Percona XtraDB Cluster Limitations¶
The following limitations apply to Percona XtraDB Cluster:
- Replication works only with InnoDB storage engine.
- Any writes to tables of other types are not replicated.
- Unsupported queries:
UNLOCK TABLESis not supported in multi-master setups
Lock functions, such as
RELEASE_LOCK(), and so on
- Query log cannot be directed to table.
If you enable query logging, you must forward the log to a file:
log_output = FILE
general_log_fileto choose query logging and the log file name.
- Maximum allowed transaction size is defined by the
LOAD DATA INFILEprocessing will commit every 10 000 rows. So large transactions due to
LOAD DATAwill be split to series of small transactions.
- Transaction issuing
COMMITmay still be aborted at that stage.
Due to cluster-level optimistic concurrency control, there can be two transactions writing to the same rows and committing in separate Percona XtraDB Cluster nodes, and only one of the them can successfully commit. The failing one will be aborted. For cluster-level aborts, Percona XtraDB Cluster gives back deadlock error code:
(Error: 1213 SQLSTATE: 40001 (ER_LOCK_DEADLOCK)).
- XA transactions are not supported
- Due to possible rollback on commit.
- Write throughput of the whole cluster is limited by the weakest node.
- If one node becomes slow, the whole cluster slows down. If you have requirements for stable high performance, then it should be supported by corresponding hardware.
- Minimal recommended size of cluster is 3 nodes.
- The 3rd node can be an arbitrator.
enforce_storage_engine=InnoDBis not compatible with
wsrep_replicate_myisamis set to
ALTER TABLE ... IMPORT/EXPORTworkloads when running Percona XtraDB Cluster in cluster mode.
- It can lead to node inconsistency if not executed in sync on all nodes.
- All tables must have a primary key.
- This ensures that the same rows appear in the same order on different nodes. The
DELETEstatement is not supported on tables without a primary key.
- Galera Documentation: Tables without Primary Keys
- Avoid reusing the names of persistent tables for temporary tables
Although MySQL does allow having temporary tables named the same as persistent tables, this approach is not recommended.
Galera Cluster blocks the replication of those persistent tables the names of which match the names of temporary tables.
With wsrep_debug set to 1, the error log may contain the following message:
... [Note] WSREP: TO BEGIN: -1, 0 : create table t (i int) engine=innodb ... [Note] WSREP: TO isolation skipped for: 1, sql: create table t (i int) engine=innodb.Only temporary tables affected.