[Marco_Polo]

I had a 580MB disk image handy, so I used that as the source and used SuperDuper to make a backup sparse image on my server. The original disk image had 60MB of free space.

The resulting sparse image was 523MB.

I then copied 60MB of files to the original image -- filling it; ran a smart backup; deleted the new files from the original image; and ran a smart backup again...

I repeated this 3 times.

Results...

First run: The sparse image was 583MB
Second run: The sparse image was 583MB
Third run: The sparse image was 583MB

I decided to switch things up a bit and so I deleted a folder, renamed some of the files and swapped a few files between different folders.

SuperDuper reported this error during the next backaup:
| 07:10:27 PM | Error | ditto: /Volumes/Test/3/XXX001.tif: No space left on device

Checking the sparse image size again, it remains at 583MB. Looking at the contents of the disk image, I see that SuperDuper did not delete a folder that I had deleted from the original image. This seems to be an unavoidable issue due to the way that it sync's. It moves down through folder hierarchy alphabetically.

Although this disk image did not swell beyond the size of the original volume, I think that I have demonstrated the mechanism by which the other disk image grows.

Assume that the sparse image is somehow made with the wrong maximum size or that over time and with repeated mounting/unmounting the maximum size somehow gets changed/corrupted... the result of this kind of file-shuffling is that occasionally the amount of data copied to the image will exceed that on the source drive. That image file will grow beyond the size of the source.

You insist that sparse image files aren't supposed to grow like that and I've been unable to reproduce it here, but it happened repeatedly when making nightly scheduled smart updates from my boot drive. I've deleted over-grown image files dozens of times and started over again with the same result.

I don't know what goes wrong with the sparse image files (maybe something records the wrong volume size from my hard drive?), but I have now demonstrated a mechanism by which those image files would grow if they could grow. And I know that they can grow because they do grow.

I'll tell you what I'll do... I've filled up the NAS drive with other stuff since I started backing up to a regular disk image. Give me a week or two and I'll clear off a few GB and start backing up to a sparse image file again. When it fills up the drive, I'll report back and I'll perform whatever reasonable tests you request.