|Oracle® TimesTen In-Memory Database Replication Guide
11g Release 2 (11.2.2)
Part Number E21635-07
Configure parallel replication. See "Configuring parallel replication".
Use asynchronous replication, which is the default. For more information, see "Making decisions about performance and recovery tradeoffs". However, if you are using active standby pairs, return twosafe (synchronous replication) has better performance than return receipt (semi-synchronous replication).
LogBufMB first connection attributes to their maximum values. For more information, see "Setting connection attributes for logging".
If the workload is heavy enough that replication sometimes falls behind, replicated changes must be captured from the transaction logs on disk rather than from the in-memory log buffer. Using the fastest possible storage for the TimesTen transaction logs reduces I/O contention between transaction log flushing and replication capture and helps replication to catch up more quickly during periods of reduced workload. Consider using a high performance, cached disk array using a RAID-0 stripe across multiple fast disks or solid state storage.
Experiment with the number of connections to the database where the updates are applied. If you need more than 64 concurrent connections, set the
Connections first connection attribute to a higher value. See "Connections" in Oracle TimesTen In-Memory Database Reference.
Adjust the transaction log buffer size and CPU power and resources. See "Adjust transaction log buffer size and CPU".
There can be performance issues after altering tables with multiple partitions and extraneous space. For more information, see "Performance considerations when altering tables that are replicated".
Increase the number of threads that apply changes from the active master database to the standby master database by altering the
RecoveryThreads first connection attribute. For details, see "Increase replication throughput for active standby pairs".
Replication and XLA operations have significant overhead with transaction logging. Replication scales best when there are a limited number of transmitters or receivers. For more information, see "Limit replication transmitters, receivers, and XLA readers".
Note:Additional recommendations can be found in "Poor replication or XLA performance" in Oracle TimesTen In-Memory Database Troubleshooting Guide.
If you are planning a replication scheme, ensure the following:
The transaction log setting for
LogBufMB should result in the value of
LOG_FS_READS in the
SYS.MONITOR table being 0 or close to 0. This ensures that the replication agent does not have to read any transaction log records from disk. If the value of
LOG_FS_READS is increasing, then increase the transaction log buffer size.
CPU resources are adequate. The replication agent on the master database spawns a thread for every subscriber database. Each thread reads and processes the transaction log independently and needs adequate CPU resources to make progress.
If the sending side and receiving side of the replication scheme are mismatched in CPU power, place the replication receiver on the faster system.
Altering a table to add or remove columns may lead to performance degradation or poor space utilization.
When you alter a table to add one or more columns, the table is allocated a new partition for the additional columns. The additional partition causes extra processing when retrieving the data, resulting in reduced performance. See "ALTER TABLE" in the Oracle TimesTen In-Memory Database SQL Reference for more information on understanding how partitions are added when using
When you alter a table to drop a column, the space is not always freed resulting in poor space utilization.
Any replication scheme defined with the
TABLE DEFINITION CHECKING EXACT attribute requires that the physical structure of the table be identical on both master databases in order to be able to replicate operations between them. When using the
EXACT table definition checking attribute, the only method to free the extraneous space resulted from dropped columns or eliminate extra partitions resulting from added columns is to drop and recreate the table, and then reload the data into the table.
However, if you create the tables with the
TABLE DEFINITION CHECKING RELAXED attribute, then (while they must have the same key definition, number of columns, and column data types) the physical structure does not need to be identical on both master databases. The
TABLE DEFINITION CHECKING RELAXED attribute can result in slightly slower performance, but only if the tables on both masters are not identical. The change in performance depends on the workload and the number of partitions and columns in the tables.
To improve performance for databases set with
RELAXED, you can use
ttMigrate -r -relaxedUpgrade to coalesce tables eliminating extraneous space from dropped columns or multiple partitions that were created when adding columns. This can be performed on one database, while the other database is still up and accepting requests on behalf of the application. You do not have to take both databases involved in replication down at the same time, but can perform
ttMigrate -r -relaxedUpgrade on each one individually one after the other. This is optimal for databases where the tables are altered often and where the database can only perform online upgrades.
You can only coalesce partitions and eliminate extraneous space with
ttMigrate -r -relaxedUpgrade on replicated tables when the table definition checking to
RELAXED. However, if your tables have been using the
EXACT attribute, then you can temporarily set table definition checking to
RELAXED, consolidate the partitions and space for your tables, and then reset it to
For more information on the
TABLE DEFINITION CHECKING RELAXED attribute, see "Column definition options for replicated tables".
Note:You can check if the table has multiple partitions. For details, see the instructions in both "Understanding partitions when using ALTER TABLE" in the Oracle TimesTen In-Memory Database SQL Reference and "Check partition counts for the tables" in the Oracle TimesTen In-Memory Database Troubleshooting Guide.
RecoveryThreads first connection attribute to increase the number of threads that apply changes from the active master database to the standby master database from 1 to 2. If you set
RecoveryThreads to 2 on the standby, you should also set it to 2 on the active to maintain increased throughput if there is a failover.
You can also set
RecoveryThreads to 2 on one or more read-only subscribers in an active standby pair to increase replication throughput from the standby master database.
Databases must be hosted on systems that are 2-way or larger to take advantage of setting this attribute to 2.
Note:For more details, see "RecoveryThreads" in the Oracle TimesTen In-Memory Database Reference.
Replication and XLA operations have significant overhead with transaction logging. Replication scales best when there are a limited number of transmitters or receivers. Check your replication topology and see if you can simplify it. Generally, XLA scales best when there are a limited number of readers. If your application has numerous readers, see if you can reduce the number.
Monitor XLA and replication to ensure they are reading from the transaction log buffer rather than from the disk. With a lot of concurrent updates, replication may not keep up. Updates are single-threaded at the subscriber. You can achieve better XLA throughput if the frequency of acknowledgements is reduced.
Estimate the number of readers and transmitters required by checking the values in the
LOG_BUFFER_WAITS columns in the
SYS.MONITOR table. The system updates this information each time a connection is made or released and each time a transaction is committed or rolled back.
LogFlushMethod=2 can improve performance of
RETURN TWOSAFE replication operations and
RETURN RECEIPT with
DURABLE TRANSMIT operations.