Re: fsync and journal_async_commit mount options

[tytso@mit.edu]

On Tue, Aug 18, 2015 at 09:59:58AM +0000, Alexey Skidanov wrote:
> 
> My question is regarding the barrier=0/1 mount option.  On the one
> hand, the "mount" man page says "This enables/disables the use of
> write barriers in the jbd code". From there I understand that this
> option changes the behaviour of journalling code only. But on the
> other hand, this option change the the behaviour of fsync() (at
> least in case of ext4): the flush request is issued only if barrier
> mount option is 1.

The bug is in the mount man page.  It may have been true for ext3 oh,
a decade or more ago when we didn't have an optimized fdatasync(2)
implementation.  (Ext4 may have predated Linux *having* fdatasync(2)
support.)  But no one has bothered to update documentation, because
all of the people who care knew what it meant, and most users don't
bother changing the defaults.

Cheers,

					- Ted