I’ve had 16 Pis running for about a week (with 16 GB SD cards) with no trouble. I’ve been running them with only 2 of 4 intake fans to maintain a power-to-airflow ratio that’s close to what I should expect from having all 40 of them running. I’ve been seeing case temperatures of roughly 31 °C (88 °F) and core temperatures of up to 77 °C (171 °F). 77 °C seems kind of hot to me, but it’s still acceptable for a BCM2835 core. I’ve read that 85 °C is supposed to be their limit, but this announcement by the Raspberry Pi foundation seems to indicate that 85 °C is an acceptable steady-state operating temperature.
Two nights ago, I decided to bring the remaining 24 Pis online. This required that I use 24 of my 8 GB SD cards . I didn’t anticipate any problems. I flashed 24 fresh 8 GB cards, brought the other 2 intake fans back online, and booted the remaining 24 Pis.
I went to the work of assigning static IPs to all of them. (Managing 40 machines through SSH without assigning static IPs is a hassle.) But, an hour or so in, I had to hard reset the whole system. As it came back online, I noticed that some machines didn’t appear to be booting up properly. (Their indicator lights were staying red.)
Since the biggest group of them failed, I’ve noticed that a couple of stragglers have also warped to the point that they no longer work. This is important to note, because I’ve been running the system with no filters and with the left side of the case off, which greatly increases airflow. So the warping is continuing, in spite of the fact that they’re probably at no more than 55 °C now.
I’m not really sure what to blame for this. The Pi’s SD card socket doesn’t back the card when it’s fully inserted, which seems like a weakness in the design that would allow this to happen. However, Kingston rates these SD4 series cards for 85 °C and I have a hard time believing that any of them got over 70 °C at any time. There’s also the fact that the 16 GB cards haven’t had the same problem, which leads me to believe this is specifically an issue with the SD4/8GB. Either way, I’m pretty sure the solution is to add a clip to the socket to back the SD card so it doesn’t warp.