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Date: Fri, 7 Oct 2016 08:16:41 -0400
From: Brian Foster <bfoster@redhat.com>
Subject: Re: [PATCH 37/63] xfs: implement CoW for directio writes
Message-ID: <20161007121641.GB58659@bfoster.bfoster>
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To: Christoph Hellwig <hch@lst.de>
Cc: "Darrick J. Wong" <darrick.wong@oracle.com>, david@fromorbit.com, linux-xfs@vger.kernel.org

On Fri, Oct 07, 2016 at 08:17:29AM +0200, Christoph Hellwig wrote:
> On Thu, Oct 06, 2016 at 06:02:25PM -0700, Darrick J. Wong wrote:
> > > Ok, kind of nasty with all of the various paths through get_blocks(),
> > > but hopefully that dies off with buffer heads.
> > 
> > It's possible Christoph might have further cleanup patches for reflink
> > that fix this?  <shrug>
> 
> I do.  It's stuck on not getting one of the corner cases right at the
> moment, and I had to take a break from it yesterday to go insane, but
> I hope I'll be able to post it soon.
> 
> > Yes, that would have been another way to implement it.  I think I
> > finally see your point about using the delalloc mechanism -- since we've
> > converted the buffered write path to iomap and therefore know exactly
> > how much userspace wants to write in both buffered and directio cases,
> > we could just allocate the cow extent right then and there, skipping the
> > overhead of writing a delalloc reservation and then changing it.
> > 
> > For buffered writes, though, it's nice to be able to use the DA
> > mechanism so that we can ask the allocator for as big of an extent as we
> > have contiguous dirty pages.  Hm.  I guess for directio then we could
> > just fill in the holes directly and convert any delalloc reservations
> > that happened already to be there, which requires only a single loop.
> > 
> > Will ponder this some more, thx for the pushback. :)
> 
> Having spent a lot of the time with the COW and non-COW I/O path
> lately here is my 2 cents:  delalloc for the buffered write path
> inherently make sense and mirrors what we do for non-COW I/O.  I
> have patches to rewrite parts of how we do it, but I see no
> reason to change that we are doing delayed allocations.
> 
> For the direct I/O path there is absolutely no point in doing delayed
> allocation and I plan to get rid of them for the next merge window.

Agreed, thanks.

Brian

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