Structure of a Mirror Chunk

The mirror chunk contains the same control structures as the primary chunk, so mirrors of dbspace chunks contain dbspace overhead pages.

For information on these structures, refer to the section on the structure of a mirror chunk in the disk structures and storage chapter of the IBM Informix: Administrator's Reference.

A display of disk-space use, provided by one of the methods discussed under Monitoring Chunks, always indicates that the mirror chunk is full, even if the primary chunk has free space. The full mirror chunk indicates that none of the space in the chunk is available for use other than as a mirror of the primary chunk. The status remains full for as long as both primary chunk and mirror chunk are online.

If the primary chunk goes down, and the mirror chunk becomes the primary chunk, disk-space allocation reports then accurately describe the fullness of the new primary chunk.