linux-kernel - Re: [RFC] extending splice for copy offloading

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Date:	Fri, 27 Sep 2013 13:50:13 -0700
From:	Zach Brown <zab@...hat.com>
To:	"J. Bruce Fields" <bfields@...ldses.org>
Cc:	Ric Wheeler <rwheeler@...hat.com>,
	Miklos Szeredi <miklos@...redi.hu>,
	Anna Schumaker <schumaker.anna@...il.com>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
	"linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
	Trond Myklebust <Trond.Myklebust@...app.com>,
	Bryan Schumaker <bjschuma@...app.com>,
	"Martin K. Petersen" <mkp@....net>, Jens Axboe <axboe@...nel.dk>,
	Mark Fasheh <mfasheh@...e.com>,
	Joel Becker <jlbec@...lplan.org>,
	Eric Wong <normalperson@...t.net>
Subject: Re: [RFC] extending splice for copy offloading


> > >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.)
> 
> Why not implement only the last flag only as  the first step?  It seems
> like the simplest one.  So I think that would mean:
> 
> 	- no worrying about cancelling, etc.
> 	- apps should be told to pass the entire range at once (normally
> 	  the whole file).
> 	- The NFS server probably shouldn't do the internal copy loop by
> 	  default.
> 
> We can't prevent some storage system from implementing a high-latency
> copy operation, but we can refuse to provide them any help (providing no
> progress reports or easy way to cancel) and then they can deal with the
> complaints from their users.

I can see where you're going with that, yeah.

It'd make less sense as a splice extension, then, perhaps.  It'd be more
like a generic entry point for the existing ioctls.  Maybe even just
defining the semantics of a common ioctl.

Hmm.

> Also, I don't get the first option above at all.  The argument is that
> it's safer to have more copies?  How much safety does another copy on
> the same disk really give you?  Do systems that do dedup provide
> interfaces to turn it off per-file?

Yeah, got me.  It's certainly nonsense on a lot of FTL logging
implementations (which are making their way into SMR drives in the
future).

> But I understand that Zach's tired of the woodshedding and I could live
> with the above I guess....

No, it's fine.  At least people are expressing some interest in the
interface!  That's a marked improvement over the state of things in the
past.

- z
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