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edoates 08-04-2005 02:29 PM

Sparse images: only use smart backup?
Once I've created a sparse image with enough extra space to handle reasonable expansion, is there ever a reason to completely refresh it by doing a full "erase and backup," or is it reasonable to do "smart backup" from then on, forever? That assumes I remember to hdiutil compact <sparse image name> from time to time.

I'm after practical recommendations from those who have used SuperDuper for some time with long periods of only using "smart backup." When I used to use Retrospect, I would do a full backup every two weeks, and incrementals between that because the backup container go very large after a while, and restoring single files took forever to list the directories (apparently it has to pass over all the deleted entries).

So far, I've been using SuperDuper in this Smart mode, and though compacting does takes some time (80GB sparse image, 56GB used), it appears to be functioning OK. I did a trial diskutil "restore" to another firewire drive and it worked perfectly: bootable and all!


dnanian 08-04-2005 04:30 PM

Well, there's no "data integrity" reason to, no. But, replacing individual files, over time, can cause fragmentation of the volume. Since it's a backup, that shouldn't matter that much, but erasing it can straighten things out.

Apart from that, though, no. I never use Erase, then copy -- other than for testing -- with my own volumes.

edoates 08-04-2005 04:58 PM

That's what I was hoping you would say.

As a side note, it is possible to defragment sparse images using TechTools 4 optimization: turn off journalling (in Tech Tool 4) and you can optimize the volume. But it seems to be actually slower than just doing a new erase and backup. And probably more risky, though Tech Tool 4 has not had any problems at all on my systems.


dnanian 08-04-2005 05:58 PM

Yes, no doubt you can do that... but, as you said, it won't really save you any time...

sjk 08-14-2005 06:15 PM


Originally Posted by edoates
That assumes I remember to hdiutil compact <sparse image name> from time to time.

I've automated that by running it in a script after the backup finishes. Still want to modify it so it only compacts when the percentage of free space exceeds a certain amount (or with some other condition; suggestions welcomed) instead of every time.


I'm after practical recommendations from those who have used SuperDuper for some time with long periods of only using "smart backup."
I've been successfully using Smart Update with sparse images for over six months.

macfeller 08-14-2005 09:39 PM

Just today have I considered sparse images and have found my way back here. I am inclined to not bother with them. I just don't get the benefit. If I have it right, a *full* sparse disk image, when unmounted is the same size as a standard disk image of the same size. That is, presently, one facet of my backups is a disk image the same size as my boot partition. There have been times that partition has been damn near full and so the backup was, too. Were I to use a sparse disk image for such, when unmounted wouldn't it be of similar size? If so, and with the fact that it doesn't shrink when things are deleted without delving into Terminal, what's the advantage?

I got into this because I was under the assumption that sparse disks were compressed. If they aren't (and in my tests they haven't), again, where's the good?

sjk 08-15-2005 12:13 AM

From Creating a disk image of a device, folder, or volume:

You can create a full disk image that uses the same amount of disk space as the disk it represents, or create a sparse disk image that includes only the data on the disk. For example, a 10MB disk may have only 5MB of data. A full disk image would be 10MB, while a sparse disk image would be 5MB.

Sparse images don't preallocate disk space in the filesystem, which definitely has advantages in certain situations; the example you gave isn't one of them. :) A drawback is that they need to be manually compacted to reclaim unused space.

In my current backup strategy I waste less disk space by using sparse images. When I store several on a single volume it means that no single image is using more space than it needs, which leaves room for the others to grow. That gives me enough flexibility to balance a limited amount of disk space until there's a real need to purchase more. And I can move images to smaller volumes which have enough free space for the sparse type.

macfeller 08-15-2005 01:17 PM

Thanks. By bed time I had gotten close to having everything staraight in my head and your answer helps greatly. I still may switch to sparse just cause their cool. Yeah, stoopid reason, but then ...

What really gets me is why they can't compact themselves. If they're smart enough to grow, why not shrink? The hdiutil thang ain't tough, but if you do get the scripting for it done do please post us.

Again, thanks.

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