On 05/24/05 21:00, James Bottomley wrote: > 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... 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. MC/S is a good thing. Luben P.S. As Eddy mentioned multipathing sits at a higher level, namely at the session level, where you open a different session to a different port (à la SCSI parlé). > 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 - : send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
