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

James Bottomley
2005-05-25T01:00:37Z

On Tue, 2005-05-24 at 20:25 -0400, open_iscsi wrote:
> The MC/S feature of iSCSI is not multi-pathing. Multi-pathing would be the
> use of multiple sessions to reach the same target. Generally the two
> sessions would use the same InitiatorName+ISID but use different Target
> Portal Groups at the target. In SCSI terms, it is the same initiator
> accessing different SCSI ports.

Well, yes, every driver vendor with a multi-path solution in-driver that
made a single presentation to the mid-layer has argued that one...

The bottom line is that implementation must be in-driver. So every
driver doing it this way has to have their own separate multi-path
implementation. Whether you call it FC/AL or MC/S (or any of the other
buzz acronyms) it's still a driver implementation of pathing.

> MC/S can be used to improve band width of a session without using
> multi-pathing and it belongs in the driver because it is hidden from the
> upper layers. Think of it like parallel wires, each carrying separate (but
> sequenced) commands in parallel.

So far, no-one has been able to produce any figures to show that MC/S is
significantly better than symmetric active dm-multipath to an iSCSI
target, but if you have them, please publish them.

Hiding something from the upper layers which the upper layers could do
equally well themselves is what's considered wrong: it adds code bloat
with no tangible benefit.

James