If you are using Time Machine to backup your Mac onto a network share, you might have noticed, that since the Snow Leopard 10.6.3 update the sparsebundle image used by Time Machine has been resized to use the whole capacity of your network share.

I use a Qnap NAS as a central network storage for backing up mulitple Macs and therefore I want to define strict limits for the maximum size of each Time Machine backup. So after noticing the new image sizes, I tried to resize them with hdiutil:

hdiutil resize -size 500g MYBUNDLENAME.sparsebundle

Unfortunately Time Machine resizes the image size every time it runs, so it is necessary to prevent it from resizing the image.

The image size of a sparsebundle is stored in the Info.plist file inside the bundle directory, so I tried to remove write permissions to that file from the terminal:

cd /Volumes/MYSHARE/
chmod a-w MYBUNDLENAME.sparsebundle/Info.*

This wasn’t working either, because OSX seems to reset the permissions to Info.plist automatically.

The solution which is finally working for me was to login to my Qnap box via SSH and change the permissions there via chmod:

chmod a-w MYBUNDLENAME.sparsebundle/Info.*

Now, when Time Machine starts, it tries to resize the image but fails, as the Qnap server is preventing any changes to the Info.plist file. You can see this behavior in the system.log:

23.05.10 22:29:13	com.apple.backupd[378] Resizing backup disk image from 500.0 GB to 989.8 GB
23.05.10 22:29:13	com.apple.backupd[378] Could not resize backup disk image (DIHLResizeImage returned 35)

After this logging message, the rest of the backup runs fine. For me this is a nice workaround until there is an official way to limit the backup size.