意外关闭对Linux的危害是否和对其他操作系统的危害一样大？请继续阅读，我们将研究灾难性系统关闭对Linux文件系统的影响。

今天的问答环节是由SuperUser提供的，SuperUser是Stack Exchange的一个分支，是一个由社区驱动的问答网站分组。

问题

超级用户读者User208554对Linux文件结构很好奇，并关心他正在开发的应用程序/安装：

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.

判决结果如何？

答案

SuperUser contributor l0b0提供了对日志/非日志文件系统的一些见解：

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.

斯图建议将操作系统和数据分离，并增加一个备用电池：

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.

最后，Jenny D扩展了日志文件系统的建议：

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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