Re: [PATCH 0/6] block atomic writes for XFS

[Christoph Hellwig <hch@lst.de>]

On Wed, Feb 21, 2024 at 08:56:15AM -0800, Darrick J. Wong wrote:
> Hmm.  For rt reflink (whenever I get back to that, ha) I've been
> starting to think that yes, we actually /do/ want to have a log item
> that tracks the progress of remap and cow operations.  That would solve
> the problem of someone wanting to reflink a semi-written rtx.
> 
> That said, it might complicate the reflink code quite a bit since right
> now it writes zeroes to the unwritten parts of an rt file's rtx so that
> there's only one mapping record for the whole rtx, and then it remaps
> them.  That's most of why I haven't bothered to implement that solution.

I'm still not sure that supporting reflinks for rtextsize > 1 is a good
idea..

> > I'm not planning to make you do it, because such a log item would
> > generally be pretty useful for always COW mode.
> 
> One other thing -- while I was refactoring the swapext code into
> exch{range,maps}, it occurred to me that doing an exchange between the
> cow and data forks isn't possible because log recovery won't be able to
> do anything.  There's no ondisk metadata to map a cow staging extent
> back to the file it came from, which means we can't generally resume an
> exchange operation.

Yeah.

> > Also if a file system supports logging data (which I have an
> > XFS early prototype for that I plan to finish), we can even do
> > the small double writes more efficiently than the application,
> > all through the same interface.
> 
> Heh.  Ted's been trying to kill data=journal.  Now we've found a use for
> it after all. :)

Well..  unconditional logging of data just seems like a really bad idea.
Using it as an optimization for very small and/or synchronous writes
is a pretty common technique.