On Mon, Jun 22, 2015 at 08:30:28AM +0200, Christoph Hellwig wrote:

quoted

Good to hear that we don't need BTT for XFS v5, can we make the
guarantee for all filesystems that may want to support DAX?  I still
think stacking is a natural fit for this problem.

I can't make any guarantees, especially not without verification.  But
if correctly implemented any filesystems that does out of place metadata
writes (and that includes a traditional log) and uses checksum to ensure
the integrity of these updates it should be fine.  You'd still have
the issue of sector atomicy of file I/O though.

Is ext4 one of the filesystems that copes with torn updates to the log?
I see there's a checksum in the tail of at least some blocks, but I'd
like someone who understands ext4 to reassure me that it also doesn't
need the ability to put its log on a BTT.