[rsyslog] ultra-reliable disk files fsync vs. fdatasync

[david at lang.hm]

On Mon, 8 Jun 2009, Rainer Gerhards wrote:

> Hi all,
>
> I am working on the ultra-reliable file writing capability. I have revisited
> the doc and am considering fsync() vs. fdatasync(). From the man pages, it
> looks like fdatasync() is sufficient if I just need to ensure that any data
> written can also be read. The advantage of fdatasync() is that it is somewhat
> faster as not all meta data is necessarily written.
>
> I would appreciate if someone could comment on this issue.

my understanding is that if you are growing the file you need to do a 
fsync of the directory, but if you are re-writing part of an already 
allocated file fdatasync is the better thing to use.

David Lang