This is a bit hacky as noted in the comment:

               * XXX we only put out the PERSIST flag if it is set.
               * Since the client-side is stupid-picky about unknown
               * attributes, this will cause an older client to fail
               * when the attribute is passed. Believe it or not,
               * that is a good thing! This will cause an older
               * client to fail if presented with a persistent
               * blockstore. If it did not fail, the client would
               * proceed to unconditionally create a filesystem on
               * the blockstore, wiping out what was previously
               * there.