Assessing disk space and memory requirements
CDC Replication requires disk space and memory when it processes change data from your source database. In order to process change data efficiently and replicate these changes to your target system, it is very important that CDC Replication has adequate disk space and memory for each of the components described.
Disk space requirements for the staging store
The CDC Replication staging store is located on your source system and is a cache of change data read from the database logs. The size of the staging store will increase as the product accumulates change data, and therefore you must plan your source environment accordingly, particularly disk space.
The disk space allocated to the staging store is controlled by the Staging Store Disk Quota value that is set when you create an instance with the IBM® Data Replication configuration tool. In most cases, the default value is appropriate for CDC Replication source systems. Since the staging store is only used on source systems, you can reduce this value to the minimum of 1 GB if you are configuring a target instance of CDC Replication.
Memory requirements for the JVM (Java Virtual Machine)
As a Java-based product, CDC Replication requires you to allocate the maximum amount of memory (RAM) to be used by the Java™ Virtual Machine (JVM). This prevents CDC Replication from using all of the available memory on the system where it is installed.
The Maximum Memory Allowed value is set on a per-instance basis for each instance you create for your target database. In most cases the default values are adequate. However, if your database is processing an extremely heavy workload, you may have to adjust the default values. The RAM allocated must be physically available on your system.
Disk space requirements for the global disk quota
The global disk quota on your source and target systems is used for all capture components including temporary files, transaction queues, and LOBs which are staged on the target before being applied. CDC Replication will manage disk space utilization across all components as required.
Most databases have a mechanism that allows you to roll back or undo changes to your database by storing uncommitted changes. Similarly, CDC Replication uses this disk quota to store in-scope change data that has not been committed in your database. Once the database transaction is committed, the disk space used by the transaction is released. Long running open transactions will contribute to the amount of disk space used.
You can configure the amount disk space that is allocated to this quota with the mirror_global_disk_quota_gb system parameter. The default setting of this system parameter is such that CDC Replication will only stop replicating after this disk quota exhausts all available disk space on your system. If you would prefer CDC Replication to stop replicating after it uses a specific amount of disk space, you can specify the value with this system parameter in Management Console.