This adds a document about nvme-multipath and policies supported by the Linux NVMe host driver, and also each policy's best scenario. Signed-off-by: Guixin Liu <kanie@xxxxxxxxxxxxxxxxx> --- Hi, We found that we should take care of the throughput of each path in service-time policy, so separate the doc patch. And continue working on service-time policy patch. Changes from v1 to v2: - Remove service-tome policy. Documentation/nvme/nvme-multipath.rst | 72 +++++++++++++++++++++++++++ 1 file changed, 72 insertions(+) create mode 100644 Documentation/nvme/nvme-multipath.rst diff --git a/Documentation/nvme/nvme-multipath.rst b/Documentation/nvme/nvme-multipath.rst new file mode 100644 index 000000000000..97ca1ccef459 --- /dev/null +++ b/Documentation/nvme/nvme-multipath.rst @@ -0,0 +1,72 @@ +.. SPDX-License-Identifier: GPL-2.0 + +==================== +Linux NVMe multipath +==================== + +This document describes NVMe multipath and its path selection policies supported +by the Linux NVMe host driver. + + +Introduction +============ + +The NVMe multipath feature in Linux integrates namespaces with the same +identifier into a single block device. Using multipath enhances the reliability +and stability of I/O access while improving bandwidth performance. When a user +sends I/O to this merged block device, the multipath mechanism selects one of +the underlying block devices (paths) according to the configured policy. +Different policies result in different path selections. + + +Policies +======== + +All policies follow the ANA (Asymmetric Namespace Access) mechanism, meaning +that when an optimized path is available, it will be chosen over a non-optimized +one. Current the NVMe multipath policies include numa(default), round-robin and +queue-depth. + +To set the desired policy (e.g., round-robin), use one of the following methods: + 1. echo -n "round-robin" > /sys/module/nvme_core/parameters/iopolicy + 2. or add the "nvme_core.iopolicy=round-robin" to cmdline. + + +NUMA +---- + +The NUMA policy selects the path closest to the NUMA node of the current CPU for +I/O distribution. This policy maintains the nearest paths to each NUMA node +based on network interface connections. + +When to use the NUMA policy: + 1. Multi-core Systems: Optimizes memory access in multi-core and + multi-processor systems, especially under NUMA architecture. + 2. High Affinity Workloads: Binds I/O processing to the CPU to reduce + communication and data transfer delays across nodes. + + +Round-Robin +----------- + +The round-robin policy distributes I/O requests evenly across all paths to +enhance throughput and resource utilization. Each I/O operation is sent to the +next path in sequence. + +When to use the round-robin policy: + 1. Balanced Workloads: Effective for balanced and predictable workloads with + similar I/O size and type. + 2. Homogeneous Path Performance: Utilizes all paths efficiently when + performance characteristics (e.g., latency, bandwidth) are similar. + + +Queue-Depth +----------- + +The queue-depth policy manages I/O requests based on the current queue depth +of each path, selecting the path with the least number of in-flight I/Os. + +When to use the queue-depth policy: + 1. High load with small I/Os: Effectively balances load across paths when + the load is high, and I/O operations consist of small, relatively + fixed-sized requests. -- 2.43.0
