From mboxrd@z Thu Jan 1 00:00:00 1970 From: "open_iscsi" Subject: Re: [PATCH RFC 2/2] implement transport scan callout for iscsi Date: Tue, 24 May 2005 21:28:18 -0400 Message-ID: <001101c560c9$0870f280$03031eac@ivivity.com> References: <42936441.0b798bab.39a4.ffff9774SMTPIN_ADDED@mx.googlegroups.com> <42939610.3070104@cs.wisc.edu> <1116976678.7710.34.camel@mulgrave> <000b01c560c0$42c59700$03031eac@ivivity.com> <1116982816.7710.58.camel@mulgrave> Mime-Version: 1.0 Content-Type: text/plain; format=flowed; charset="Windows-1252"; reply-type=original Content-Transfer-Encoding: 7bit Return-path: Received: from rwcrmhc13.comcast.net ([204.127.198.39]:419 "EHLO rwcrmhc13.comcast.net") by vger.kernel.org with ESMTP id S262227AbVEYB2U (ORCPT ); Tue, 24 May 2005 21:28:20 -0400 Sender: linux-scsi-owner@vger.kernel.org List-Id: linux-scsi@vger.kernel.org To: James Bottomley Cc: open-iscsi@googlegroups.com, Mike Christie , 'SCSI Mailing List' But it is not multi-pathing. Multi-pathing belongs at a higher layer. Yes, you could make multi-pathing perform a similar action but being at a higher layer, it means more operations to achieve the same thing. Also, multi-pathing is better suited for failover than multi-connections. There is another point here ... an HBA will probably use multi-connections irrespective of what higher layers want. Regarding the numbers, we get 400,000 IOPS with our hardware solution using multiple connections and multiple micro-engines. I have not tried multi-pathing but I can tell you that I had to count clocks to get that number and found that even a few extra clocks could mean a lot. So since multi-pathing takes a lot of extra clocks, then I think there is a benefit. However with a software solution the extra clocks for the multi-pathing may not be significant. I would think that you would want to let the lower layers do their best to get the best thruput and leave the failover logic to the upper layers. Eddy ----- Original Message ----- From: "James Bottomley" To: "open_iscsi" Cc: ; "Mike Christie" ; "'SCSI Mailing List'" Sent: Tuesday, May 24, 2005 9:00 PM Subject: Re: [PATCH RFC 2/2] implement transport scan callout for iscsi > 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 > >