Q and A

Asked and Answered

Without using SMS, DB2 has an object size limit of 4GB. According to the documentation I can find, this would equate to about 4,704 cylinders of 3390 storage. Yet when we see physical DB2 objects reach this limit, they always max out at 5,826 cylinders. Do you know why this is?

Robert Catterall responds:

Basically, you see those numbers because DB2 doesn't use all the space on a 3390 track in a disk subsystem. DB2 space is typically managed in physical chunks of 4KB (what we call a page). Twelve pages can fit onto a 3390 track, and 15 tracks comprise a cylinder. Here's the way the calculation works:

• 4GB is 1,048,576 pages (each of which is 4KB in size).
• It takes 87,382 tracks to hold these pages (divide 1,048,576 by 12 and round up to the nearest integer value).
• The 87,382 tracks are equivalent to 5,826 cylinders (divide 87,382 by 15 and round up to the nearest integer value).

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