意外關閉對Linux的危害是否和對其他作業系統的危害一樣大？請繼續閱讀，我們將研究災難性系統關閉對Linux檔案系統的影響。

今天的問答環節是由SuperUser提供的，SuperUser是Stack Exchange的一個分支，是一個由社群驅動的問答網站分組。

問題

超級使用者讀者User208554對Linux檔案結構很好奇，並關心他正在開發的應用程式/安裝：

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.

判決結果如何？

答案

SuperUser contributor l0b0提供了對日誌/非日誌檔案系統的一些見解：

This would depend on

1. whether you’re using a journaling file system and
2. how well the applicati*** are able to handle aborted processing.

C***ider 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.

斯圖建議將作業系統和資料分離，並增加一個備用電池：

To help minimise the possibility of OS corruption, it’s probably best to have separate “system” and “data” partiti*** 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.

最後，Jenny D擴充套件了日誌檔案系統的建議：

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 ********* 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 c***ider 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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