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

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@oracle.com>

-- 
Martin K. Petersen	Oracle Linux Engineering