Shadow column disk space

If you plan to use shadow columns, make sure to allow additional disk space for their values.

If you plan to use any conflict-resolution rule except ignore or always-apply, you must allow for an additional 8 bytes for the CRCOLS shadow columns, cdrserver and cdrtime, which store the server and time stamp information that Enterprise Replication uses for conflict resolution.

If you want to speed consistency checking by indexing the REPLCHECK shadow column, you must allow for an additional 8 bytes for the ifx_replcheck shadow column.

If you want to use ERKEY shadow columns as the replication key, or you create your replicated tables through a grid, you must allow of an additional 10 bytes for the ERKEY shadow columns, ifx_erkey_1, ifx_erkey_2, and ifx_erkey_3. When you create replicated tables through a grid, these ERKEY columns are automatically added.ERKEY columns also require disk space for the index that is created on them. In addition to the standard partition and page overhead, for each row in the table the ERKEY index uses 14 bytes for non-fragmented tables and 18 bytes for fragmented tables for each row in the table.

The following table shows the amount of space used by each shadow column.

Table 1. Shadow column size
Shadow column name Data type Size
cdrserver INTEGER 4 bytes
cdrtime INTEGER 4 bytes
ifx_replcheck BIGINT 8 bytes
ifx_erkey_1 INTEGER 4 bytes
ifx_erkey_2 INTEGER 4 bytes
ifx_erkey_3 SMALLINT 2 bytes

The shadow columns claim disk space immediately, except when CRCOLS and ERKEY columns are added to an existing table.


Copyright© 2020 HCL Technologies Limited