Shutting Down a Robot-Mounted Pi

[viggy96]

Originally Posted by Jared

Please don't give out misleading information on the internet based off of personal experience. The raspberry pi has documented issues that occur when power is cut, and it runs Linux.

The issues is more about hardware than software. The FTL (flash transition layer) will remap storage on the fly as part of its wear leveling feature. If you kill power during a remapping, you pretty much lose the whole disk. This is different from flash storage on something like a roboRIO or a wireless router, which doesn't have that additional layer of abstraction.

Linux does care when its file system is no longer usable.

Originally Posted by Foster

/sigh. No computer system that I've used from mainframes to Raspberry Pi, from MCP to Unix, including Windows and Linux deals well with hard power off when there are IOs in process. You've been lucky.

Lots of people have borked the SD card in a Raspberry Pi. The OP asked a good question, and people are trying to help. Your "whistling in the dark" isn't productive and I predict that someday you will either spend the night hand sewing inodes to restore a file system, or spend most of the day hoping your backup system really works while you boss yells at you about lost orders.

Hmmmm... Let's see, first, was referring purely to the software when I made those statements, not the hardware, and yes you can run into issues dealing with the SD card if the power is suddenly cut, IN SOME CASES. If you have set it up, correctly, and are using it as a vision co-processor, all you are doing is cutting the power while a programme is running. I have done exactly this hundreds, actually thousands, of times, with a Pi B+. I have used it for vision, and just unplugged it to turn it off. Every time, it booted correctly the next time I needed it. Of course, during setup, I made sure to never to that.

So please folks, don't question my intelligence.

[techhelpbb]

Originally Posted by viggy96

Hmmmm... Let's see, first, was referring purely to the software when I made those statements, not the hardware, and yes you can run into issues dealing with the SD card if the power is suddenly cut, IN SOME CASES. If you have set it up, correctly, and are using it as a vision co-processor, all you are doing is cutting the power while a programme is running. I have done exactly this hundreds, actually thousands, of times, with a Pi B+. I have used it for vision, and just unplugged it to turn it off. Every time, it booted correctly the next time I needed it. Of course, during setup, I made sure to never to that.

So please folks, don't question my intelligence.

It is possible if you are running Arch Linux with the swap file OFF that your systems will only be writing to storage when and if your application writes to storage. Raspbain quite often has swap enabled and written to the SD card. Depending on how swappiness (yes that's not made up it's a real thing) is setup your kernel may or may not use that swap if the memory space of your application starts to use up too much of the system's actual RAM.

This is not to question to you, or anyone's intelligence, just a point that currently I have 50,000+ copies of Linux online and running in various public and private clouds and datacenters. The smart thing to do is do a proper shutdown unless you know for a fact you've controlled the risk or have no better option.

For example I used to take Dell OptiPlex systems I had as leftovers from upgrades and put Ubuntu on them. Mess with the disk settings to spin down the hard drives and spin them up only on demand or every 6 hours (which ever was first and then spin them back down in 5 minutes). I would increase the write cache large enough that casual writes would come back from memory. I would remove the swap so that would not qualify to spin up the hard drive. Most of the drives lasted beyond 8 years and those that did not were already questionable (had de-allocated bad sectors that weren't mapped at the factory). These systems were used for print server farms like a giant version of the HP JetDirect driving down the cost of the printers because we could print to CUPS raw queues from Windows happily.

So again - there's plenty of simple ways shown here to accomplish this task from using a button and a script or SSH.
Why run any risk you don't need to?
Unlike other applications out there you really might take the chance you have an issue in the next match that hurts your competitive edge.

[apalrd]

As long as you never write the SD card, then the SD card's wear leveling feature shouldn't try to move blocks around.

This requires:
-Disabling swap entirely (it's generally a good idea to disable virtual memory entirely on real-time systems anyway)
-Mount the SD card as read-only
-Mount the temp directory onto a RAM disk

There are some tutorials on the internet explaining this, there are a lot of nuances since a number of automatically installed packages will try to write to disk (including syslog and fake-hwclock among others). Once you take care of all of those, you can set the fstab to mount the partition as read only so no program can write to the disk.

It's certainly possible for a carefully designed system to be turned off by pulling power. Virtually all embedded systems including the roboRIO operate in this way.