SCA list structure size

The shared communications area (SCA) is a list structure in the coupling facility. Your storage estimate for your data sharing group should include an estimate of SCA size.

Tip: You can use the coupling facility structure sizer (CFSizer) tool to help you calculate CF structure storage sizes for Db2 data sharing. For more information, see The CFSizer tool .

The SCA contains the following information:

  • Member names
  • BSDS names
  • Database exception status conditions about object in the database
  • Recovery information, such as log data set names and the list of indoubt XA transactions

The following table shows how to estimate the size of the SCA. The SCA size can be specified in 1 KB increments.

Table 1. Estimating storage for the SCA
Site size Databases Tables INITSIZE SIZE
Small 50 1000 16 MB 32 MB
Medium 200 4000 32 MB 64 MB
Large 400 10,000 64 MB 128 MB
Extra Large 600 50,000 128 MB 256 MB

Running out of space in the SCA can cause Db2 to fail. Because much of the space in the SCA is taken up with exception information, you can reclaim space by correcting database exception conditions. Each indoubt XID entry takes approximately 250 bytes. The additional SCA storage required to be reserved is dependent on the total number of database access threads (DBATs) that can be active for the data sharing group.If the data sharing group supports XA transactions, be sure to reserve additional space in the SCA for the indoubt transaction ID (XID) list. (DRDA XA protocol support enables Db2 servers to participate in WebSphere® coordinated two-phase commit transactions that have multiple connections to more than one data source.) After a failure, indoubt XA transactions are resolved by WebSphere, or you can reclaim space by resolving indoubt XA transactions manually.