local copies
Many users have huge image collections stored on extra hard drives in their desktop computer, or on an external storage medium (RAID NAS, external hard drives etc.).
It is a common requirement to develop a number of images while travelling using a laptop and then later synchronize them back to the original storage medium. However, copying images manually from the main storage to the laptop and back is cumbersome and prone to errors. The “local copies” feature of darktable has been designed to directly support these use cases.
You can create local copies of selected images from within the lighttable. Local copies are always used when present, giving continued access to images even if the external storage is no longer connected. At a later point, when your primary storage medium has been reconnected, you can synchronize the XMP sidecar files back to this storage, deleting any local copies. These operations can be found in the actions on selection module in the lighttable.
For safety reasons, if local copies exist and the external storage is available, the local XMP sidecars are automatically synchronized at start up.
Local copies are stored within the $HOME/.cache/darktable directory and named img-<SIGNATURE>.<EXT> (where SIGNATURE is a hash signature (MD5) of the full pathname, and EXT is the original filename extension).
Local copies can be identified in the lighttable view by a white marker on the top right of the thumbnail. In addition, all local copies carry the darktable|local-copy tag to allow them to be easily selected.