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This means that nr_active is > maintained per controller. If multiple NVMe namespaces are created and > attached to the same controller, their I/O activity is accumulated in > the single ctrl->nr_active counter. > > In contrast, libmultipath defines nr_active in struct mpath_device, > which is referenced from struct nvme_ns. Even if we add code to update > mpath_device->nr_active, that accounting would effectively be per > namespace, not per controller. Right, I need to change that back to per-controller. > > The nr_active value is used by the queue-depth policy. Currently, > mpath_queue_depth_path() accesses mpath_device->nr_active to make > forwarding decisions. However, if mpath_device->nr_active is maintained > per namespace, it does not correctly reflect controller-wide load when > multiple namespaces share the same controller. Yes > > Therefore, instead of maintaining a separate nr_active in struct > mpath_device, it may be more appropriate for mpath_queue_depth_path() to > reference ns->ctrl->nr_active directly. In that case, nr_active could be > removed from struct mpath_device entirely. > I think so, but we will need scsi to maintain such a count internally to support this policy. And for NVMe we will need some abstraction to lookup the per-controller QD for a mpath_device. Thanks for checking!