When folks need a small embedded linux machine for control applications, a Raspberry Pi is usually the first thought. I’ve made good use of Raspberry Pi Zeros and 3Bs but have been reluctant to adopt the RPi 4 due to the apparent need for active cooling, high power consumption, very poor availability, and high pricing (it makes little sense to use an RPi when you could use a much more powerful x86-family platform).
With RPis out of stock for months and being scalped everywhere, I decided to try a Libre Computer ROC-RK3328-CC which is footprint/form factor compatible with the Raspberry Pi and can run Ubuntu, Raspberry Pi OS, Armbian, Debian, Android, and many other OS. The docs are here. The board comes in two versions: 2GB for $45 and 4GB for $55 – those prices are with free one-day shipping via amazon prime and they are available immediately. I bought the 4GB version which is 4x the memory of an RPi 3B+; the memory is also DDR4 vs. the DDR3 used on the Pi. The board is easily passively cooled; I bought the custom heat sink ($10) although any similarly sized heatsink should work fine.
I tried Ubuntu desktop but was disappointed by the bloat and installed Raspberry Pi OS (a Debian derivative) instead and am very happy with it; I installed the desktop (not lite) version. The board is DIN-rail mounted using this high-quality mounting solution. It runs several minicom sessions monitoring/logging other embedded boards as well as a Postgres database and Java backend data collection application. Even over TightVNC, it feels snappy and doesn’t break a sweat (stays between 45 and 47C); it is using less than 1/4 of the available RAM (but would have used nearly all of the RAM on an RPi3).
Other upsides: 4K video (mainly of value for HTPC applications) and USB 3.0 – much more important because it makes it worthwhile to connect an external SSD which will be much faster and more reliable than uSD storage. The main downsides relative to the Raspberry Pi are: no WiFi/Bluetooth and no Pi-compatible camera connector. I didn’t need those for my application (which is rack-mounted and connected to Ethernet), but if you need either, you can easily solve them via USB connection.
For storage, I use Sandisk Extreme uSD cards. 64GB costs $11 and is plenty of storage for my application (I’m only using 6%); if I need more, storage or speed, I’ll use an external M.2 card connected via USB 3.0. Note: there is a huge difference in performance and reliability between SD storage cards used in RPi applications; some cards won’t work at all, some will work but at half the speed of others (see this performance comparison). I’ve tried a bunch and settled on the Sandisk Extreme which offer good speed with a cost only slightly higher than lesser cards; the benchmarks bear this out. If I were doing something more disk-intensive, I’d consider either a board with a native M.2 interface (like the Odroid M1) or an x86 board with a native SATA or M.2 interface.
Note: uSD cards aren’t meant for frequent writing (as in linux logs), so if you want your card to last, I strongly recommend using a utility like log2ram that creates a small RAM disk for the /var/log partition (you can add others) and then periodically flushes that partition to SD storage. This will dramatically lengthen the life of your SD card; see here for more info.