linux-kernel - Re: [patch 0/3] make splice more generic

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Date:	Mon, 11 May 2009 17:17:00 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	torvalds@...ux-foundation.org
CC:	miklos@...redi.hu, jens.axboe@...cle.com, max@...mpel.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 0/3] make splice more generic

On Thu, 7 May 2009, Linus Torvalds wrote:
> On Thu, 7 May 2009, Miklos Szeredi wrote:
> > 
> > One more generalization would be to allow splice to work on two
> > non-pipes, using an internal intermediate pipe, a-la do_splice_direct().
> 
> You can't do that without some painful issues.
> 
> Or rather, you can only do it trivially for the one case where we 
> _already_ do that, namely "sendfile()". That's exactly what sendfile() is 
> now.

Hmm yeah, almost exactly: sendfile doesn't have a "destination offset"
argument.

> That said, maybe we could allow it in a few more cases. Or maybe people 
> think the simplification in user interfaces is worth making the IO be 
> non-interruptible (but still killable, for example, at which point the 
> buffered data really is dropped - but that's not different from having 
> the buffers in user space, so at that point it's ok).

That's a hard decision.  I think it's better to leave it in its
current form.  That means more complexity in the apps, but less
unexpected nastiness from being non-interruptible.

One more interesting use of splice() would be for things like speeding
up "cp" on network filesystems and similarly for fs which can
refcount/COW data blocks.  Seems like BTRFS already has an ioctl for
this, wouldn't it be nice to do it with splice()?

Thanks,
Miklos
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