IBM i Disk Growth and Capacity Forecast Calculator
Running out of auxiliary storage on IBM i is a serious outage risk, and the warning signs are slow. This calculator estimates when your used storage will cross a threshold and how much disk you need to add to stay under it for a year.
Your numbers
Total auxiliary storage in the system pool you plan against.
Current used storage, not including free space.
Average monthly increase in used storage, measured over at least six months.
Data you expect to archive or delete once.
Use your quote, including installation and support.
Your results
Planning estimate using compound monthly growth. Real growth is lumpy because of month-end, journals, spooled output and projects. Use measured data from your own system and leave headroom for save operations and temporary storage.
Used storage after 12 months vs monthly growth rate
Sweeps Monthly growth rate from 0 to 10, holding your other inputs fixed.
Optional data growth analysis
Share a library size extract and we can show which data drives your growth.
Optional. The tool and every export work without this. No spam.
How to use it
Enter total capacity and current used storage from your system status. For growth, measure used storage monthly for at least six months and use the average percentage change. Pick a planning threshold. Many sites plan to stay below roughly eighty percent because performance and save operations suffer as disks fill. Add a planned purge if you have an archive project. The calculator returns months to threshold with and without that purge.
- Use measured growth, not vendor estimates
- Include journal receivers and spooled output in used storage
- Add headroom for restores and temporary objects
- Recalculate quarterly
The formula
Growth is modeled as compound monthly growth. Months to threshold equals the natural logarithm of threshold storage divided by used storage, divided by the natural logarithm of one plus the monthly growth rate. Used storage after twelve months is used storage times one plus growth to the power of twelve. Disk to add is that figure divided by the threshold, minus current capacity.
Reading the results
If months to threshold is below your budget cycle or lead time for hardware, act now. A purge can buy time, but the date moves only once because growth resumes afterward. Compare the cost of added disk with the cost of an archive project and the cost of a risky outage. The chart shows how sensitive the twelve-month figure is to growth rate.
Common mistakes
Using one very good or bad month as the growth rate gives a misleading answer. Another mistake is ignoring journal receivers, which can dominate growth when journaling is turned on without a cleanup policy. Also remember that a fuller system makes backups, reorganizations and restores harder. Plan for the worst day, not the average day.
Frequently Asked Questions
What utilization is too high on IBM i?
There is no single rule, but many administrators plan to stay under about eighty percent and treat above ninety as urgent. Performance, saves and reorganizations get harder as storage fills, so confirm your own threshold with your support provider.
How do I measure monthly growth?
Record used storage at the same point each month for six to twelve months, compute the percentage change between months, and average the results. Remove one-off events such as a large data load so they do not distort the trend.
Does a purge fix the problem?
It moves the date out once. If growth continues at the same rate, you reach the threshold again. Combine purges with retention rules so that growth itself slows.
Is the result a guarantee?
No. It is a planning estimate based on a steady growth rate. Actual use changes with business cycles and projects, so review the forecast regularly.
Netray can help analyze which libraries and files are driving growth and which are safe archive candidates.
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