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

open_iscsi
2005-05-25T00:25:32Z

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.

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.

Eddy

----- Original Message -----
From: "James Bottomley" <James.Bottomley@SteelEye.com>
To: "Mike Christie" <michaelc@cs.wisc.edu>
Cc: <open-iscsi@googlegroups.com>; "'SCSI Mailing List'"
<linux-scsi@vger.kernel.org>
Sent: Tuesday, May 24, 2005 7:17 PM
Subject: Re: [PATCH RFC 2/2] implement transport scan callout for iscsi

>
> On Tue, 2005-05-24 at 14:01 -0700, Mike Christie wrote:
>> > It's a leading transport connection of the session1 (above). Quoting
>> > the
>> > spec (http://www.faqs.org/rfcs/rfc3720.html):
>> >
>> > - According to [SAM2], the I_T nexus is a relationship
>> > between a SCSI Initiator Port and a SCSI Target Port. For iSCSI,
>> > this relationship is a session, defined as a relationship between
>> > an iSCSI Initiator's end of the session (SCSI Initiator Port) and
>> > the iSCSI Target's Portal Group.
>> >
>> > The session itself is not a "physical connection"; it aggregates one or
>> > more
>> > transport connections between a given SCSI Initiator Port and a SCSI
>> > Target
>> > Port. There is always at least one (leading) connection.
>> >
>>
>> So just to be clear, open-iscsi can support multiple connections per
>> session. Do you want us to completely remove this feature for mainline?
>> I know you and christoph have given me this answer many times before,
>> but not seeing a reply to Nicholas's question about just disabling may
>> have created some doubt as to the extent people have to go? Since
>> open-iscsi pushed the connection management code to userspace, removing
>> MCS from the driver will not be too terrible a job for us though.
>>
>> The connection dir for single connection sessions though is just a nice
>> way to export the kernel structure's info and have it also reflect the
>> iSCSI RFC's definitions at the same time. For sfnet we used to throw
>> everything in one dir becuase it did not have a connection structure so
>> it simplified refcounting.
>
> My current position on MC/S is that it runs counter to the no multi-
> pathing in the drivers policy, so should not be done.
>
> As far as I can see it's an optional add on to the iSCSI standard which
> doesn't improve the feature set or provide any value add over the
> mandatory explicit multi-path support in rfc3720, which is easy to do
> via dm-multipath.
>
> James
>
>