SOLVED: FreeBSD nfsv4 client/server protocol prob err=10026

While working with high-load and high-lock contention NFSv4 with FreeBSD as the NFS client you may see this error and have the NFS mount hang:

nfsv4 client/server protocol prob err=10026

Make sure your mount parameters do *not* include the intr keyword, per this PR: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=178231.

We found this was especially true when using NFS appliances such as NetApps. A good starting mount options would be:

rw,nfsv4,soft,retrycnt=0

You may adjust retrycnt up a bit but note it can create hangs in programs trying to access a down NFS link.