I tend to keep one or two clones of each template some number of weeks of updates behind, just in case an update (especially a *-testing update) goes awry. I think this approach is useful for most folks who are trying to balance "more secure by updating regularly" and "able to manually recover when a template stops working". So: a backup regime that can dedupe on some level would be very welcome.
Q: Speaking of hashes (this is regardless of the encryption question): are the hashes in sparsebak salted per qubes system (or backup set?)...or would the same hash on two different (non-cloned) Qubes systems match for the same content?
And Chris: thanks for all your contributions to Qubes usability, I really appreciate it.
Brendan
I've been using sparsebak since the announcement - here's my experience with it so far:
Before using sparsebak I used custom rsync scripts for frequent incremental backup of the data in my VMs' private volumes. It was quick and efficient and despite the many obvious drawbacks of that approach it was a thousand times better than using qvm-backup/-restore (those programs are so resource hungry that I only ran them once or twice a year - the suggestion of leaving my laptop on at night with a scheduled unattented backup task is a no go for me).
Sparsebak was a huge improvement over both qvm-backup/-restore and my custom script approach. Despite the alpha (beta?) status I haven't run into a single problem and Chris has been very diligent in replying to some usage questions I had. Since the announcement he has improved the tool without breaking anything and he also implemented deduplication which definitely saves disk space when backuping cloned VMs (interestingly this also works pretty well cloned VMs that have been heavily customized/updated compared to the original VM).
I currently backup 24 volumes (a mix of -root and -private volumes from linux and windows VMs) with a total size of ~100GB, and I use my laptop without any problem while sparsebak is running in the background (incremental backups are fast anyway so even if it used a bit more resources it would be for a short time).
Now, as Chris pointed out the only hassle is that you have to set up an encrypted volume to encrypt your backups but it's a pretty easy thing to do.
So, I recommend you give the tool a try, maybe in addition to using qvm-backup every now and then until you're confident it's working properly for your setup.