zfs vdev cache consumes excessive memory
See OpenSolaris CR 6684116
but note that the work-around suggested there is less than ideal.
Better to set zfs_vdev_cache_size to zero, i.e. in /etc/system
set zfs:zfs_vdev_cache_size = 0
After some performance verification, it might make sense to
change the default to zero in uts/common/fs/zfs/vdev_cache.c