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Old 10-21-2005, 06:27 PM
Winston Winston is offline
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Join Date: Oct 2005
Posts: 9
Space estimate for disk image backup?

Originally Posted by dnanian
Thanks for the suggestion, Winston. But, again, we just *don't know* how much space things are going to take. Compression can act differently on every image, and trying to guess means we'll get it wrong as much as we get it right.

Yes, we could probably do it for uncompressed images. But, given that's just one setting, and not the default, it just doesn't seem worth it...
Now I am confused! Is this correct for making a DMG backup:

1. SD! makes a complete copy of the original to a sparse image. So, with the exception of any space saved by de-fragmenting the files on the orignal drive, the sparse image takes up the same amount of disk space as the original files.

2. The sparse image is not compressed. But it is a complete copy that ccould be used to restore the original. The sparse image can also be used for incremental backups.

3. If the user set up SD! to do compression during the backup the compression takes place while converting the sparse image to the DMG image. Because of this, the exact size of the DMG image cannot be predicted exactly, even when the size of the sparse image is known.

4. If there is no compression the DMG image is the same size as the sparse image.

5. Once the DMG image is complete, SD! removes the sparse image. However, if the backup is stopped after the sparse image is finished, but before the DMG image is done, the sparse image is left in place.

Assuming the above is correct -
Even if SD! cannot estimate exactly how large the final DMG image will be it should be able to estimate the size of the (uncompressed) sparse image. At a minimum, it would help me to get a warning that there may not be enough space on the backup drive to complete the DMG creation.

Even if estimating the sparse image size is too difficult it would help to have a warning any time backup to a disk image is requested - it is the only type of backup SD! does that might fail because of a lack of disk space on a drive that has a lot more available space than the original.

Note that while there is one warning in the User's Manual about the space needed to do a disk image backup, it is not obvious.

A search of the User's Manual for "disk image" gives 6 references, on pp. 23,29 and 30. None of these pages contain the warning. A search for "image" turns up what must be over a hundred references. The relevant one is the fifth reference to p. 21:
"the original image, and the"

Not an obvious link.

Here is the sentence, from the middle of p.21:
"This operation requires approximately twice the disk space taken by the original image, and the conversion can take a few hours to process."

Not having a backup because of not enough disk space seems worse to me than having to get a warning, especially since the failed backup may not happen for several hours (since SD! goes through most of the process before the backup fails).

Think about someone about to leave on a trip, who only has time for one backup run. (This has happened to me.) When it fails, no backup.

I came to SD! from Carbon Copy Cloner. It always worked, even if it had fewer options. I think CCC only makes sparse image backups when a disk image backup is asked for. Maybe this is why I was surprised at the difficulty in doing this with SD!.


- Winston

Last edited by Winston; 10-21-2005 at 06:29 PM.
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