Re: [Scst-devel] Fwd: Re: linuxcon 2010...

[James Bottomley <James.Bottomley@suse.de>]

On Mon, 2010-08-23 at 19:44 +0200, Bart Van Assche wrote:
> On Mon, Aug 23, 2010 at 6:59 PM, James Bottomley
> <James.Bottomley@suse.de> wrote:
> >
> > On Sun, 2010-08-22 at 18:10 -0400, Gennadiy Nerubayev wrote:
> > > [ ... ]
> > > Hi James,
> > >
> > > (disclaimer: I'm a hoi polloi SCST user)
> > >
> > > I'm not sure if I understand why there is a need for a replacement
> > > target to reuse existing code, and would definitely appreciate a brief
> > > explanation or a pointer to an earlier one.
> >
> > The best thread on the topic is this massive one:
> >
> > http://marc.info/?t=120109820100005
> >
> > I want replacement because evidence suggests that multiple things doing
> > the same thing don't get as much attention as a single one.  We need to
> > support STGT because it's the one that has the in-kernel user base.
> > Just breaking them constitutes an ABI problem under the new kernel
> > rules.
> >
> > > But even that aside, I'm
> > > curious if the criteria for what a replacement target must have for
> > > (at least potential) inclusion into the kernel were ever clearly
> > > outlined in the past. If they were, then there probably would have
> > > been things like interested contenders, deadlines, feature
> > > comparisons, code reviews, and so on, right?
> >
> > Yes, in that thread.
> >
> > My basic conclusion was that there's no incredible discriminator between
> > LIO and STGT (although there are reams written on which performs better
> > in which circumsances, is useful for clustering, supports ALUA, etc.
> > each with partisans for the features).  If the two communities can't
> > work together (as seems to be the case) and I have to choose one, I'll
> > go by what helps me which, as I've said before, are:
> >
> >     1. That it would be a drop in replacement for STGT (our current
> >        in-kernel target mode driver), since he only wanted a single
> >        SCSI target infrastructure.
> >
> >     2. That it used a modern sysfs based control and configuration
> >        plane.
> >
> >     3. That the code was reviewed as clean enough for inclusion.
> >
> >
> > > Now, I can't claim familiarity with the kernel development process, or
> > > any "political" workings in it. The aforementioned however would seem
> > > like a logical way of doing this since I assume that for whatever
> > > reason, there is a strict limit to only one generic SCSI target in the
> > > Linux kernel, and obviously as per this thread the current one is
> > > being replaced.
> >
> > Well, my preference would be to keep STGT.  However, I indicated to both
> > target infrastructures that if they could satisfy the above, I'd be OK
> > with replacing STGT, so I'm not about to go back on that after causing
> > quite a large amount of work.
> 
> And when did you indicate that ?

So 1. comes directly from what you quote below.  The sysfs thing is
buried in another long thread that I can't be bothered to dig through
and 3. is a basic requirement from anything for inclusion.

> Sorry James, but the above looks to me like an interesting attempt to
> rewrite history. What you repeated a few times in that thread from
> January 2008 is that you were not convinced that a kernel-based
> storage target could outperform a user-space target implementation. So
> at least the impression was created that you were not going to accept
> a kernel-based storage target for inclusion in the Linux kernel. In
> another thread from December 2008 you repeated that view . A literal
> quote from http://lkml.indiana.edu/hypermail/linux/kernel/0812.1/02168.html:

To be honest, I can't see how you arrive at that interpretation from the
quote below.  It specifically says "what it comes down to is that if we
had a kernel side protocol accelerator for STGT, the project would no
longer suffer from this theoretical failing. *Both* of you have such a
thing embedded in your respective submissions (all 74k LOC of them) so
can't you just enhance STGT with whichever one is better?"  That's
reluctant acceptance, not the blanket refusal you ascribe to it.

James


> <quote>
> The only identified failing of STGT (and it's theoretical, not
> demonstrated, although I can agree the theory looks correct) is that the
> user space packet processing may cause performance problems on high
> speed networks. We know from practical tests that these networks have
> to be above 1Gbit because the results were identical for STGT and SCST
> on a 1G network, so it's infiniband or 10Gbit ethernet.
> 
> So, what it comes down to is that if we had a kernel side protocol
> accelerator for STGT, the project would no longer suffer from this
> theoretical failing. *Both* of you have such a thing embedded in your
> respective submissions (all 74k LOC of them) so can't you just enhance
> STGT with whichever one is better ... actually, if you'd both bury the
> hatchet and work on the enhancement together taking the best of each
> project, we'd have something that worked much better and a unified user
> base and neither side would be able to claim sole credit ... just a
> thought.
> </quote>
> 
> While about two years ago you were not convinced that a kernel-based
> storage target could outperform a user-space based storage target,
> recently you announced that a kernel-based storage target is going to
> be integrated. I have no problem that someone changes his opinion, but
> you should not try to make people believe that you have always been in
> favor of a kernel-based storage target.
> 
> Bart.