Re: [RFC] extending splice for copy offloading

From: Zach Brown
Date: Wed Sep 25 2013 - 15:07:02 EST

On Wed, Sep 25, 2013 at 03:02:29PM -0400, Anna Schumaker wrote:
> On Wed, Sep 25, 2013 at 2:38 PM, Zach Brown <zab@xxxxxxxxxx> wrote:
> >
> > Hrmph. I had composed a reply to you during Plumbers but.. something
> > happened to it :). Here's another try now that I'm back.
> >
> >> > Some things to talk about:
> >> > - I really don't care about the naming here. If you do, holler.
> >> > - We might want different flags for file-to-file splicing and acceleration
> >>
> >> Yes, I think "copy" and "reflink" needs to be differentiated.
> >
> > I initially agreed but I'm not so sure now. The problem is that we
> > can't know whether the acceleration is copying or not. XCOPY on some
> > array may well do some shared referencing tricks. The nfs COPY op can
> > have a server use btrfs reflink, or ext* and XCOPY, or .. who knows. At
> > some point we have to admit that we have no way to determine the
> > relative durability of writes. Storage can do a lot to make writes more
> > or less fragile that we have no visibility of. SSD FTLs can log a bunch
> > of unrelated sectors on to one flash failure domain.
> >
> > And if such a flag couldn't *actually* guarantee anything for a bunch of
> > storage topologies, well, let's not bother with it.
> >
> > The only flag I'm in favour of now is one that has splice return rather
> > than falling back to manual page cache reads and writes. It's more like
> > O_NONBLOCK than any kind of data durability hint.
> For reference, I'm planning to have the NFS server do the fallback
> when it copies since any local copy will be faster than a read and
> write over the network.

Agreed, this is definitely the reasonable thing to do.

- z
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