Pool subscription limit
The following figure depicts setting the subscription limit of pool capacity.
The value of using a subscription limit is to manage the maximum amount of over-provisioning that is acceptable for a pool. By managing the pool subscription limit, you can control the potential demand for storing data that might exceed the pool capacity.
The subscription limit is the ratio (%) of the total DP-VOL capacity that has been configured to the total capacity of the pool. When the subscription limit is set, you cannot configure another DP-VOL if the new DP-VOL capacity would cause the subscription limit to be exceeded.
The subscription limit includes pages required to store user data and control information. The total capacity of DP-VOLs that are created from the pool is smaller than the subscription limit capacity. The formula used to calculate the required pages for one DP-VOL includes the control information. To determine the total pages required in a pool, multiply the number of calculated pages by the number of DP-VOLs. The value enclosed in ceil( ) must be rounded up to the nearest whole number. The number of pages for the DP-VOL including the control information equals:
Number of pages for the
DP-VOL including the control information = ceil((One
DP-VOL capacity (MB) + ceil(One
DP-VOL capacity (MB) / 3,145,548 (MB)) × 4 (Pages) × 42 (MB)) / 42 (MB))
For example, if the pool capacity is 100 GB and the subscription limit is 150%, you can configure up to a total of 150 GB of capacity to the DP-VOLs related to the pool.
If you are using a pool comprised of pool-VOLs assigned to accelerated compression-enabled parity groups, the pool subscription limit is defined with respect to the pool capacity not reserved for writing. In this case, the free area of the pool must be monitored even if the subscription limit is defined to 100%.
If you do not want to monitor the free area of the pool, specify the subscription limit conforming to the following formula:
Pool capacity / Pool capacity reserved for data writing × Subscription limit = 100%
For example, if 100 TB of the pool capacity and 80 TB of the pool capacity reserved for writing exist, specify 80% as the subscription limit.
- For a Dynamic Provisioning pool that contains Thin Image pairs and snapshot data, the pool subscription limit should be defined as unlimited because in most cases the pool used capacity by the snapshot data is smaller than the used capacity permitted by the pool subscription limit.
- For a Dynamic Provisioning pool that contains Thin Image pairs and snapshot data, the required pool capacity as a subscription limit is the total capacity of all DP-VOLs and Thin Image pairs. For details about calculating the Thin Image pair capacity for Dynamic Provisioning pools, see the Thin Image User Guide.