====================================
 Percona XtraDB Cluster Limitations
====================================

There are some limitations which you should be aware of. Some of them will be eliminated later as product is improved and some are design limitations.

 - Currently replication works only with |InnoDB| storage engine. Any writes to tables of other types, including system (mysql.*) tables, are not replicated. However, DDL statements are replicated in statement level, and changes to mysql.* tables will get replicated that way. So, you can safely issue: CREATE USER..., but issuing: INSERT INTO mysql.user..., will not be replicated.

 - DELETE operation is unsupported on tables without primary key. Also rows in tables without primary key may appear in different order on different nodes. As a result SELECT...LIMIT... may return slightly different sets.

 - Unsupported queries:
    * LOCK/UNLOCK TABLES cannot be supported in multi-master setups.
    * lock functions (GET_LOCK(), RELEASE_LOCK()... )

 - Query log cannot be directed to table. If you enable query logging, you must forward the log to a file: log_output = FILE. Use general_log and general_log_file to choose query logging and the log file name.

 - Maximum allowed transaction size is defined by wsrep_max_ws_rows and wsrep_max_ws_size. LOAD DATA INFILE processing will commit every 10K rows. So large transactions due to LOAD DATA will be split to series of small transactions.

 - Due to cluster level optimistic concurrency control, transaction issuing COMMIT may still be aborted at that stage. There can be two transactions writing to same rows and committing in separate XtraDB Cluster nodes, and only one of the them can successfully commit. The failing one will be aborted. For cluster level aborts, XtraDB Cluster gives back deadlock error code: 
   (Error: 1213 SQLSTATE: 40001  (ER_LOCK_DEADLOCK)).

 - XA transactions can not be supported due to possible rollback on commit.

 - The write throughput of the whole cluster is limited by weakest node. If one node becomes slow, whole cluster is slow. If you have requirements for stable high performance, then it should be supported by corresponding hardware (10Gb network, SSD).

 - The minimal recommended size of cluster is 3 nodes.

 - DDL statements are problematic and may stall cluster. Later, the support of DDL will be improved, but will always require special treatment.