short version first: I'm looking for Linux compatible software which is able to transparently cache HDD writes using an SSD. However, I only want to spin up the HDD once or twice a day (to write the cached data to the HDD). The rest of the time, the HDD should not be spinning due to noise concerns.
Now the longer version: I have built a completely silent computer running Xubuntu. It has a A10-6700T APU, huge fanless cooler, fanless PSU, SSD. The problem is: it also has (and needs) a noisy HDD and I want to forbid spinning it up during the night. All writes should be cached on the SSD, reads are not needed in the night.
Throughout every day, this computer will automatically download about 5 GB of data which will be retained for about a year, giving a total needed disk capacity of slightly less than 2 TB. This data is currently stored on a 3 TB noisy hard disk drive which is spinning day and night. Sometimes, I'll need to access some data from several months ago. However, most times I'll only need data from the last 14 days, which would fit on the SSD. Ideally, I'd like a transparent solution (all data on one filesystem) which caches all writes to the SSD, writing to the HDD only once a day. Reads would be served by the cache if they were still on the SDD, else the HDD would have to spin up.
I have tried bcache without much success (using cache_mode=writeback, writeback_running=0, writeback_delay=86400, sequential_cutoff=0, congested_write_threshold_us=0 - anything missing?) and I read about ZFS ZIL/L2ARC but I'm not sure I can achieve my goal with ZFS. Any pointers?
If all else fails, I will simply use some scripts to automatically copy files over to the big drive while deleting the oldest files from the SSD.
Edit: Cache really is the wrong term for what I wanted. It seems the original intended solution is not possible for me at this time. Thanks to everybody who contributed!