PDA

View Full Version : How can a startup volume faithfully back up itself?


Syzygies
08-10-2006, 05:42 PM
How can a startup volume faithfully back up itself? Isn't this exactly like taking a picture of someone while they're moving? The picture blurs = the backup cannot possibly represent the state of the system at any particular instant.

I believe that one generally "gets away" with this, but I don't understand why. As a mathematician, I'm constantly confronting the principle "Lack of imagination isn't a proof of anything". So the fact that I can't imagine specifically what should go wrong when a volume backs itself up, doesn't impress me. I still worry.

I've been booting into an aux partition, then running an Applescript which calls SuperDuper! for each partition I want backed up. I've been thinking of automating the restarts into and out of this aux partition, to make this a "one click" solution as I go to bed.

First, I'd like to see if anyone can convince me that I'm worrying too much! Thanks.

dnanian
08-10-2006, 08:45 PM
If you're worried about files changing after you've backed them up... well, they're going to do that once you start back up from the drive, too. But, most definitely: if you want to capture the drive state exactly as it was when you hit "Copy Now", you can't run from it, because things could change. But those things are typically not troublesome -- you're the user, after all, and you know what you're changing that's significant.

Syzygies
08-11-2006, 01:43 AM
I realize time passes after I back up, and I'm not worried about my user files. As you say, I understand them.

I bet that 99% of your user base (myself included) would flunk a quiz on what system processes are even going on in a typical OS X, let alone which ones are changing files. A system backup that takes 20 minutes is "blurred": Some files are as they existed at the start, and some files are as they existed 20 minutes later. If all files were as they were at the start, we understand the system would function just fine. If all files were as they were 20 minutes later, we also understard the system would function just fine. But a blurred graft of files at various different times? The software debugger in me can imagine disaster.

It feels like you're either ducking my question, or you didn't understand it. Imagine grafting my 30 year old head onto my 50 year old body. Sure, I'd be fine being my 30 year old self again, although I wouldn't have any recollection of what happened since. And I'm coping just fine with being 50. But the combination? That's an entirely different question. That's my question. My question is about a blurred photograph, not about life happening after the photograph is taken!

dnanian
08-11-2006, 04:37 AM
I understand your question. You're asking me to convince you to not do something you feel is necessary. I don't know why I would want to do so!

No doubt there are situations where you could have a problem, but they're incredibly rare, and would typically be easily resolved with an archive-and-install or similar. It's just not a serious problem: nearly all backups, regardless of platform or program, are done on live volumes.

jfritz_drfritz
08-15-2006, 03:04 PM
FWIW, powerquest solved this problem in DriveImage 7 on the windows platform. It essentially involves taking a "snapshot" of the drive. It allowed them to perform hot backups without having to boot to dos. THe technology was good enough it induced symantec to buy them out so it could be incorporated into Ghost. There used to be a white paper on the technology on the powerquest website, I don't know if it was transferred to the symantec site. Whether this technology is possible for the macintosh I do not know

dnanian
08-15-2006, 06:21 PM
Well, there's something called the Volume Shadow Service on Windows, too, that helps with this kind of thing... but at present, there's no easy way to do it for the Mac.