In the computer business, everybody is smarter than the average bear, but… Abode tries to be smart and make Lightroom faster, but when it comes down to it, it’s all reads and writes. And Lightroom does a lot of disk activities.

  1. Lightroom reads the whole catalog upon starting. The catalog is itself very small. A 10,000 photos catalog is between 25 Megs to 35 Megs depending on the number of keywords, develop revisions, history… It’s very small. The actual catalog is the file that end with the .LRCAT and the original photos are the only important things.
  2. Lightroom writes to the catalog during:

    • Imports
    • Updates to the metadata, flags, keywords, copyright…
    • Develop module processing: every single step to apply the changes and the history…
    • History of the various operations for printing, web and slide shows.
    • Lightroom writes to the sidecar, that’s the XMP file, if you have set the Catalog SettingsMetadataAutomatically write changes into XMP. The writes to the sidecar are not synchronous, meaning that it doesn’t happens as you are doing it, but Lightroom writes to sidecars when it has time.
    • Lightroom uses a lot of other files, but they are used only for speeding up its operations and mostly related to the previews. The previews are stored in the Previews.lrdata directory. If you take a look, it’s divided in 16 directories with hundreds of directories underneath. Lightroom write the previews either as on a as needed basis or as pre-built. Because of all the split file operations, Lightroom can keep the previews closer to its “core” but keeping another copy of the preview in the Camera Raw Cache. If the data isn’t in the Camera Raw Cache, Lightroom will read the original, the previews and write them the Camera Raw Cache.