linux-kernel - Re: [Patch] ltt-relay-alloc mmap support (due to NFS lack of splice support)

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Message-ID: <20090323175820.GH24084@Krystal>
Date:	Mon, 23 Mar 2009 13:58:20 -0400
From:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
To:	Masahiro Tamori <masahiro.tamori@...il.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	ltt-dev <ltt-dev@...ts.casi.polymtl.ca>,
	linux-kernel@...r.kernel.org, linux-embedded@...r.kernel.org,
	linux-nfs@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
	Nick Piggin <npiggin@...e.de>
Subject: Re: [Patch] ltt-relay-alloc mmap support (due to NFS lack of
	splice support)

* Masahiro Tamori (masahiro.tamori@...il.com) wrote:
> 2009/3/21 Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>:
> > * Masahiro Tamori (masahiro.tamori@...il.com) wrote:
[...]
> >> Our customer will use NFS to store trace data not storage device for
> >> embedded devices.
> >> Since newer lttng use splice() even if NFS can not support splice(),
> >> I create a patch to support to mmap of ltt-relay-alloc.
> >>
[...]
> >> ltt-relay-alloc mmap support
> >>
> >> Splice syscall does not support NFS. We can not save trace data to
> >> NFS directory. If this feature is enabled, you can use mmap()
> >> instead of splice().
> >>
> >
> >
> > Hi Masahiro,
> >
> > Maybe we should consider implementing splice() support in NFS instead ?
> >
> > I removed the mmap support from the ltt-relay-alloc files because splice
> > is more efficient and does not require to vmap the pages.  Unless there
> > is a strong argument telling what in NFS makes it impossible to
> > implement splice(), I don't really see the gain in putting back the
> > old mmap() mechanism we had.
> >
> > Maybe the NFS people will have some information about this ?
> >
> > Thanks,
> >
> > Mathieu
> >
> 
> Hello Mathieu,
> 
> I think that the best solution is NFS can support splice() even if
> it cannot zero copy.
> 
> The splice() will be used by any other tools, hence NFS should support
> it sooner or later.
> 
> If technical issue is remained to support splice() in NFS,
> we should support mmap in LTTng until the problem is resolved.
> Embedded people will want to use LTTng with NFS,
> though this is a bad choice.
> 
> Thank you,
> Masahiro Tamori
> 

There was a LKML thread on NFS splice support back in 2006 :

http://lkml.indiana.edu/hypermail/linux/kernel/0603.3/2102.html

I don't know what happened with this ? I have tested NFS v2 and v3 and
have seen they do not support splice, but haven't tested NFSv4.

Mathieu

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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