[PATCH separate v3] docs, nvme: introduce nvme-multipath document

[Guixin Liu <kanie@linux.alibaba.com>]

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@linux.alibaba.com>
---
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 v2 to v3:
- Move this new rst file to admin-guide dir, and add it to index.rst(Jonathan).

Changes from v1 to v2:
- Remove service-tome policy.

 Documentation/admin-guide/index.rst          |  1 +
 Documentation/admin-guide/nvme-multipath.rst | 72 ++++++++++++++++++++
 2 files changed, 73 insertions(+)
 create mode 100644 Documentation/admin-guide/nvme-multipath.rst

diff --git a/Documentation/admin-guide/index.rst b/Documentation/admin-guide/index.rst
index e85b1adf5908..15a522a96e76 100644
--- a/Documentation/admin-guide/index.rst
+++ b/Documentation/admin-guide/index.rst
@@ -136,6 +136,7 @@ configure specific aspects of kernel behavior to your liking.
    vga-softcursor
    video-output
    xfs
+   nvme-multipath
 
 .. only::  subproject and html
 
diff --git a/Documentation/admin-guide/nvme-multipath.rst b/Documentation/admin-guide/nvme-multipath.rst
new file mode 100644
index 000000000000..97ca1ccef459
--- /dev/null
+++ b/Documentation/admin-guide/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