Re: [PATCH v9 00/29] Native SCSI Multipath support

[Xose Vazquez Perez <xose.vazquez@gmail.com>]

On 11/10/26 10:05 AM, John Garry wrote:

> On 11/9/26 20:19, Laurence Oberman wrote:
>> 3. Smaller points
>>
>> - numa with equal NUMA distance uses one path per LUN, so with two AO
>>    paths through two HBAs, half the optimized bandwidth is idle (sdl:2
>>    above under numa). That matches NVMe, but users coming from
>>    dm-multipath service-time will notice. Would round-robin or
>>    queue-depth be a better default for SCSI?
> 
> Maybe, but round-robin may not be always ideal considering only a bio mode of operation is supported and no merging like dm-mpath RQ mode.

iopolicy can easily be customized per array/device via udev rules.
Just like it's done for NVMe:
https://github.com/linux-nvme/nvme-cli/blob/master/nvmf-autoconnect/udev-rules/71-nvmf-hpe.rules.in

Additionally, "service-time" and "latency" I/O policies are currently under
development:
https://lore.kernel.org/linux-nvme/20261026111019.3724465-1-kanie@linux.alibaba.com/T/
https://lore.kernel.org/linux-nvme/20260815173502.1185929-1-nilay@linux.ibm.com/T/

>> - There is no userspace tool yet to show heads, paths and ALUA state
>>    (the equivalent of multipath -ll or nvme list-subsys). I used a
>>    small sysfs script for this testing. Ben, is native SCSI multipath
>>    support in multipath-tools planned?
> 
> Well, the whole point is that we don't need userspace tools. However, I appreciate that there is no general SCSI tooling which could help here. I know that Himanshu M did work on a SCSI version of 
> nvme-cli tool previously. Martin may know the status of that.

Agreed that userspace tools are not strictly needed for configuration, but
observability is key.
A standard utility to visualize the device topology, path health and ALUA
states at a glance, similar to what multipath-tools provides, would save a
lot of friction for sysadmins migrating away from dm-multipath.
Relying solely on ad-hoc scripts or manual sysfs parsing can become quite
tedious in production environments.