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:
LOCK TABLES
andUNLOCK TABLES
is not supported in multi-source setupsLock functions, such as
GET_LOCK()
,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
Use
general_log
andgeneral_log_file
to choose query logging and the log file name. -
Maximum allowed transaction size is defined by the
wsrep_max_ws_rows
andwsrep_max_ws_size
variables.LOAD DATA INFILE
processing will commit every 10 000 rows. So large transactions due toLOAD DATA
will be split to series of small transactions. -
Transaction issuing
COMMIT
may 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 message
(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=InnoDB
is not compatible withwsrep_replicate_myisam=OFF
wsrep_replicate_myisam
is set toOFF
by default. -
Avoid
ALTER TABLE ... IMPORT/EXPORT
workloads 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
DELETE
statement is not supported on tables without a primary key. -
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:
Error 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.
As of version 8.0.21, an INPLACE ALTER TABLE query takes an internal shared lock on the table during the execution of the query. The LOCK=NONE
clause is no longer allowed for all of the INPLACE ALTER TABLE queries due to this change.
This change addresses a deadlock, which could cause a cluster node to hang in the following scenario:
-
An INPLACE
ALTER TABLE
query in one session or being applied as Total Order Isolation (TOI) -
A DML on the same table from another session
Do not use one or more dot characters (.) when defining the values for the following variables:
MySQL and XtraBackup handles the value in different ways and this difference causes unpredictable behavior.