How can I batch-compress DNG files in my Lightroom library?

Question

I shoot with my Pentax K10d in the DNG format. The camera produces the files in an uncompressed format. I would like to save some disk space by compressing all in a batch process too in Lightroom.

Can I compress all files in my lightroom library at once?

Answer 1

According to Attila Acs:

You can compress new DNGs:

if I import them using the Lightroom “convert to DNG” command, 
they are compressed

You can also compress DNGs already in your Lightroom database:

Select the photos in your catalog and choose
“Metadata -> Update DNG Previews & Metadata”

Answer 2

The Lightroom Queen explains how to make Lightroom apply lossy or lossless DNG compression to raw files (including DNGs) that are in your photo library:

1. Select the images. (This means LR can only do this one folder at a time.)
2. Go to Library menu > Convert Photos to DNG... [not a memorable command name for this purpose]
3. In the dialog, pick:
   • Only convert RAW files [that includes DNGs]
   • Delete originals after successful conversion
   • Pick lossy or lossless compression as desired
   • Not "Embed Original RAW File".

Note: This Adobe forum thread explains what "Update DNG Preview and Metadata" does:

1. It does not compress the image files.
2. Maybe takes hours.
3. Updates the preview that's embedded in the DNG so other programs (that don't process Lightroom's non-destructive edit list) can get at least a version of the edited image.
4. Updates the metadata stored in the DNG file, although it's unclear from this thread if/when that includes keywords, ratings, color labels, and the edit list.