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The onconfig parameters TAPESIZE and LTAPESIZE are used in connection with TAPEDEV and LTAPEDEV to determine the size for each tape or file to be used for an ontape backup. At times, the backup size requirement may exceed the size configured for an ontape backup. This is usually not a problem if you are standing by, ready to handle the situation. If you want the backup to fit into a single tape or file, you have to make sure your size estimate is reasonable.

The size needed for an Informix Server Backup using ontape is based on several internal factors. The most important considerations are: 

• when the last backup was done
• the page size used for each dbspace
• how much activity has been transacted since the last backup
• whether it is a level 0, 1, or 2 backup.

To determine backup size requirements, generate an 'onstat -d' listing, and examine the chunk column references for "Size" and "Free". Work with one group of page size at a time to determine a value (K):

1. Review the dbspaces with similar page size (2k, 4k, 8k, etc). Each collection of dbspaces with the same page size is a group (group-P). Add all the totals for (group-P) 'Size' and place it in 'N'.
2. Add all the totals for (group-P) 'Free' and place it in 'F'.
   • Ignore 'size' and 'free' for temp dbspace and sbspace chunks.
3. Subtract F from N (N-F).
   • This will provide the count of pages which are in use (M).
4. Multiply M times the page size (for group-P) to get the number of page units (K ) needed for storage:
   • M * P = K (kb).
5. Repeat steps 1-4 for other page sizes, and add all of the K values. The final result provides a maximum size estimate for the next backup