A chunk is the largest unit of physical disk dedicated to database server data storage.

Chunks provide administrators with a significantly large unit for allocating disk space. The maximum size of an individual chunk is 4 TB. The number of allowable chunks is 32,766. If you have upgraded from a version before version 10.00, you must run the onmode -BC 2 command to enable the maximum size of a chunk and the maximum number allowable, otherwise, the maximum chunk size is 2 GB.

The following storage spaces are comprised of chunks:
  • Dbspaces
  • Blobspaces
  • Sbspaces
  • Temporary dbspaces
  • Temporary sbspaces

When you create a chunk, you specify its path, size, and the associated storage space name.

The database server also uses chunks for mirroring. When you mirror a chunk, the database server maintains two copies of the data on the chunk. Every write operation to a primary chunk is automatically followed by an identical write operation to the mirror chunk. Read operations are evenly divided between the two chunks. If either the primary chunk or the mirror chunk fails, the chunk that failed is marked as down, and the other chunk performs all operations without interrupting the user access to data.

When you create tables, indexes, and other database objects, chunk space is allocated, or assigned, to those objects. Space that is allocated is not necessarily used. For example, when you create a table, you allocate space for it, but that space is not used until you add data to the table. When all the chunks in a dbspace report 0 free pages, you cannot create new database objects in that dbspace. However, you can continue to add data to existing database objects as long as they have unused space. You can monitor chunks by using the onstat -d command or the OpenAdmin Tool (OAT) for Informix®.

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