ZFS ARC cache and Linux Buffer cache contention (Ubuntu 16.04)

Set a minimum ARC value. That's all that's necessary.

The ARC will grow/shrink as needed. Whatever is eating buffet cache must not be on a ZFS file system. What is this server doing?

Try mounting your NFS directory with the sync mount option.

Moreover, to limit Linux pagecache usage, you can periodically run echo 3 > /proc/sys/vm/drop_caches

EDIT: as by eewhite comment, the last command seems to drop not only the pagecache, but the ARC also. So, issue it with care and only when necessary.

echo 1 >/proc/sys/vm/drop_caches will touch Linux buffer cache, but leave the ARC alone