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

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Date:	Mon, 30 Sep 2013 14:20:30 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Ric Wheeler <rwheeler@...hat.com>
Cc:	"Myklebust, Trond" <Trond.Myklebust@...app.com>,
	Zach Brown <zab@...hat.com>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	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>,
	"Schumaker, Bryan" <Bryan.Schumaker@...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

On Sat, Sep 28, 2013 at 11:20 PM, Ric Wheeler <rwheeler@...hat.com> wrote:

>>> I don't see the safety argument very compelling either.  There are real
>>> semantic differences, however: ENOSPC on a write to a
>>> (apparentlíy) already allocated block.  That could be a bit unexpected.
>>> Do we
>>> need a fallocate extension to deal with shared blocks?
>>
>> The above has been the case for all enterprise storage arrays ever since
>> the invention of snapshots. The NFSv4.2 spec does allow you to set a
>> per-file attribute that causes the storage server to always preallocate
>> enough buffers to guarantee that you can rewrite the entire file, however
>> the fact that we've lived without it for said 20 years leads me to believe
>> that demand for it is going to be limited. I haven't put it top of the list
>> of features we care to implement...
>>
>> Cheers,
>>     Trond
>
>
> I agree - this has been common behaviour for a very long time in the array
> space. Even without an array,  this is the same as overwriting a block in
> btrfs or any file system with a read-write LVM snapshot.

Okay, I'm convinced.

So I suggest

 - mount(..., MNT_REFLINK): *allow* splice to reflink.  If this is not
set, fall back to page cache copy.
 - splice(... SPLICE_REFLINK):  fail non-reflink copy.  With this app
can force reflink.

Both are trivial to implement and make sure that no backward
incompatibility surprises happen.

My other worry is about interruptibility/restartability.  Ideas?

What happens on splice(from, to, 4G) and it's a non-reflink copy?
Can the page cache copy be made restartable?   Or should splice() be
allowed to return a short count?  What happens on (non-reflink) remote
copies and huge request sizes?

Thanks,
Miklos
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