Re: fsync() after close() and re-open -- insights appreciated

[Amir Goldstein <amir73il@gmail.com>]

On Thu, May 3, 2018 at 3:30 PM, Niklas Hambüchen <mail@nh2.me> wrote:
> Hello,
>
> here's a simple question
> I'm having trouble finding answers to that is
> probably trivial to answer for file system developers.
>

Not trivial :)

Are you asking the question without being aware of all the discussions
that led to this patch https://lkml.org/lkml/2018/4/23/994

> Does a sequence of close()/re-open()/fsync() provide the same durability
> guarantees as fsync()/close()?

The short answer is no, the latter provides a better guaranty.
The longer answer is that durability guarantees depends on kernel version,
because situation has been changing in v4.13, v4.14 and now again in
v4.17-rc and stable kernels.

>
> http://pubs.opengroup.org/onlinepubs/009695399/functions/fsync.html
> specifies that
>
>   "The fsync() function shall request that all data for the open file
> descriptor named by fildes is to be transferred to the storage device"
>
> which seems to restrict the statement to "for the open file descriptor",
> possibly suggesting that close()/open()/fsync() may not have the desired
> effect.
>
> Being able to link to an authorative answer would be very appreciated.
>

Sadly, there is no documentation at the level that you desire.

Thanks,
Amir.