Users Guide

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Emulex Drivers for Linux User Guide
Appendix C: Native NVME Multipathing
Two multipathing options can be used with NVMe storage: Device Mapper (DM) and Native NVMe Multipathing.
DM is a general block-device multipathing framework that can work with devices of any protocol. DM supports the addition
of hardware-specific handlers to help manage the storage device and its path management. DM has been used extensively
with SCSI storage. DM can be used in its generic form with NVMe storage. However, DM currently does not support
advanced NVMe features, such as ANA support. Refer to the operating system release documentation for the level of DM
and NVMe storage integration supported.
Native NVMe Multipathing is a kernel-level feature that may be enabled to add an NVMe-specific multipath module to the
kernel, which automatically detects subsystems and manages paths to the subsystems. The NVMe-specific module
supports the NVMe ANA feature. Inclusion and enablement of Native NVMe Multipath module is operating system-specific.
Refer to the operating system release documentation for the level of Native NVMe Multipathing supported.
Native NVMe Multipathing is included, but is not enabled by default, in SLES12 SP4 and SLES15.
To enable NVMe Multipathing on SLES12 SP4 and SLES15 and later initiators, add nvme-core.multipath=Y in the
kernel command line, either in the grub boot loader or during bootup.
To verify if NVMe Multipathing is enabled, run the following command:
# cat /sys/module/nvme_core/parameters/multipath
If NVMe Multipathing is enabled, the output displayed is Y.
If NVMe Multipathing is not enabled, the output displayed is N.
When enabled, Native NVMe Multipathing changes the manner in which NVMe devices are presented and related in
/sys/class/nvme, especially in cases where multiple paths to target namespaces exist. These changes significantly
affect the views generated by the kernel and system utilities, such as nvme-cli. This appendix describes these new views.
When Native NVMe Multipathing is not enabled, if an NVMe device is connected to a subsystem, a controller element is
created. If the subsystem is connected using multiple paths, multiple controller elements are created. Each controller
element is considered a unique and separate storage entity, even though the subsystem and namespaces the controller
could access might be the same.
When Native NVMe Multipathing is enabled, controller elements are still created for each connection to a subsystem.
However, when a controller is created, the subsystem is compared to the list of subsystems that have already been found
in the system, and one of the following actions occurs:
 If the subsystem is not found, a new subsystem element is created and added to the system list, with the controller
linked to it as a path.
 If the subsystem is found, the controller is linked to the subsystem as an additional path.
Namespaces are elements of the NVMe subsystem, and not the NVMe controller. A controller acts as a communication port
to a subsystem. If a subsystem contains two controllers (communication ports), and namespace 1 is changed by controller A,
controller B sees this change as an action on namespace 1, because the namespace is a singular component of the
subsystem.