The Wiki has description-how-to for RAM Drive and for HDD Backup Drive.
However, for the latter solution also see this thread.
The settings for the RAM Drive may get lost at shutdown of the Raspberry (by own experience, and as written in this Wiki-section), requiring that you again have to walk the road to get it:
not difficult, but nuisance, and after a few times it is 'forgotten' ........
First, shortterm solution might be that in the Wiki a clear&short instruction is added how to add the required lines to domoticz' start-file, bearing in mind what has been written at the bottom section of the elaborate description.
Longer term it would be nice&easy if the Domoticz' GUI under Setup would get an option
1) to set a RAM Drive for 'working files' of selectable size [1M ~20M], with 0 = no RAM Drive
1a) with option to include the logging & temp files
2) for backup to External HDD to set & activate path & timing with RSync to a second, external location either at LAN, or at available USB-port
With the recent experience in mind (as mentioned in first line) would be nice if patch for 1), 1a) and 2) would be available for Domoticz working with Raspian Jessie, Stretch and Buster, serving all present & future users of Domoticz.