Under Leopard, if you turn off iDisk Syncing and then turn it back on, your local copy of your iDisk is recreated as a SparseBundle. The advantage to this is that when a file is deleted from a SparseBundle, the system is able to immediately reduce the file size of the image because it's actually a series of 8 meg files segmented to make up the entire image. (Right click on a SparseBundle and choose "Show Package Contents" and in the folder Bands will be these files)

Now, my hunch is that Time Machine works with SparseBundles to back up only those tiny bits of the image that have change and not the whole thing, but I can't be 100% without knowing exactly what it is Time Machine is archiving.

I've tried forcing Time Machine right after modifying only my iDisk and the size of whatever Time Machine is moving is significantly smaller than my entire iDisk, though I don't want to speak with authority without being certain.

Anyone else want to weigh in on this?

-dennis

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You are correct. It is easy to check. Right click time machine and back up now. Then go to your time machine volume, check the amount of space taken up by used space. Then, make a tiny change on the idisk.. say add a byte to some text file. Then backup again with time machine. My idisk has about 1GB of space used but after this test, the time machine volume is only using about 1MB of extra space from the first backup.. it does not backup the entire volume, but only the *bands* within the package of the sparsebundle which have changed.

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Now that (iDisk SparseBundle feature) is seriously cool. Thanks for the tip!

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