LFAREA calculation example 1

Table 1 shows an example of the maximum amount of LFAREA that can be configured for various amounts of online real storage using the LFAREA=(1M=(target,minimum),2G=(target,minimum)) syntax. The maximum amount is calculated using the formula: 80% of (online storage at IPL – 4GB). Also shown are the smallest percentages that can be specified for an LFAREA request for 1 MB or 2 GB pages to reserve at least one 1 MB page or one 2 GB page, respectively.

For example, on a z/OS® system with 16 GB of online storage at IPL, LFAREA=(2G=17%) must be specified to reserve at least one 2 GB page. This is calculated as percentage * (online storage at IPL – 4GB) = 0.17 * (16GB – 4GB) = 0.17 * 12GB = 2.04 GB, which is rounded down to the 2 GB boundary at 2 GB. To satisfy this request, the system must have 2 GB of contiguous real storage on a 2 GB boundary above the bar. The request will be refused if there are offline storage increments that create discontiguous areas which prevent contiguous 2 GB pages from being formed.

Table 1. LFAREA calculation example 1
Amount of online real storage at IPL in gigabytes (GB)	Maximum amount available for LFAREA in gigabytes (GB)	Minimum percentage request for at least one 1 MB page	Minimum percentage request for at least one 2 GB page
4	0	Not applicable	Not applicable
4.25	0.2	1% (results in two 1 MB pages)	Not applicable
6.5	2	1% (results in 25 1 MB pages)	80%
6.75	2.2	1% (results in 28 1 MB pages)	79%
8	3.2	1% (results in 40 1 MB pages)	50%
16	9.6	1% (results in 122 1 MB pages)	17%
32	22.4	1% (results in 286 1 MB pages)	8%
64	48	1% (results in 614 1 MB pages)	4%
128	99.2	1% (results in 1269 1 MB pages)	2%
208	163.2	1% (results in 2088 1 MB pages)	1%
256	201.6	1% (results in 2580 1 MB pages)	1%
512	406.4	1% (results in 5201 1 MB pages)	1% (results in two 2 GB pages)
1024	816	1% (results in 10444 1 MB pages)	1% (results in five 2 GB pages)
2048	1635.2	1% (results in 20930 1 MB pages)	1% (results in ten 2 GB pages)
4096	3273.6	1% (results in 41902 1 MB pages)	1% (results in twenty 2 GB pages

Note the following additional points regarding Table 1:

4 GB is not enough online real storage to form an LFAREA.
4.25 GB is the next incremental size up from 4096 MB (using an increment size of 256 MB) and the smallest amount of online real storage that can be configured for LFAREA=(1M=1%) to reserve at least one 1 MB page (in this case two 1 MB pages).
6.5 GB is the smallest amount of online real storage that can be configured to reserve at least one 2 GB page. Note that LFAREA=(2G=80%) is required to reserve that one page, leaving no storage available for 1 MB pages (because the sum of 1 MB and 2 GB pages must not exceed 80% of online real storage).
6.75 GB is the next incremental size up from 6656 MB (using an increment size of 256 MB) and the smallest amount of online real storage that can be configured for LFAREA=(1M=1%,2G=79%) to reserve at least one 1 MB page and one 2 GB page (in this case 28 1 MB pages).
208 GB is the smallest amount of online real storage for LFAREA=(2G=1%) to reserve one 2 GB page (below 208 GB, a percentage higher than 1% is required to reserve at least one 2 GB page).