[PATCH RFC 0/1] Add visibility for native NVMe miltipath using debugfs

[Keith Busch]

On Mon, Jul 22, 2024 at 03:01:08PM +0530, Nilay Shroff wrote:
> # cat /sys/kernel/debug/block/nvme1n1/multipath
> io-policy: queue-depth
> io-path:
> --------
> node  path          ctrl    qdepth      ana-state
> 2     nvme1c1n1     nvme1   1328        optimized
> 2     nvme1c3n1     nvme3   1324        optimized
> 3     nvme1c1n1     nvme1   1328        optimized
> 3     nvme1c3n1     nvme3   1324        optimized
> 
> The above output was captured while I/O was running and accessing 
> namespace nvme1n1. From the above output, we see that iopolicy is set to 
> "queue-depth". When we have I/O workload running on numa node 2, accessing
> namespace "nvme1n1", the I/O path nvme1c1n1/nvme1 has queue depth of 1328 
> and another I/O path nvme1c3n1/nvme3 has queue depth of 1324. Both paths 
> are optimized and seems that both paths are equally utilized for 
> forwarding I/O.

You can get the outstanding queue-depth from iostats too, and that
doesn't rely on queue-depth io policy. It does, however, require stats
are enabled, but that's probably a more reasonable given than an io
policy.

> The same could be said for workload running on numa 
> node 3.

The output for all numa nodes will be the same regardless of which node
a workload is running on (the accounting isn't per-node), so I'm not
sure outputting qdepth again for each node is useful.