Author: Kerry Scharfglass / Source: Hackaday
Filesystems for computers are not the best bet for embedded systems. Even those who know this fragment of truth still fall into the trap and pay for it later on while surrounded by the rubble that once was a functioning project. Here’s how it happens.
The project starts small, with modest storage needs.
It’s just a temperature logger and you want to store that data, so you stick on a little EEPROM. That works pretty well! But you need to store a little more data so the EEPROM gets paired with a small blob of NOR flash which is much larger but still pretty easy to work with. Device settings go to EEPROM, data logs go to NOR. That works for a time but then you remember that people on the Internet are all about the Internet of Things so it’s time to add WiFi. You start serving a few static pages with that surprisingly capable processor and bump into storage problems again so the NOR flash gets replaced with an SD card and now the logs go there too. Suddenly you’re dealing with multiple files and want access on a computer so a real filesystem is in order. FAT is easy, so the card grows a FAT filesystem. Everything is great, but you start to notice patches missing from the logs. Then the SD card gets totally corrupted. What’s going on? Let’s take a look at the problem, and how to reach embedded file nirvana.
How Filesystems Organize Files
You’ve had a surprise Learning Opportunity (AKA, you screwed something up). In the cold garden shed the power supply for your little device isn’t quite as reliable as you though and it tends to suddenly shut down. It turns out FAT (and many other filesystems) aren’t especially tolerant of the kinds of faults you find in these environments. You know what is? littleFS, a BSD licensed open source filesystem by ARM, designed for small devices.
Let’s get pedantic for a moment; what is a filesystem anyway? It’s nothing more than the way data is organized on a storage medium. For a very simple system with simple storage needs, like our original EEPROM temperature logger, there might not really be a “system” per-se. EEPROMs are typically good at byte-resolution addressing so the most direct way to store temperature data might be as simple as “each byte is a sample.” Coupled with knowledge of the sampling interval and time of first boot that would be sufficient for a simple application. In such a system you might say that the EEPROM contains one “file” — the single list of temperature records.
In more complex systems it wouldn’t be surprising to discover that more organization is needed. The advanced temperature logger had a small static website, a file of configuration settings, and the data logs. You could certainly keep all of these in one “file” and hard code the address offsets…
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