Re: [LSF/MM TOPIC] iSCSI MQ adoption via MCS discussion

From: Bart Van Assche <Bart.VanAssche@sandisk.com>
To: "open-iscsi@googlegroups.com" <open-iscsi@googlegroups.com>,
	Hannes Reinecke <hare@suse.de>,
	"lsf-pc@lists.linux-foundation.org"
	<lsf-pc@lists.linux-foundation.org>
Cc: linux-scsi <linux-scsi@vger.kernel.org>,
	target-devel <target-devel@vger.kernel.org>,
	Oren Duer <oren@mellanox.com>, Or Gerlitz <ogerlitz@mellanox.com>
Subject: Re: [LSF/MM TOPIC] iSCSI MQ adoption via MCS discussion
Date: Fri, 9 Jan 2015 13:31:29 +0000	[thread overview]
Message-ID: <5EE87F5E6631894E80EB1A63198F964D040A6A8F@SACMBXIP03.sdcorp.global.sandisk.com> (raw)
In-Reply-To: 54AFBDDC.1000107@dev.mellanox.co.il

On 01/09/15 12:39, Sagi Grimberg wrote:
> On 1/8/2015 4:11 PM, Bart Van Assche wrote:
>> On 01/08/15 14:45, Sagi Grimberg wrote:
>>> Actually I started with that approach, but the independent connections
>>> under a single session (I-T-Nexus) violates the command ordering
>>> requirement. Plus, such a solution is specific to iSER...
>>
>> Which command ordering requirement are you referring to ? The Linux
>> storage stack does not guarantee that block layer or SCSI commands will
>> be processed in the same order as these commands have been submitted.
>
> I was referring to the iSCSI session requirement. I initially thought of
> an approach to maintain multiple iSER connections under a single session
> but pretty soon I realized that preserving commands ordering this way
> is not feasible. So independent iSER connections means independent
> iSCSI sessions (each with a single connection). This is indeed another
> choice, which we are clearly debating on...
>
> I'm just wandering if we are not trying to force-fit this model. How
> would this model look like? We will need to define another entity to
> track and maintain the sessions and to allocate the scsi_host. Will that
> be communicated to user-space? How will error recovery look like?

Hello Sagi,

As you probably remember scsi-mq support was added in the SRP initiator
by changing the 1:1 relationship between scsi_host and RDMA connection
into a 1:n relationship. I don't know how much work it would take to
implement a similar transformation in the SCSI initiator. Maybe we
should wait until Mike's workday starts such that Mike has a chance to
comment on this.

Bart.

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