[PATCH v2 2/2] docs, nvme: add a nvme-multipath document

[Guixin Liu]

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 at linux.alibaba.com>
---
 Documentation/nvme/nvme-multipath.rst | 83 +++++++++++++++++++++++++++
 1 file changed, 83 insertions(+)
 create mode 100644 Documentation/nvme/nvme-multipath.rst

diff --git a/Documentation/nvme/nvme-multipath.rst b/Documentation/nvme/nvme-multipath.rst
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+.. 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,
+queue-depth and service-time.
+
+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.
+
+
+Service-Time
+------------
+
+The service-time policy distributes I/O requests based on the total size of
+in-flight I/Os, selecting the path with the least total size.
+
+When to use the service-time policy:
+  1. Highly Variable Workloads: Adapts to unpredictable and varying I/O patterns
+     by directing requests to the most responsive paths.
-- 
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