udev: Add custom udev rules for HPE Alletra Storage

[rmvasuki]

HPE Alletra Storage arrays support NVMe-oF block access.

1. The "round-robin" iopolicy is preferred for performance benefits.
2. Setting ctrl_loss_tmo to -1 for NVMe/TCP controllers enables indefinite reconnect attempts after a path loss, and disables purging of the path on the host.

[igaw]

I don't have any problems with adding this rule, but are you sure about round-robin? Did you have a look at queue-depth? round-robin has severe limits, e.g. it doesn't check the utilization of a link, if one link fails, all others are taken out as well.

[igaw]

I'll merged it for now. If someone complains, it's easy to change :)

[rmvasuki]

I don't have any problems with adding this rule, but are you sure about round-robin? Did you have a look at queue-depth? round-robin has severe limits, e.g. it doesn't check the utilization of a link, if one link fails, all others are taken out as well.

Sorry, I missed responding to this. I will investigate queue-depth further and report back here.
Could you elaborate a bit on the single link failure taking down other links? We haven’t seen that kind of behaviour in our testing so far.

thanks!

[igaw]

Ah, no worries.

It's been a while since I last played with round-robin, but IIRC, the failing path significantly impacted the good paths for a while. It might also be related to the transport used, e.g., FC. Anyway, I can't remember the details, but generally, we (SUSE) advise using queue-depth as it results in better performance and handles error cases more effectively. @mwilck has a bit more experience with this.