IBM i (AS/400) Disk Capacity Runway Calculator
Disk full conditions on IBM i are painful because the system slows and can stop at high ASP usage. This calculator gives you a quick runway in months based on current usage and growth.
Your numbers
System ASP capacity as shown in WRKSYSSTS or WRKDSKSTS.
Used portion of the system ASP.
Compare used space a year ago with now.
Many shops treat 80 to 90 percent as the line for planning an expansion.
Your results
A planning estimate using steady compound growth. Real growth is lumpy, so add margin for projects, journaling changes and backups held on disk.
Months of runway by annual growth rate
Sweeps Annual growth rate from 1 to 60, holding your other inputs fixed.
Capacity monitor for IBM i
Optional: an automated read-only report showing which libraries and files drive growth.
Optional. The tool and every export work without this. No spam.
How to use it
Enter total and used disk for the system ASP, the growth rate you have observed and your alert threshold. Find used disk with WRKSYSSTS, which shows the percent of system ASP used, or with WRKDSKSTS for unit level detail. To estimate growth, compare usage from the same month last year. The primary result is the number of months until you hit the threshold.
The formula
With compound growth, usage after t years is used times (1 + g) raised to t. Solving for the time to reach the threshold T gives t = ln(T / used) / ln(1 + g). The tool multiplies by 12 to give months. If you are already past the threshold the result is zero. The chart sweeps the growth rate so you can see how sensitive the runway is to the assumption.
- t = ln(T / used) / ln(1 + g)
- T = total disk times threshold
- g = annual growth as a decimal
- Result shown in months
Reading the result
Under 12 months means start the purchase or cleanup now, because hardware lead times and approvals take time. Between 12 and 24 months is a planning zone. Over 36 months is comfortable unless a project is about to add data. Before buying disk, check for cheap wins such as old journal receivers, spool files, save files and unused libraries, which the Db2 for i services query tool can help find.
Common mistakes
Using total installed disk when part of it belongs to an independent ASP or is reserved leads to false comfort. Taking growth from a single busy month overstates it. Forgetting that journaling, new reporting databases or a larger backup retention on disk can change growth sharply is another mistake. Recalculate every quarter and after major projects.
Frequently Asked Questions
What disk usage level is too high on IBM i?
IBM i performance and stability can suffer when the system ASP gets very full, and the system has its own storage threshold limit. Many shops plan expansion at 80 to 90 percent. Check IBM guidance for your release and set your threshold lower than the system limit.
How do I find what is using the disk?
Query QSYS2.OBJECT_STATISTICS by library and sort by size, check journal receivers, and look at spool files and save files. The IBM i Navigator and Access Client Solutions tools can also show storage by library and object type.
Is linear or compound growth better?
Compound is a cautious choice for databases that grow in proportion to business activity. If your growth is a steady number of GB per month, a linear estimate can be simpler: headroom divided by monthly GB added.
Does this include backups on disk?
Only if they are in the used figure you enter. Save files and image catalogs on the system ASP count towards usage, so include them or remove them first for a clean base number.
If you want automated capacity alerts that explain what is growing, Netray can build a read-only monitor over your IBM i catalog data on your own servers.
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