The SAN I’m currently working with connects a pair of Netapp FAS3270 filers running ONTAP 8.0.2 7-Mode.

If you’re running VMware ESXi in your environment in front of a Fibre Channel SAN, path selection is discovered more or less in a first-come-first-served fashion.

I got this message on my Netapp filer:

FCP Partner Path Misconfigured:
Host I/O access through a non-primary and non-optimal path was detected.

Since the LUNs mounted by ESXi were residing on the A-side filer, the paths going through the B-side filer would just be sent over the partner link to the A-side, which is less efficient than going directly through A. However, since ESXi doesn’t know which is more efficient, it uses whatever comes first.

If you want to optimize this (and I recommend it), then hop over to Netapp’s support site - they’ve provided the ESX Host Utility toolkit for optimizing Fibre Channel on ESXi and includes a utility called config_mpath. This will allow you to set paths correctly.

The toolkit is available here:

http://support.netapp.com/NOW/download/software/sanhost_esx/ESX/

Matt Oswalt

Principal Systems Engineer at Cloudflare focused on traffic engineering, Rust, and highly available distributed systems.

Writes primarily about systems programming, Linux networking, machine learning, and lessons learned from building and operating large-scale production systems.

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