The GlusterFS documentation mentions how to set volume options, but there’s no obvious way to unset them. In the case that you’ve set something like performance.cache-size and you just want to return to the defaults, you need to reset the setting:

[root@storage1 ~]# gluster volume reset homes performance.cache-size
reset volume successful

Hopefully some SEO-fu will take place and this will help some other confused GlusterFS admins figure this out.