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

["Jörn Engel" <joern@logfs.org>]

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().


Jörn

-- 
Joern's library part 10:
http://blogs.msdn.com/David_Gristwood/archive/2004/06/24/164849.aspx