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

[James Bottomley <James.Bottomley@SteelEye.com>]

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).

James