#fsync

PostgreSQL 對 fsync() 的行為傷腦筋…

FOSDEM 2019 上的演講，討論 PostgreSQL 在確保 ACID 特性中的 Durability 時遇到 fsync() 的行為跟預想的不一樣 (主要是當 fsync() 失敗的行為)：「PostgreSQL vs. fsync」。

在「PostgreSQL vs. fsync. How is it possible that PostgreSQL used fsync incorrectly for 20 years, and what we’ll do about it.」這邊的 Q&A 形式的訪談有快速描述了短期的計畫與長期的想法：

The short-term solution is ensuring that we detect fsync errors reliably at least on sufficiently recent kernels (since…

Use FileChannel.force(boolean) or FileDescriptor.sync() to force data to be persistent on disk. Either of them can work. FileChannel.force use FileDispacther.force[1] and it calls fdatasync or fsync in Java 8. 

When you use OutputStream.flush, it does not guarantee the data to be written to disk, just flush it to OS. Better to use FileOutputStream.getChannel().force(true) or FileOutputStream.getFD().sync() to guarantee the persistency, performance might not be good.

Special Thanks to Yongkun. He wrote very good blog post. [2]

[1] http://hg.openjdk.java.net/jdk8/jdk8/jdk/file/687fd7c7986d/src/solaris/native/sun/nio/ch/FileDispatcherImpl.c#l141