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I do know that fiddling with stuff in LR is non-destructive to the original raw data, as only the settings themselves are changed.

With CR2 files, LR writes an XML file with the same base name, and doesn't write to the image file.

But with DNG, the settings are saved in the DNG file itself. Does this give anyone else a less-confidant feeling about the image data preservation? I'm thinking that it will affect the volume snapshot (backups) size if the large file is rewritten instead of only a small file being written. Does it resave the entire file thus taking up more network bandwidth, or is that info arranged at the end or have a fixed position to overwrite, or otherwise only change some of the bytes in the existing file as opposed to saving the whole file? This is also a problem if I make the original image files read-only!

Does anyone know more about it? Can I somehow keep it separate in DNG like it does with other formats?

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I don't think there's any way around the issue of the larger DNG file size causing more issues with backups, since any change to the image's metadata/processing instructions will write out the entire file again. However, you can create a .xmp sidecar file from a DNG file with exiftool:

exiftool -xmp -b -w xmp <path_containing_dngs>

And, in fact, if you open a DNG file with Microsoft Word, you can copy and paste out the XMP data, according to this website.

So no, I don't feel any less secure with the XMP information embedded within a DNG file than outside it in an external .xmp file. And, actually, I don't really feel that insecure with all the information in the .lrcat library file, either, since it's just an SQLite database, and works with all the SQLite tools for data browsing and dumping. Sure, databases go corrupt, but if you need to can opener data out of one, better it be a widely-adopted open source SQL-based one with a bunch of tools out there for you to do so. ;)

Also, apparently, if you make the DNG file read-only, that's the perfect solution, as Lightroom will then make a .xmp sidecar file for the DNG.


Addendum: I just tried Metadata -> Save Metadata to File in Lightroom 5.6 (on OSX, 10.8.5), and got .xmp sidecar files for a folder's worth of DNG files.

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  • Ah, I'll try that. – JDługosz Jan 13 '15 at 3:36
  • Actually, I just tried Metadata -> Save Metadata to File with some DNGs, and got .xmp files without having to make them read-only. I'm running Lr 5.6. Will edit the answer. – inkista Jan 13 '15 at 3:44
  • But did that still re-write the DNG file too? That's the point, not having the xmp for its own sake. – JDługosz Jan 13 '15 at 4:52
  • Don't think it did--the datetime stamps on the DNGs are unchanged. – inkista Jan 13 '15 at 5:57
  • I mean this presupposes that the dng had already been updated with xmp info, if you are later splitting it out. I don't care about having the xmp, I just want the dng to be read-only once it's put on the filesystem. The volume snapshot works on the specific file sectors, so even rewriting with identical contents (which git considers unchanged, for example) will cause it to make another backup. – JDługosz Jan 13 '15 at 7:26

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