On 01/07/2015 08:25 AM, 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. > > Cheers, > Sagi. I started looking at this last year (and Hannes' suggestion), and would love to join the discussion. Please add me to the list of those that wish to attend. -- Lee Duncan SUSE Labs -- To unsubscribe from this list: 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
