Commit 80568f47 authored by Guixin Liu's avatar Guixin Liu Committed by Jonathan Corbet
Browse files

docs, nvme: introduce nvme-multipath document 



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: default avatarGuixin Liu <kanie@linux.alibaba.com>
Reviewed-by: default avatarChristoph Hellwig <hch@lst.de>
Signed-off-by: default avatarJonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20241209071127.22922-1-kanie@linux.alibaba.com
parent dfddf353

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

Documentation/admin-guide/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 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.