I came across this article:
which tested the performance of various Mac backup programs on metadata such as file creation dates, HFS+ extended attributes, BSD flags and ACL’s, and concluded that the only one tested that performed OK on all of them was SuperDuper.
Does DropDMG (which wasn’t tested for that article) preserve all of these correctly? I admit I’m not even sure why one needs all of them :-o but I want a program to do full backups with and don’t want to be missing anything important.
When you use DropDMG to create a tar archive, it should copy all the metadata that’s supported by the tar format, which I think is everything except the Finder locked flag.
When creating a device image from a read-only volume, DropDMG will preserve everything, including the inodes.
When creating a disk image from a file or folder, DropDMG relies on Apple’s hdiutil and ditto tools. At present, these copy the ownership, permissions, Finder flags, modification and creation dates, Finder comments, and resource forks. (In order to preserve the owners when restoring, you’ll need to mount the image with DropDMG rather than the Finder.) The BSD flags, symlink ownership, Finder locked flag, and extended attributes are not copied. However, in general I don’t think this is a cause for concern; if you need that particular metadata, you’ll probably know that you need it.
I would like for DropDMG to preserve all the metadata, as that removes any guesswork. However, there are tradeoffs. Because of the lack of support in the underlying Apple tools, creating an image with DropDMG would take roughly twice as long and require roughly three times as much temporary disk space in order to preserve all the metadata. My guess is that for most people this is simply not worth it, and I believe you are only the second person to even ask whether DropDMG preserves extended attributes.
My plan for future versions of DropDMG is to add support for creating device images from read-write volumes; this would allow you to create a full backup of an entire drive, preserving everything. For file/folder images, I will probably add an option so that those who don’t mind the speed and disk usage tradeoff can opt to preserve all the metadata.
Thanks for the detailed reply!
So if I created an image of my disk with SuperDuper, then used DropDMG to convert that image to a segmented image, I would lose metadata (because the convert operation would be done by hdiutil); but if I mounted the image read-only and told DropDMG to make a segmented image from the mounted volume, it would be preserved?
Somewhere on that article I linked to there’s a discussion of what the various forms of metadata are used for. As I said I’m not sure I do really need any of them, but if I’m setting up a backup system it’s nice to know that everything is being stored.
No—converting an image using DropDMG will preserve all the metadata. The limitations I described above only apply when creating a new image from a folder.