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

Jörn Engel
2008-02-26T17:03:44Z

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: