[lustre-devel] [PATCH 00/24] lustre - more cleanups including module reduction.

[NeilBrown]

On Wed, Jun 27 2018, Patrick Farrell wrote:

> Neil,
>
> We do indeed have such functionality (it’s called DVS and it’s
> basically a high speed file system projection framework, ala NFS but
> faster), so the ability to build lnet separately is valuable to us.
> While it is being open sourced under the GPL, I don’t think there’s
> any intention to try to upstream it.  The current code isn’t even
> usable off of Cray systems as it depends on info from user space (that
> is provided, in the end, from Cray proprietary hardware) to keep its
> connection/routing tables up to date.  That’s supposedly in the
> pipeline to get fixed, but it’s still pretty far from generally
> usable.
>
> But we’d still really appreciate it if lnet stayed separate.  Don’t
> know if that’s enough for you - I know sometimes *small* stuff is done
> for out of tree users.  Hopefully this meets that standard.
>

Ahh - DVS.  That answers a question I just asked in another email.
My google-skills don't seem to be up to locating the source code though
:-(

While I wouldn't knowingly break an interface used by some out-of-tree
code without good reason, it is hard to avoid if you don't know what the
out-of-tree code does.  It can be very tempting to remove something that
isn't being used, but that can certainly hurt out-of-tree code
sometimes.

A particular example I'm exploring at present is the dual data paths in
LNet.  Or maybe it is dual types of Memory Descriptors.
There is 'kiov' which uses kernel-virtual addresses and 'iovec' which
uses page+offset.
The kiov option isn't used in the client code and it seems likely that
the server-side code could be converted to use iovec without problems.

I'd like to remove the kiov as I wouldn't be able to justify its
existence when submitting the client-only code upstream.  But I don't
want to remove the option of having an alternate MD type if it really is
significantly more efficient in some context.
If I know whether DVS used kiov or iovec - and in what way - that would
help me to know if I might break something, and to be able to assess the
cost.

In my mind, the "standard" that you mention is always about
practicality.   Code needs to be maintainable - easy to understand and
hard to break.  If the LNet interface is clean and well documented in
the kernel, then I don't see why we would not at least attempt to
preserve it.

Thanks,
NeilBrown
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