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Phil 10-12-2004 02:12 PM

Sparse Disk Image track record?
I've purchased SuperDuper for personal use and it looks very promising.

Additionally I support several digital photographers that are looking for a backup solution. They tend to need "versioning" less than authors or software developers for whom the file can change significantly day to day.

Given the cost, complexity, and declining robustness of Retrospect (which I've used for over a decade in businesses) I'm looking to develop an operating model for this group of friends using SuperDuper.

In general, I like the split of backuping up the System separately from User files. The System would be backed up to a bootable partition. As for User files, I like the idea of Smart Backups of the /User/xyz to a sparse disk image created with Disk Utility. (descibed in the FAQ The reason I prefer the sparse disk image, is that typically in would not be mounted, lessening potential confusion as which is the primary file and which is the backup. I realize that once it is mounted it looks just like a disk and confusion is possible. This group in particular varies quite widely in their awareness of the file system. Perhaps the distinction is too minor to matter.

Sorry for the long introduction. What I was wondering is does anyone have extensive experience with the sparse disk images? Are they reliable? Can they shrink, if the content is removed? When were they introduced to OS X and do they draw on a tested code base?

Any perspective would be greatly appreciated.


dnanian 10-12-2004 02:52 PM

Hi, Phil -- thanks for your post!

Sparse disk images have been around for quite some time, although they're rarely talked about. Apple themselves leveraged sparse images to implement FileVault, so it's been quite well tested.

I don't believe they shrink 'automatically', but since FileVault images will shrink, there's likely a command that could be used to compress them.

I believe, using a combination of saved settings and our upcoming AppleScript support (see your other question for more), you'll be able to quite easily develop a model that will unmount the sparse image and keep it 'out of mind'...

SpinUp 01-05-2005 04:13 AM

Shrinking Sparse Images
I use a sparse image to back up and recently noticed that the file size has been growing far beyond the size of the data on the disk image. The sparse image had max size of 7 GB and contained only 4.95 GB of data, but the file size was 6.45 GB!

That seemed unreasonable, so I decided to look into shrinking it. Reading from here,

I found the hdiutil command "compact", which can be used in the terminal as follows:


hdiutil compact imagename.sparseimage
This removes the parts of the image file that aren't being used in the volume. The process ran for a couple of minutes, and the file size of the sparse image was subsequently reduced to a more reasonable 5.3 GB. Note that the above link is just the man page for hdiutil in Panther, and has this to say on the subject of reliability:


As of OS X 10.3, partially-updated SPARSE images are now properly handled such that they can be safely used for persistent storage. SPARSE images are not recommended for persistent storage on earlier versions of the operating system.
I was using sparse images for backup in Jaguar, when perhaps I shouldn't have been...

dnanian 01-05-2005 01:45 PM

Yes: sparse images are used for FileVault folders in Panther, and I know that they -- along with lots of other things -- took fixes for Panther (and later releases). But I believe sparse images work quite well as long as you don't get any internal corruption due to crashes and the like...

In any case, I absolutely recommend that people update to Panther. There are a lot of improvements in it, as there will be in Tiger. And we don't guarantee that any version past v2.0 will be Jaguar compatible: we just can't maintain backward compatibility forever -- it constrains new features too much, and makes our testing matrix far, far too difficult to execute.

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