The default logging mode is delaylog. In delaylog mode, the effects of most system calls other than
pwrite(2) are guaranteed to be persistent approximately 3 seconds after the system call returns to the application. Contrast this with the behavior of most other file systems in which most system calls are not persistent until approximately 30 seconds or more after the call has returned. Fast file system recovery works with this mode.
rename(2) system call flushes the source file to disk to guarantee the persistence of the file data before renaming it. In the log and delaylog modes, the rename is also guaranteed to be persistent when the system call returns. This benefits shell scripts and programs that try to update a file atomically by writing the new file contents to a temporary file and then renaming it on top of the target file.