Verified with PXC 5.6.21, can't run query OR use I_S but can use SHOW and SET commands. 

Related: https://bugs.launchpad.net/percona-xtradb-cluster/+bug/1409618

mysql> show global status like 'wsrep_ready%';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| wsrep_ready   | OFF   |
+---------------+-------+
1 row in set (0.00 sec)

mysql> use information_schema
ERROR 1047 (08S01): Unknown command
mysql> 
mysql> select * from information_schema.tables;
ERROR 1047 (08S01): Unknown command
mysql> 
mysql> show plugins;
+-----------------------------+----------+--------------------+---------+---------+
| Name                        | Status   | Type               | Library | License |
+-----------------------------+----------+--------------------+---------+---------+
| binlog                      | ACTIVE   | STORAGE ENGINE     | NULL    | GPL     |
| wsrep                       | ACTIVE   | STORAGE ENGINE     | NULL    | GPL     |
| mysql_native_password       | ACTIVE   | AUTHENTICATION     | NULL    | GPL     |
| mysql_old_password          | ACTIVE   | AUTHENTICATION     | NULL    | GPL     |
| sha256_password             | ACTIVE   | AUTHENTICATION     | NULL    | GPL     |
| MEMORY                      | ACTIVE   | STORAGE ENGINE     | NULL    | GPL     |
| MRG_MYISAM                  | ACTIVE   | STORAGE ENGINE     | NULL    | GPL     |
| MyISAM                      | ACTIVE   | STORAGE ENGINE     | NULL    | GPL     |
| CSV                         | ACTIVE   | STORAGE ENGINE     | NULL    | GPL     |
| InnoDB                      | ACTIVE   | STORAGE ENGINE     | NULL    | GPL     |
| XTRADB_READ_VIEW            | ACTIVE   | INFORMATION SCHEMA | NULL    | GPL     |
| XTRADB_INTERNAL_HASH_TABLES | ACTIVE   | INFORMATION SCHEMA | NULL    | GPL     |
| XTRADB_RSEG                 | ACTIVE   | INFORMATION SCHEMA | NULL    | GPL     |
| INNODB_TRX                  | ACTIVE   | INFORMATION SCHEMA | NULL    | GPL     |
| INNODB_LOCKS                | ACTIVE   | INFORMATION SCHEMA | NULL    | GPL     |
| INNODB_LOCK_WAITS           | ACTIVE   | INFORMATION SCHEMA | NULL    | GPL     |
| INNODB_CMP                  | ACTIVE   | INFORMATION SCHEMA | NULL    | GPL     |
| INNODB_CMP_RESET            | ACTIVE   | INFORMATION SCHEMA | NULL    | GPL     |
| INNODB_CMPMEM               | ACTIVE   | INFORMATION SCHEMA | NULL    | GPL     |
| INNODB_CMPMEM_RESET         | ACTIVE   | INFORMATION SCHEMA | NULL    | GPL     |
| INNODB_CMP_PER_INDEX        | ACTIVE   | INFORMATION SCHEMA | NULL    | GPL     |
| INNODB_CMP_PER_INDEX_RESET  | ACTIVE   | INFORMATION SCHEMA | NULL    | GPL     |
| INNODB_BUFFER_PAGE          | ACTIVE   | INFORMATION SCHEMA | NULL    | GPL     |
| INNODB_BUFFER_PAGE_LRU      | ACTIVE   | INFORMATION SCHEMA | NULL    | GPL     |
| INNODB_BUFFER_POOL_STATS    | ACTIVE   | INFORMATION SCHEMA | NULL    | GPL     |
| INNODB_METRICS              | ACTIVE   | INFORMATION SCHEMA | NULL    | GPL     |
| INNODB_FT_DEFAULT_STOPWORD  | ACTIVE   | INFORMATION SCHEMA | NULL    | GPL     |
| INNODB_FT_DELETED           | ACTIVE   | INFORMATION SCHEMA | NULL    | GPL     |
| INNODB_FT_BEING_DELETED     | ACTIVE   | INFORMATION SCHEMA | NULL    | GPL     |
| INNODB_FT_CONFIG            | ACTIVE   | INFORMATION SCHEMA | NULL    | GPL     |
| INNODB_FT_INDEX_CACHE       | ACTIVE   | INFORMATION SCHEMA | NULL    | GPL     |
| INNODB_FT_INDEX_TABLE       | ACTIVE   | INFORMATION SCHEMA | NULL    | GPL     |
| INNODB_SYS_TABLES           | ACTIVE   | INFORMATION SCHEMA | NULL    | GPL     |
| INNODB_SYS_TABLESTATS       | ACTIVE   | INFORMATION SCHEMA | NULL    | GPL     |
| INNODB_SYS_INDEXES          | ACTIVE   | INFORMATION SCHEMA | NULL    | GPL     |
| INNODB_SYS_COLUMNS          | ACTIVE   | INFORMATION SCHEMA | NULL    | GPL     |
| INNODB_SYS_FIELDS           | ACTIVE   | INFORMATION SCHEMA | NULL    | GPL     |
| INNODB_SYS_FOREIGN          | ACTIVE   | INFORMATION SCHEMA | NULL    | GPL     |
| INNODB_SYS_FOREIGN_COLS     | ACTIVE   | INFORMATION SCHEMA | NULL    | GPL     |
| INNODB_SYS_TABLESPACES      | ACTIVE   | INFORMATION SCHEMA | NULL    | GPL     |
| INNODB_SYS_DATAFILES        | ACTIVE   | INFORMATION SCHEMA | NULL    | GPL     |
| INNODB_CHANGED_PAGES        | ACTIVE   | INFORMATION SCHEMA | NULL    | GPL     |
| PERFORMANCE_SCHEMA          | ACTIVE   | STORAGE ENGINE     | NULL    | GPL     |
| FEDERATED                   | DISABLED | STORAGE ENGINE     | NULL    | GPL     |
| BLACKHOLE                   | ACTIVE   | STORAGE ENGINE     | NULL    | GPL     |
| ARCHIVE                     | ACTIVE   | STORAGE ENGINE     | NULL    | GPL     |
| partition                   | ACTIVE   | STORAGE ENGINE     | NULL    | GPL     |
+-----------------------------+----------+--------------------+---------+---------+
47 rows in set (0.00 sec)

mysql> set @a=1;
Query OK, 0 rows affected (0.00 sec)

mysql> 
mysql> show global variables like 'innodb_file_per%';
+-----------------------+-------+
| Variable_name         | Value |
+-----------------------+-------+
| innodb_file_per_table | ON    |
+-----------------------+-------+
1 row in set (0.00 sec)

mysql> SET GLOBAL innodb_file_per_table=OFF;
Query OK, 0 rows affected (0.00 sec)

mysql> show global variables like 'innodb_file_per%';
+-----------------------+-------+
| Variable_name         | Value |
+-----------------------+-------+
| innodb_file_per_table | OFF   |
+-----------------------+-------+
1 row in set (0.00 sec)

mysql>