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

[Hannes Reinecke <hare-l3A5Bk7waGM@public.gmane.org>]

On 01/07/2015 05:25 PM, Sagi Grimberg wrote:
> Hi everyone,
> 
> Now that scsi-mq is fully included, we need an iSCSI initiator that
> would use it to achieve scalable performance. The need is even greater
> for iSCSI offload devices and transports that support multiple HW
> queues. As iSER maintainer I'd like to discuss the way we would choose
> to implement that in iSCSI.
> 
> My measurements show that iSER initiator can scale up to ~2.1M IOPs
> with multiple sessions but only ~630K IOPs with a single session where
> the most significant bottleneck the (single) core processing
> completions.
> 
> In the existing single connection per session model, given that command
> ordering must be preserved session-wide, we end up in a serial command
> execution over a single connection which is basically a single queue
> model. The best fit seems to be plugging iSCSI MCS as a multi-queued
> scsi LLDD. In this model, a hardware context will have a 1x1 mapping
> with an iSCSI connection (TCP socket or a HW queue).
> 
> iSCSI MCS and it's role in the presence of dm-multipath layer was
> discussed several times in the past decade(s). The basic need for MCS is
> implementing a multi-queue data path, so perhaps we may want to avoid
> doing any type link aggregation or load balancing to not overlap
> dm-multipath. For example we can implement ERL=0 (which is basically the
> scsi-mq ERL) and/or restrict a session to a single portal.
> 
> As I see it, the todo's are:
> 1. Getting MCS to work (kernel + user-space) with ERL=0 and a
>    round-robin connection selection (per scsi command execution).
> 2. Plug into scsi-mq - exposing num_connections as nr_hw_queues and
>    using blk-mq based queue (conn) selection.
> 3. Rework iSCSI core locking scheme to avoid session-wide locking
>    as much as possible.
> 4. Use blk-mq pre-allocation and tagging facilities.
> 
> I've recently started looking into this. I would like the community to
> agree (or debate) on this scheme and also talk about implementation
> with anyone who is also interested in this.
> 
Yes, that's a really good topic.

I've pondered implementing MC/S for iscsi/TCP but then I've figured my
network implementation knowledge doesn't spread that far.
So yeah, a discussion here would be good.

Mike? Any comments?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
hare-l3A5Bk7waGM@public.gmane.org			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg)

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