Debian :: Cache A Directory Tree?

I have a home storage of many drives which are seldom accessed, and extremely seldom written to. So, I made several RAID1 arrays with "write-mostly" drives in each one, so that all the drives spin down after a while, and if they are accessed, only one drive in each array has to spin up. This way, I hope to minimize the risk of losing anything due to mechanical shock or wear.

But it was not so easy. First, the write-mostly drives did spin up on every read request due to a bug in the kernel, so I managed to get the fix accepted into the newer kernels (which is my largest contribution to the Linux development so far. See [URL] .....) Now, if I read directly from the array, it works as expected (only the first drive spins up); but if I create a filesystem there (ext4) and read from it, the second drive still spins up anyway. (Does ext4 write something when it reads a file?.. I have it mounted with relatime, so AFAIK it shouldn't...)

Well, I thought, I'll just mount the filesystems read-only and remount them if I need to modify the data there, which is not very often, to say the least. Also, this way the drives don't have to spin up every system shutdown to unmount the filesystems; during a power outage, it used to take a lot of time for them to spin up one by one just to unmount, while my UPS had to survive long enough for them to finish. That's one more problem solved.

But I decided to improve even further. Sometimes I just want to see the contents of those arrays without even accessing some files (for example, when I accidentally click the wrong network drive button in Windows, or when I just want to see if I have something there or not). If I read the directory trees during startup and keep them cached, then those drives won't even have to spin up at those moments! The question is, is there a way to do it? I remember Linus boasting about our kernel being the best at caching the filesystem trees, and I know there is vmtouch [URL] .... which allows us to keep some files cached, but what about the directory tree itself?.. If I simply "find" all the files there, it seems to work, but they get evicted from cache sooner or later...