[stgt] [PATCH RFC] add support of sheepdog backing store

[Hitoshi Mitake]

At Thu, 26 Sep 2013 19:29:06 -0700,
ronnie sahlberg wrote:
> 
> On Thu, Sep 26, 2013 at 6:59 PM, Dan Mick <dan.mick at inktank.com> wrote:
> >
> >>> Some might say this had better to be a dynamic loadable
> >>> library. Either is fine by me because it doesn't depend on any
> >>> original C header files.
> >>
> >>
> >> I'm ccing people who would be interested in modular backing
> >> stores, Ronnie Sahlberg and Andy Grover. I think the sheepdog backing
> >> store support doesn't have to be a module because it doesn't create
> >> any new dependencies. Any comments?
> >
> >
> > It seems that you've accomplished this by including structure and constant
> > definitions (and presumably protocol-marshalling routines) in the tgt source
> > file itself, which seems to me to be the wrong way to do it, but it does
> > free you from requiring headers or libraries, so
> > technically speaking, true.
> >
> > I wouldn't write the plugin this way (copying code), but that's me.
> 
> 
> What Dan said!
> Since you don't add any new dependencies it shouldn't matter if this
> is built-in or a module.
> 
> However, you do this at the cost of having a full blown
> re-implementation (or it is a copy?) of a sheepdog client in the
> backing store file.
> I assume there are client libraries for sheepdog available?
> 
> So I think from a maintenance viewpoint, it would probably be better
> if you had the backend just be a thin layer that would call out to and
> link to the sheepdog client library.
> I.e. link to a sheedog library instead of reimplementing it in the backend.
> And in that case then this should be a module.

Thanks for your comment, Dan and Ronnie.

Currently, sheepdog doesn't have its client library. We have a plan to
implement it. But it will take time.

In addition, the backing store support of sheepdog isn't a simple copy
(from QEMU). We can't re-implement it by simple copy because current
sheepdog users, QEMU and tgt, have very different I/O models. e.g.
 QEMU is a heavy user of coroutine.

After implementing the library, the backing store support of sheepdog
can be a module. But for now, this patch would be optimal because it
doesn't increase the cost of tgt maintenance (the patch simply adds a
single file).

Thanks,
Hitoshi
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