[toast0]

At some level, it doesn't make a big difference if you've got a file open or not once the file's data falls out of the disk cache, you'll have the same kinds of latency to get it back. Sure, you'll avoid a round or two of latency pulling in the filesystem data to get to the file, but probably not a big deal on SSD.

Chances are, the popular databases stay in cache and are quick to open anyway; and the unpopular ones are rarely accessed so delay is ok. But you'd also be able to monitor for disk activity/latency on a system level and add more disks if you need more throughput; possibly disks attached to other machines, if you also need more cpu/ram to go with it. Should be relatively simple to partition the low use databases, because they're low use.