In linux how do I have a writeback cache on disks? Basically, I'd like to setup a 100MB RAM cache for writing that writes out every 3-5 minutes or when full. Is this possible? Is there a
sysctl setting to tweak?
Linux already does some writeback caching. Look into pdflush and how it works. Unfortunately it takes some time to understand all the details, since it's quite complicated, but if you're wanting to tune it (e.g. for laptops) then that's the place to start.
You could look into the Eventually-Persistent Ram Disk (EPRD), but the best way to handle this is in hardware. You want something independent of the operating system in order to protect your data through OS crashes, power events and other disruptions.
Is there a reason that you're not interested in using the non-volatile cache available on modern RAID controllers?