View Full Version : Comparing Two SD Sparse images

10-27-2016, 11:18 AM
I (try) to do a daily backup to sparse images which I cycle through. I noticed one day one image was 90GB smaller and I became concerned. I know what happened: Sierra's System Preference->iCloud->iCloud Drive -> Options checked Optimize Mac Storage.

My question is how can one compare the sparse images to see which files are missing. A launched a couple of instance of Terminal, mounted the images, changed the directories of each instance of Terminal to a mounted image. I then did sudo du -sh */ and ls -lh on each image in each instance. Both were identical from the information reported. The images were 690GB and 780GB.

Shouldn't I have seen the difference or am I clueless?

How can I figure out the differences between two images?


10-27-2016, 12:45 PM
The external image file size isn't really relevant. Is the internal drive size significantly different?

10-28-2016, 06:16 AM
Is the internal drive size significantly different?
No, he wrote as much.

I too want to compare whole volumes. Acutely a Time Machine restore vs an older SuperDuper clone. But also to verify backups.

I've used rsync --dry-run, but it's a pain to exclude all the stuff that breaks it, like /Volumes and /dev.

VisualDiffer is on the App Store, but doesn't cover all metadata and has problems with permissions.

10-28-2016, 07:21 AM
Well, it wasn't entirely clear, even though he did a "du". But in his case, external size difference isn't relevant.

With millions of files on a typical copy, and the fact that you're copying from a live drive in most cases, "verifying" the backup is always going to find many mismatches and doesn't really help a lot...

10-28-2016, 04:53 PM
Well, if there's a mismatch in the Documents folder, or some folder completely missing (because I messed up excludes), I want to know.

10-29-2016, 10:16 AM
I think I'd generally recommend not being overly selective in your copies, for sure, that way you can't mess up the excludes. But, my general philosophy is that trying to put verification in the same code that does the copy means that while you can catch things like copy errors (that would be caught anyway, since we stop on errors), you can't catch "algorithmic error" - that is, if we have somehow made a mistake, we're going to make the same mistake in our "check" (since the "check" is, in essence, the same as the check-to-copy).

So, using another product, written by a whole different team/company, helps to ensure you're really "checking".

You'll note I also encourage people to use multiple products to back up, and multiple devices. Again: my primary focus is to try to get people to back up. It's a big reason SuperDuper's free version doesn't expire, and why I provide the same support to our free users (and there are a lot of them) that I do to our paid users.

Hope that makes some sense.

10-29-2016, 06:46 PM
So does anybody know another product that can compare two volumes?

I mentioned two. ChronoSync might work, but I'm not smart enough to operate it.