Disk compression and capacity
Capacity gains might vary and are determined by the amount of compression that can be performed on the data.
An overall 2:1 compression ratio should be expected for a disk unit. A maximum 4:1 compression ratio for a disk unit might be achievable in certain circumstances. Additionally, a compression ratio for a set of data on that disk unit might reach a maximum of 8:1. The IBM i software enforces the overall disk unit maximum compression ratio of 4:1. Because capacity is dependent on how well the data compresses, the capacity that is displayed will change as data is written to or deleted from the disk. The capacity that is displayed for a compressed disk unit is the amount of data on the disk plus an estimate of how much additional data can fit on the disk.
The following example shows the calculation and display of capacity by the system for compressed disk units. The disk unit capacities are available on the Display Disk Configuration Capacity display under the DST menus and the SST menus. The capacities are also available on the Work with Disk Status (WRKDSKSTS) display.
- Before starting compression, a nonconfigured 6602 model 050 has
a capacity of 1031 megabytes.
Display Non-Configured Units Serial Resource Number Type Model Name Capacity Status 83-0135199 6602 050 DD005 1031 Non-configured 83-0306044 6602 050 DD006 1031 Non-configured - After starting compression, the 6602 model number changes to 060,
and the capacity doubles.
Display Non-Configured Units Serial Resource Number Type Model Name Capacity Status 83-0135199 6602 060 DD005 2062 Non-configured 83-0306044 6602 060 DD006 2062 Non-configured - Two compressed disk units are added into user ASP 2.
Display Disk Configuration Capacity ----Protected---- ---Unprotected--- ASP Unit Type Model Threshold Overflow Size % Used Size %Used 1 90% No 0 0.00% 21372 17.26% 1 6607 050 0 0.00% 4194 29.25% 2 6713 050 0 0.00% 8589 14.33% 3 6713 050 0 0.00% 8589 14.34% 2 90% No 0 0.00% 4124 0.10% 5 6602 060 0 0.00% 2062 0.10% 6 6602 060 0 0.00% 2062 0.10% - After writing data to the user ASP, capacities and percentages
that are used are displayed.
Display Disk Configuration Capacity ----Protected---- ---Unprotected--- ASP Unit Type Model Threshold Overflow Size % Used Size %Used 1 90% No 0 0.00% 21372 17.26% 1 6607 050 0 0.00% 4194 29.25% 2 6713 050 0 0.00% 8589 14.33% 3 6713 050 0 0.00% 8589 14.34% 2 90% No 0 0.00% 5812 61.06% 5 6602 060 0 0.00% 2905 61.04% 6 6602 060 0 0.00% 2907 61.08% - The following calculations can be performed to determine how well
the data is being compressed, and the estimated disk unit capacity.
These calculations can be performed on a user ASP basis, as well as
on individual disk units.
Amount Calculation Logical data written Size * (% Used / 100) Physical free space (Size * (1-(% Used / 100)) / 2 Physical space used Non-compressed size - Physical free space Compression ratio of data written Logical data written / Physical space used Estimated disk capacity Logical data written + (2 * Physical free space)
Using the values listed for Unit 5 in the preceding step with these formulas produces the following table:
| Amount | Calculation |
|---|---|
| Logical data written | 2905 * .6104 = 1773 |
| Physical free space | (2905 * (1-(.6104)) / 2 = 566 |
| Physical space used | 1031 - 566 = 465 |
| Compression ratio | 1773 / 465 = 3.8 |
| Estimated disk capacity | 1773 + (2 * 566) = 2905 |