[PATCH 4/7] nvme: implement multipath access to nvme subsystems

[Martin K. Petersen]

Christoph,

> This patch adds native multipath support to the nvme driver.  For each
> namespace we create only single block device node, which can be used
> to access that namespace through any of the controllers that refer to
> it.  The gendisk for each controllers path to the name space still
> exists inside the kernel, but is hidden from userspace.  The character
> device nodes are still available on a per-controller basis.  A new
> link from the sysfs directory for the subsystem allows to find all
> controllers for a given subsystem.
>
> Currently we will always send I/O to the first available path, this
> will be changed once the NVMe Asynchronous Namespace Access (ANA) TP
> is ratified and implemented, at which point we will look at the ANA
> state for each namespace.  Another possibility that was prototyped is
> to use the path that is closes to the submitting NUMA code, which will
> be mostly interesting for PCI, but might also be useful for RDMA or FC
> transports in the future.  There is not plan to implement round robin
> or I/O service time path selectors, as those are not scalable with the
> performance rates provided by NVMe.
>
> The multipath device will go away once all paths to it disappear, any
> delay to keep it alive needs to be implemented at the controller
> level.

Beautiful!

Reviewed-by: Martin K. Petersen <martin.petersen at oracle.com>

-- 
Martin K. Petersen	Oracle Linux Engineering