Embedded File System and power-off
I am working on an embedded application without any OS that needs the use of a File System. I've been over this many times with the people in the project and some agree with me that the system must make a proper shut down of the system whenever there is a power failure or else the file system might go crazy.
Some people say that it doesn't matter if you simply power off the system and let nature run its course, but I think that's one of the worst things to do, especially if you know this will bring you a problem and probably shorten your product's life span.
In the last paragraph I just assumed that it is a problem, but my question remains:
Does a power down have any effect on the file system?
Answer
Here is a list of various techniques to help an embedded system tolerate a power failure. These may not be practical for your particular application.
Use a Journaling File System - Can tolerate incomplete writes due to power failure, OS crash, etc. Most modern filesystems are journaled, but do your homework to confirm.
Unless your application needs the write performance, disable all write caching. Check your disk drivers for caching options. Under Linux/Unix, consider mounting the filesystem in sync mode.
Unless it must be writable, make it read-only. Try to keep your application executables and operating system files on their own partition(s), with write protections in place (e.g. mount read only in Linux). Your read/write data should be on its own partition. Even if your application data gets corrupted, your system should still be able to boot (albeit with a fail safe default configuration).
3a. For data that is only written once (e.g. Configuration Settings), try to keep it mounted as read-only most of the time. If there is a settings change mount is as R/W temporarily, update the data, and then unmount/remount it as read-only.
3b. Use a technique similar to 3a to handle application/OS updates in the field.
3c. If it is impractical for you to mount the FS as read-only, at least consider opening individual files as read-only (e.g. fp=fopen("configuration.ini", "r")).
If possible, use separate devices for your storage. Keeping things in separate partitions provides some protection, but there are still edge cases where a partition table may become corrupt and render the entire drive unreadable. Using physically separate devices further isolates against one corrupt device bringing down the whole system. In a perfect world, you would have at least 4 separate devices:
4a. Boot Loader
4b. Operating System & Application Code
4c. Configuration Settings
4e. Application Data
Know the characteristics of your storage devices, and control the brand/model/revision of devices used. Some hard disks ignore cache flush commands from the OS. We had cases where some models of CompactFlash cards would corrupt themselves during a power failure, but the "industrial" models did not have this problem. Of course, this information was not published in any datasheet, and had to be gathered by experimental testing. We developed a list of approved CF cards, and kept inventory of those cards. We periodically had to update this list as older cards became obsolete, or the manufacturer would make a revision.
Put your temporary files in a RAM Disk. If you keep those writes off-disk, you eliminate them as a potential source of corruption. You also reduce flash wear and tear.
Develop automated corruption detection and recovery methods. - All of the above techniques will not help you if the application simply hangs because a missing config file. You need to be able to recover as gracefully as possible:
7a. Your system should maintain at least two copies of its configuration settings, a "primary" and a "backup". If the primary fails for some reason, switch to the backup. You should also consider mechanisms for making backups whenever whenever the configuration is changed, or after a configuration has been declared "good" by the user (testing vs production mode).
7b. Did your Application Data partition fail to mount? Automatically run chkdsk/fsck.
7c. Did chkdsk/fsck fail to fix the problem? Automatically re-format the partition and get it back to a known state.
7d. Do you have a Boot Loader or other method to restore the OS and application after a failure?
7e. Make sure your system will beep, flash an LED, or something to indicate to the user what happened.
Power Failures should be part of your system qualification testing. The only way you will be sure you have a robust system is to test it. Yank the power cord from the system and document what happens. Try yanking the power at multiple points in the system operation (during runtime, while booting, mid configuration, etc). Repeat each test multiple times.
If you cannot mitigate all power failure problems, incorporate a battery or Supercapacitor into the system - Keep in mind that you will need a background process in your OS to initiate a graceful shutdown when power gets low. Also, batteries will require periodic testing and replacement with age.