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

[Nilay Shroff <nilay@linux.ibm.com>]

Hi,

This patch propose adding a new debugfs file entry for NVMe native 
multipath. As we know NVMe native multipath today supports three different
io-policies (numa, round-robin and queue-depth) for selecting optimal I/O
path and forwarding data. However we don't have yet any visibility to find
the I/O path being selected by NVMe native multipath code.

IMO, it'd be nice to have this visibility information available under 
debugfs which could help a user to validate the I/O path being chosen is 
optimal for a given io policy. This patch propose adding a debugfs file 
for each head disk node on the system. The proposal is to create a file 
named "multipath" under "/sys/kernel/debug/nvmeXnY/".

Please find below output generated with this patch applied on a system 
with a multi-controller PCIe NVMe disk attached to it. This system is also
an NVMf-TCP host which is connected to NVMf-TCP target over two NIC cards. 
This system has two numa nodes online when the below output was captured:

# cat /sys/devices/system/node/online
2-3

# nvme list -v
Subsystem        Subsystem-NQN                                                                                    Controllers
---------------- ------------------------------------------------------------------------------------------------ ----------------
nvme-subsys1     nvmet_subsystem                                                                                  nvme1, nvme3
nvme-subsys2     nqn.2019-10.com.kioxia:KCM7DRUG1T92:3D60A04906N1                                                 nvme0, nvme2

Device           Cntlid SN                   MN                                       FR       TxPort Address        Slot   Subsystem    Namespaces
---------------- ------ -------------------- ---------------------------------------- -------- ------ -------------- ------ ------------ ----------------
nvme0    2      3D60A04906N1         1.6TB NVMe Gen4 U.2 SSD IV               REV.CAS2 pcie   0524:28:00.0   U50EE.001.WZS000E-P3-C4-R1 nvme-subsys2 nvme2n2
nvme2    1      3D60A04906N1         1.6TB NVMe Gen4 U.2 SSD IV               REV.CAS2 pcie   0584:28:00.0   U50EE.001.WZS000E-P3-C4-R2 nvme-subsys2 nvme2n2
nvme1    1      a224673364d1dcb6fab9 Linux                                    6.9.0    tcp    traddr=10.0.0.200,trsvcid=4420,src_addr=10.0.0.100        nvme-subsys1 nvme1n1
nvme3    2      a224673364d1dcb6fab9 Linux                                    6.9.0    tcp    traddr=20.0.0.200,trsvcid=4420,src_addr=20.0.0.100        nvme-subsys1 nvme1n1

Device            Generic           NSID       Usage                      Format           Controllers
----------------- ----------------- ---------- -------------------------- ---------------- ----------------
/dev/nvme1n1 /dev/ng1n1   0x1          5.75  GB /   5.75  GB      4 KiB +  0 B   nvme1, nvme3
/dev/nvme2n2 /dev/ng2n2   0x2          0.00   B /   5.75  GB      4 KiB +  0 B   nvme0, nvme2


# cat /sys/class/nvme-subsystem/nvme-subsys2/iopolicy
numa

# cat /sys/kernel/debug/block/nvme2n2/multipath
io-policy: numa
io-path:
--------
node  current-path  ctrl    ana-state
2     nvme2c2n2     nvme2   optimized
3     nvme2c0n2     nvme0   optimized

The above output shows that current selected iopolicy is numa. And when we 
have workload running I/O on numa node 2, accessing namespace "nvme2n2", 
it uses path nvme2c2n2 and controller nvme2 for forwarding data. Moreover
the current ana-state for this path is optimized. Similarly, for I/O 
workload running on numa node 3 would use path nvme2c0n2 and controller
nvme0.

Now changing the iopolicy to round-robin,

# echo "round-robin" > /sys/class/nvme-subsystem/nvme-subsys2/iopolicy

# cat /sys/kernel/debug/block/nvme2n2/multipath
io-policy: round-robin
io-path:
--------
node  rr-path       ctrl    ana-state
2     nvme2c2n2     nvme2   optimized
2     nvme2c0n2     nvme0   optimized
3     nvme2c2n2     nvme2   optimized
3     nvme2c0n2     nvme0   optimized

The above output shows that current selected iopolicy is round-robin, and
when we have I/O workload running on numa node 2, accessing namespace 
"nvme2n2", the I/O path would toggle between nvme2c2n2/nvme2 and 
nvme2c0n2/nvme0. And the same is true for I/O workload running on node 3. 
Both I/O paths are currently optimized.

The namespace "nvme1n1" is accessible over fabric(NVMf-TCP).

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

Nilay Shroff (1):
  nvme-multipath: Add debugfs entry for showing multipath info

 drivers/nvme/host/multipath.c | 92 +++++++++++++++++++++++++++++++++++
 drivers/nvme/host/nvme.h      |  1 +
 2 files changed, 93 insertions(+)

-- 
2.45.2