mirror of
git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
synced 2025-08-05 16:54:27 +00:00
73 lines
2.5 KiB
ReStructuredText
73 lines
2.5 KiB
ReStructuredText
|
.. 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.
|