Oracle Internals Notes

Controlfile Resizing

The slots in some control file record sections can be reused circularly. The most obvious examples are the log history, archived logs and offline ranges, but the various backup related record types are also cyclically reusable.

The control_file_record_keep_time parameter sets the minimum number of days that must have elapsed before a reusable controlfile record slot can be reused. The default is 7 days. If all the slots in a record section are in use and that number of days has not yet elapsed since the timestamp on the earliest entry, then Oracle will dynamically expand the record section (and thus the controlfile too) to make more slots available, up to a maximum of 65535 slots per section, or the controlfile size limit. (The controlfile size limit is based on the number of blocks that can be represented in the block version bitmap, and is thus most unlikely to be reached.) Informational "kccrsz" messages about the dynamic expansion of the controlfile (or the failure to do so) may be seen in the alert.log.

There are V$ views for each reusable controlfile record section, each with a timestamp column. These views can be used to estimate the number of controlfile slots that would be required in each record section for a particular keep time setting. The control_file_record_keep_time parameter can also be set to zero to prevent keep time related controlfile expansion. However, dynamic controlfile expansion may nevertheless be required for the non-reusable record sections. For example, the controlfile may grow when creating a tablespace if either of the datafile or tablespace record sections are already full.

Controlfile resizing happens in a special controlfile transaction under the protection of the CF enqueue. An instance or system failure during the resizing of the controlfile can result in its corruption. Therefore controlfile backups are, as always, important.