Re: [RFC] extending splice for copy offloading

[Ric Wheeler]

On 09/26/2013 02:55 PM, Zach Brown wrote:
> On Thu, Sep 26, 2013 at 10:58:05AM +0200, Miklos Szeredi wrote:
> > On Wed, Sep 25, 2013 at 11:07 PM, Zach Brown <zab@xxxxxxxxxx> wrote:
> > > > A client-side copy will be slower, but I guess it does have the
> > > > advantage that the application can track progress to some degree, and
> > > > abort it fairly quickly without leaving the file in a totally undefined
> > > > state--and both might be useful if the copy's not a simple constant-time
> > > > operation.
> > > I suppose, but can't the app achieve a nice middle ground by copying the
> > > file in smaller syscalls?  Avoid bulk data motion back to the client,
> > > but still get notification every, I dunno, few hundred meg?
> > Yes.  And if "cp"  could just be switched from a read+write syscall
> > pair to a single splice syscall using the same buffer size.  And then
> > the user would only notice that things got faster in case of server
> > side copy.  No problems with long blocking times (at least not much
> > worse than it was).
> Hmm, yes, that would be a nice outcome.
>
> > However "cp" doesn't do reflinking by default, it has a switch for
> > that.  If we just want "cp" and the like to use splice without fearing
> > side effects then by default we should try to be as close to
> > read+write behavior as possible.  No?
> I guess?  I don't find requiring --reflink hugely compelling.  But there
> it is.
>
> > That's what I'm really
> > worrying about when you want to wire up splice to reflink by default.
> > I do think there should be a flag for that.  And if on the block level
> > some magic happens, so be it.  It's not the fs deverloper's worry any
> > more ;)
> Sure.  So we'd have:
>
> - no flag default that forbids knowingly copying with shared references
>    so that it will be used by default by people who feel strongly about
>    their assumptions about independent write durability.
>
> - a flag that allows shared references for people who would otherwise
>    use the file system shared reference ioctls (ocfs2 reflink, btrfs
>    clone) but would like it to also do server-side read/write copies
>    over nfs without additional intervention.
>
> - a flag that requires shared references for callers who don't want
>    giant copies to take forever if they aren't instant.  (The qemu guys
>    asked for this at Plumbers.)
>
> I think I can live with that.
>
> - z

This last flag should not prevent a remote target device (NFS or SCSI array) 
copy from working though since they often do reflink like operations inside of 
the remote target device....

ric


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