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Spørsmålet
SuperUser-leser User208554 er nysgjerrig på Linux-filstrukturer og bekymret for en app / installasjon han jobber med:
I am developing an application on a Linux embedded board (runs Debian) e.g. Raspberry Pi, Beagle Board/Bone, or olimex. The boards works on an environment that the electricity is cut unexpectedly (it is far too complicated to place PSU, etc.) and it would happen every day a couple times. I wonder if the unexpected power cuts would cause problem on the Linux Operation System? If it is something that I should worry about, what would you suggest to prevent the damages on OS against the unexpected power cuts?
PS. The application needs to writes some data to the storage medium (SD card), I think it would not be suitable to mount it as read-only.
Så hva er dommen?
Svaret
SuperUser bidragsyter l0b0 gir litt innsikt i journaling / ikke-journaling filsystemer:
This would depend on
- whether you’re using a journaling file system and
- how well the applications are able to handle aborted processing.
Consider for example an application which processes a file and writes the results as they are computed (one output line per input line) to another file. If the power is cut during processing, and the same application is run after restarting, it can’t just restart processing from the start of the input file – that would mean the output file would contain duplicate information.
It could be very difficult to say anything definite about a hypothetical complex system, but most stable Linux software seems to be able to handle crashes quite nicely.
Stu foreslår å skille operativsystemet og dataene, samt legge til et batteri backup:
To help minimise the possibility of OS corruption, it’s probably best to have separate “system” and “data” partitions on the SD card. That way you can mount the “system” partition read-only and use a highly-resilient FS on the “data” partition.
Additionally, most of those boards have very low power requirements, so a battery backup is possible. The “LiPo rider” board for the Raspberry Pi can be used as a basic UPS to provide a clean shutdown on power loss.
Til slutt utvider Jenny D på journaling filsystem forslaget:
Unexpected power cuts may cause corruption of file system data – e.g. if a process has started writing to a file, but not yet finished writing it, the file may end up only half written. Now imagine if the power cut happens when you’re half way through a kernel upgrade…
As l0b0 wrote, using a journaling file system will help, since it will be able to keep track of what has actually gotten done. In addition to the wikipedia info that l0b0 linked, you may be interested in Do Journaling Filesystems Guarantee Against Corruption After a Power Failure as well.
You as a programmer obviously need to consider carefully how to handle writing to files so that it becomes an atomic process (i.e. it’s either fully done or not done at all, but never ever half done). It’s a fairly complex issue.
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