vfs.nfsd.highwater tuning

[scook2000]

A couple of weeks ago, NFS on our FREENAS-CERTIFIED-4U-24 running FreeNAS-11.2-U7 became unresponsive.
These messages were logged multiple time in /var/log/messages:
Apr 3 05:40:14 hetnas1 nfsd server cache flooded, try increasing vfs.nfsd.tcphighwater
Apr 3 05:40:14 hetnas1 nfsrv_cache_session: no session

So, I increased vfs.nfsd.tcphighwater in the Web Interface by going to System->Tunables and clicking the Add Button. Then I created a sysctl variable named vfs.nfsd.tcphighwater with a value of 300,000.

This fixed the problem.

My question to the community is what are the possible repercussions of this change. I have searched the documentation and Internet, and have been unable to find information about the vfs.nfsd.highwater variable. I picked a value of 300,000 because somebody on the Internet was having an identical problem, and this value fixed it. Any recommendations on the value of this setting would be appreciated.

Thanks,
Steve

 

[hokan]

I can't answer your question, but I'll share my ill-remembered experience. I say ill-remembered because I don't recall the reason I had to adjust this, but I do have a note that shows I initially set it to 100,000 and when that wasn't enough, set it to 200,000. That must have worked because the whole thing has now been purged from my mind!