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docs, nvme: introduce nvme-multipath document

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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>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20241209071127.22922-1-kanie@linux.alibaba.com
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Guixin Liu 2024-12-09 15:11:27 +08:00 committed by Jonathan Corbet
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Documentation/admin-guide/index.rst
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@ -136,6 +136,7 @@ configure specific aspects of kernel behavior to your liking.
vga-softcursor
video-output
xfs
nvme-multipath
.. only:: subproject and html

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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 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.