Re: [PATCH RFC 2/2] implement transport scan callout for iscsi

Dmitry Yusupov
2005-05-25T18:32:16Z

On Wed, 2005-05-25 at 14:04 -0400, James Bottomley wrote:
> On Wed, 2005-05-25 at 11:18 -0400, Luben Tuikov wrote:
> > MC/S in iSCSI can be seen as a "wide port" in SAS.
> >
> > That is, commands are ordered, nexus is the same, going to the same port,
> > etc, etc, etc. MC/S, has nothing to do with multipathing, which sits above
> > the nexus level. With MC/S the nexus is the same.
>
> When I use the term "multi-pathing" I mean multiple virtual paths may be
> traversed to get a command from an application to a target device.
> Under that definition, dm-multipath, MC/S and even network bonding are
> all examples of multi-pathing.
>
> The visibility of the coding is what I have an issue with. bonding
> could be inherited invisibly from the network but MC/S has to be
> explicitly coded in the software initiator whereas dm-multipath is done
> above the driver: one code base for all multi-path implementations.
>
> > MC/S is a good thing.
>
> a) It's optional, so you can't rely on it.
> b) it requires explicit coding in the driver which is a big negative
> since you can't leverage our existing multi-path code (i.e. more bug
> prone)
> c) The feature set it provides to Linux is identical to the feature set
> that dm-multipath provides.
>
> It's pointless to add support for an optional feature that provides no
> additional benefit (and its detrimental when the only addition is a
> potential negative impact to the code quality).

Those are very strong arguments against having MC/S implemented for the
software iSCSI initiator. Especially true is (a). Not that many targets
offers MC/S, since according to RFC it implies ERL=2.

Now, back to open-iscsi initiator business and its mainline acceptance.
We are going to met your requirement and will remove multi-connection
support before next patch-set submission. As Mike C. mentioned before,
it is fairly easy to do since the whole connection management resides in
user-space. Though it will exists as a kernel patches on our web site
(http://www.open-iscsi.org) for those who wonders.

Are there are some other requirements we need to met besides MC/S
removal ?

Thanks for the good explanation of your point of view!
Dima

> James
>
>