I am running MySQL with its data dir on a 128Gb SSD. I am dealing with large datasets (~20Gb) that are loaded and processed weekly, each stored in a separate DB for the purposes of time point comparisons. Putting all the data into a single database in unfeasible because the performance on such large databases is already a problem. However, I cannot keep more than 6 datasets on the SSD at a time. Right now I am manually dumping the oldest to much larger 2Tb spinning disk every week, and dropping the database to make space for the new one. But if I need one of the 'archived' databases (a semi regular occurrence) I have to drop a current one (after dumping), reload it, do what I need to, then reverse the results.

Is there a way to configure MySQL to use multiple data directories, say one on the SSD and one on the 2Tb spinning disk, and 'merge' them transparently? If I could do this, then archiving would no longer mean "moved out of the database entirely", but instead would mean "moved onto the slow physical device". The time taken to do my queries on a spinning disk would be less than that taken to completely dump, drop, load, drop, reload two entire databases, so this is a win.

I thought of using something like unionfs but I can't think of a way to control which database gets stored on which physical drive, because it works by merging on a directory level (from what I understand) so I'm still stuck with using multiple directories.

Any help appreciated, thanks in advance