Simple question about backing up images

[Alaska Jack]

Dave, or other expert, I have a simple question.

I use SD to back up files from my iBook to an external firewire drive.

On the ibook hard drive is a relatively large, encrypted disk image. I don't modify the contents very often.

I normally back up with a script that excludes the image. The rationale was, if I changed one of the image's content files, I didn't want SD to take the time to write an entire copy of the image to the backup hard drive.

Now I'm wondering, though, is that the case? If I add, delete or modify just one file within the encrypted disk image, does SD need to rewrite the entire file? Or does it just rewrite the little bit of the image that changed?

Thanks, and great program -- it's worked like a charm for me.

- Alaska Jack

[dnanian]

It depends on what type of image you're using. A 'sparse bundle' (Leopard only) would only rewrite the 2MB or so 'around' the change. A 'sparse image' would rewrite the whole thing.

[sumguy]

hi, i have a similar question i'd love to find an answer for... if i have a large file that's stored on a sparse bundle disk image, and i make a small change to that file, will every part of the bundle ("band" i guess it's called) that contains this file be changed and need to be rewritten? or just the few bands that actually had new data written to them?

to be specific, i use thunderbird for e-mail, which stores all my mail in one giant text file. so every time i get a new e-mail appended to it, i have to re-write several hundred megabytes to the backup. what if i stored my thunderbird mail folder on a sparse bundle? would it then back up only the small part of the mail file that changed? or every part of the bundle that contained part of the mailbox file? if someone's already tried this, it sure would save me a lot of "experimenting" time...

i know i could avoid this by switching to apple's mail.app, or by sorting all my mail into subfolders, but i just don't have time. i use superduper because i want something fast i can really rely on without a lot of fussing around.

one more thing that i'd really like to find out, is whether superduper backs up sparse bundles while they are open and mounted, and if so, whether that is safe? in researching how time machine works, i found that it won't back up your filevault-protected home folder (just a big sparse bundle) unless you're logged out and the image is unmounted. however, it seems that it will happily back up mounted sparse bundles you've created yourself with disk utility, if you're not using filevault. this seems like a contradiction - and another reason i don't trust time machine. does superduper back up open/mounted sparse bundle disk images? is that really safe? wouldn't the image become corrupted, if you backed up only some parts/bands just before an adjacent part was changed?

in the past i've never bothered unmounting disk images, and closing all other open files/programs before doing a superduper backup in the background. but switching to this new file format - which will be great if it works - i'm worried that's no longer good practice. again, it would save me a lot of time and guessing, if someone has an informed answer about this, 'cause i've spent half the day searching support sites and blogs, and still don't feel confident that my backup strategy remains reliable.

thanks...

[dnanian]

It's really unlikely that'll help, sumguy, because Thunderbird will rewrite that file, and all its bands will be modified.

We suggest, in our user's guide, that you back up a FileVault volume from a separate account, with the FV volume closed -- for the same reason Time Machine suggests it. It's not safe to do otherwise.

[sumguy]

i understand about filevault, but i don't use it. i wanted to know about the disk images i create manually with disk utility, for sensitive documents. with tiger, i never unmounted them before doing a superduper backup. i assumed that when i saved a file on one, the changes were immediately written to disk. was that dumb? if not, do i need to start doing that now in leopard, if i use the new sparse bundle format? i want to make sure superduper is never backing up corrupted copies of the disk images, but at the same time it's a pain in the butt to unmount/mount them all the time if it's not necessary.

with regards to thunderbird, it only appends new mail to the mailbox text file. it never removes deleted mail or rewrites the whole file, unless you do "compact folders". well i guess it's up to me to experiment with that one, i'll let you know!

[dnanian]

No, you should unmount those images: there's no guarantee that the on-disk structures are updated when you copy files to it.

[sumguy]

ok, that's good to know. i would have thought time machine and/or superduper would give some kind of warning, or refuse to back up mounted disk image files, if it wasn't safe to do so. i'll start unmounting all my disk images before doing a backup. thanks!

[dnanian]

We don't really know that the copy of the data on the drive is potentially not complete. That's why we generally suggest not running programs during a backup -- although we don't explicitly mention "eject any disk images you've been writing to" since, well, it's kind of a marginal case.