How does Retrospect handle Mac OS X Encrypted Disk Images?

[a_riot]

Hi,

We have some users that have encrypted disk images in the directories that are being backed up by Retrospect Server. I am curious as to how Retro deals with these because they change all the time and since they are encrypted they can only be backed up in their entirety. Does Retro back up the entire file repeatedly overwriting the previous contents or does it somehow do a byte by byte comparison and take snapshots of the image everytime it gets changed?

Thanks for your help,

Paul

[CallMeDave]

Quote:

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Does Retro back up the entire file repeatedly overwriting the previous contents or does it somehow do a byte by byte comparison and take snapshots of the image everytime it gets changed?

 

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Retrospect never overwrites anything, unless you do a Recycle backup (in which case it overwrites everything). Disk image files are just like any other file; if it's changed since the last backup, Retrospect will back it up again, appending the new version of the file to the Backup Set.

 

Dave

[ghoffman]

A question suggested by the original query:

 

Would it be possible to mount the disk image and have Retrospect back-up the individual (changed) files within?

 

Of course, one would also want to tell Retrospect not to backup the disk image file, for the original reasons.

 

Glenn

[a_riot]

Thanks for your reply. The poster above asks a good question. I have a user with a large encrypted disk image, say 50 MB, that changes daily. In a week that means that even though they might have made only small changes, I have 5 x 50 MB worth of backups. I was under the assumption that Retro only backs up the changed parts of the file, not the entire file itself. Is this not correct? Anyway to change this behavior and still keep all recovery options the same?

Thanks,

Paul

[CallMeDave]

Quote:

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I was under the assumption that Retro only backs up the changed parts of the file, not the entire file itself. Is this not correct?

 

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From where did this assumption come?

 

Retrospect's "Normal" backup behavior has always been to copy any files that have changed, matching an assortment of criteria such as mod date, size, location, etc. If you change a pixel on a 50 MB Photoshop file, Retrospect will copy the file again. It has no logic to inspect every layer, mask, chanel and pixel of such a complex file to copy only the changed bit. Same for a disk image, and especially an encrypted one. Would you even _want_ a third-party program to have the ability to have access to the contents of such an image?

 

With respect to the question of backing up _mounted_ disk images then yes, if the volume is mounted at /Volumes then Retrospect can see it and treat it as any other volume (making copies of any files that have changed). Of course, this would completely defeat the purpose of having encrypted images.

 

Apple appears to have made their new FileFault feature even more secure then just mounting an image at /Volumes. Retrospect sees the contents of /Users/your_home_directory/ as the single disk image file (named something like ".EncryptedDiskImage"). There does not appear to be a way for Retrospect to gain access to the files inside (this probably has to do with the issue that has been part of Retrospect on OS X since it shipped; the program runs as root while most users are running the Finder as a different user).

 

Dave