Operating system in RAM

[gilphilbert]

I agree with you.  If you want to avoid corruption of a drive (MicroSD in this case) you need to have some kind of graceful shutdown in the event of an AC power failure.  There are way too many ways to inadvertently lose power -- storm outage, utility work, someone accidentally trips on a cord or bumps a PDU switch, etc.

I may be purchasing one of the Hypnic devices since your posts brought it to my attention.  My sole complaint concerns sequencing or, more specifically, lack thereof; it doesn't charge its supercaps first and then apply power to the Pi.

You could always ask the designer, I hear he's pretty good about answering questions...

The Hypnic was designed specifically to address SD card corruption - kids pulling power cords, power outages, etc. - these are fairly common occurrences in my house! It won't protect against wear out, of course. For that you need a high-endurance card if you want to perform lots of writes to the SD card.

You've raised an interesting question around concurrency. The default firmware on the Hypnic will power the device and charge the supercaps at the same time - as soon as power is connected. This is the most common use-case and - as long as your power supply can supply enough current - it works faily well, even when loads are high. There are two downsides though: obviously it requires a larger power supply that provides more current, and your device is not fully protected until the supercaps are charged, so an outage within the first few minutes could lead to an ungraceful power down. I say could, as it depends on load, previous supercap charge levels, how long it takes for the SBC to power down, etc.

The Hypnic actually has all the hardware to charge the supercaps to a given level before powering on the device. There are downsides here too though, the biggest one being that you need to wait for the supercaps to be charged before you can power on the device. Depending on your power supply and the size of the supercaps, this may be a few minutes. For many applications, this may well be fine - but as a general rule of thumb, users expect devices to power on when they press the power button : I suppose you could "long-press" the button to force the device to power on even when the supercaps are not charged fully.

Writing firmware to make this work would be fairly trivial, I just haven't got to it yet (I have other features in the works and this hasn't been a top priority).