Messages in this thread | | | From | Baegjae Sung <> | Date | Fri, 30 Mar 2018 13:57:25 +0900 | Subject | Re: [PATCH] nvme-multipath: implement active-active round-robin path selector |
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2018-03-29 4:47 GMT+09:00 Keith Busch <keith.busch@intel.com>: > On Wed, Mar 28, 2018 at 10:06:46AM +0200, Christoph Hellwig wrote: >> For PCIe devices the right policy is not a round robin but to use >> the pcie device closer to the node. I did a prototype for that >> long ago and the concept can work. Can you look into that and >> also make that policy used automatically for PCIe devices? > > Yeah, that is especially true if you've multiple storage accessing > threads scheduled on different nodes. On the other hand, round-robin > may still benefit if both paths are connected to different root ports > on the same node (who would do that?!). > > But I wasn't aware people use dual-ported PCIe NVMe connected to a > single host (single path from two hosts seems more common). If that's a > thing, we should get some numa awareness. I couldn't find your prototype, > though. I had one stashed locally from a while back and hope it resembles > what you had in mind:
Our prototype uses dual-ported PCIe NVMe connected to a single host. The host's HBA is connected to two switches, and the two switches are connected to a dual-port NVMe SSD. In this environment, active-active round-robin path selection is good to utilize the full performance of a dual-port NVMe SSD. You can also fail over a single switch failure. You can see the prototype in link below. https://youtu.be/u_ou-AQsvOs?t=307 (presentation in OCP Summit 2018)
I agree that active-standby closer path selection is the right policy if multiple nodes attempt to access the storage system through multiple paths. However, I believe that NVMe multipath needs to provide multiple policy for path selection. Some people may want to use multiple paths simultaneously (active-active) if they use a small number of nodes and want to utilize full capability. If the capability of paths is same, the round-robin can be the right policy. If the capability of paths is different, a more adoptive method would be needed (e.g., checking path condition to balance IO).
We are moving to the NVMe fabrics for our next prototype. So, I think we will have a chance to discuss about this policy issue in more detail. I will continue to follow this issue.
> --- > struct nvme_ns *nvme_find_path_numa(struct nvme_ns_head *head) > { > int distance, current = INT_MAX, node = cpu_to_node(smp_processor_id()); > struct nvme_ns *ns, *path = NULL; > > list_for_each_entry_rcu(ns, &head->list, siblings) { > if (ns->ctrl->state != NVME_CTRL_LIVE) > continue; > if (ns->disk->node_id == node) > return ns; > > distance = node_distance(node, ns->disk->node_id); > if (distance < current) { > current = distance; > path = ns; > } > } > return path; > } > --
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