminal (on one single line):
  2. defaults write com.apple.systempreferences TMShowUnsupportedNetworkVolumes 1

  3. You’ll now see all mounted network disks in the Time Machine choose disk list. Pick one, then initiate a backup
  4. This will fail, that’s OK, we did this for a reason. (And if it doesn’t fail .. I guess you can stop now)
  5. Open up Console, you should see backupd logging the failure to create a backup image: copy out the name of the sparse bundle it tried to create – this saves you having to find out your machine’s name and MAC address to name the sparse image!
  6. Create a sparse bundle with that name – on a local disk. I just used Disk Utility, but you can do it in terminal if you want. You do need to make it a big size, but don’t fret about space: an empty 200G sparse bundle was 100MB, I didn’t need a whole 200G free to do this
  7. Copy that image file to your unsupported share
  8. Star Time Machine – it now works!

After one week…

It’s all working rather well! My initial 150G backup survived several wake/sleeps, several more deliberate “stop backing up”, and most incredibly, two kernel panics on the server hosting the share, and now the hourly incremental takes just a few minutes. I now have both household backups going to the shared disk. I even restored one whole tiny file, and it worked

What remains to be seen is whether the system will cope gracefully when the disk fills up – this was the issue with the 10.5.2 hacks – once full, the system did not “thin” the backup data properly – it seemed to delete it but not actually free the space; and then it would try to throw away some more in a rapidly spiralling deletion-of-death.

What also remains to be seen, of course, is a large-scale restoration after some sort of disaster. This is the $64K question not just for hacked Time Machine, but for TM itself.

And later again ..

Tonight I finally reached the 200G limit on my sparse bundle (nothin beats VMs to waste a pile of space in a short time!). And my MBP did not grow the disk image, so it looks like you can use this method to store the backups from several systems on one drive (as speculated below).

Other thoughts:

Those who have tried to use one disk for more than one Time Machine user will know that it doesn’t really work: TM is built to fill up all the available disk it can, then start getting rid of old stuff, so the disk fills up very quickly with the most profligate user’s stuff, and then you can’t add any more (users) to it. The workaround I and others have used is to partition the backup disk. This works but you then have to share out n TM disks, set them as time machine disks, and individually connect everyone. And you still have to buy another drive and do it again once you have n + 1 users.

It would seem that by creating the sparse image to a size of your own choosing, you could manage this much better. However, the system may grow the sparse bundle later on if it thinks it can. It is certainly telling me the amount of free space on the (whole) disk as being the amount it has left to use.

And lastly, it’s pretty cool (and well out of character) for Apple to allow this. I’m personally very glad and let’s just hope they don’t taketh it away. (Like, for example, the video out on iPods)

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