Re: Proposal for "proper" durable fsync() and fdatasync()

[Jamie Lokier <jamie@shareable.org>]

Jörn Engel wrote:
> On Tue, 26 February 2008 15:28:10 +0000, Jamie Lokier wrote:
> > 
> > > One interesting aspect of this comes with COW filesystems like btrfs or
> > > logfs.  Writing out data pages is not sufficient, because those will get
> > > lost unless their referencing metadata is written as well.  So either we
> > > have to call fsync for those filesystems or add another callback and let
> > > filesystems override the default implementation.
> > 
> > Doesn't the ->fsync callback get called in the sys_fdatasync() case,
> > with appropriate arguments?
> 
> My paragraph above was aimed at the sync_file_range() case.  fsync and
> fdatasync do the right thing within the limitations you brought up in
> this thread.  sync_file_range() without further changes will only write
> data pages, not the metadata required to actually access those data
> pages.  This works just fine for non-COW filesystems, which covers all
> currently merged ones.
> 
> With COW filesystems it is currently impossible to do sync_file_range()
> properly.  The problem is orthogonal to your's, I just brought it up
> since you were already mentioning sync_file_range().

You're right.  Though, doesn't normal page writeback enqueue the COW
metadata changes?  If not, how do they get written in a timely
fashion?

-- Jamie