Pool subscription limit

The value of using 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.

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.

The following figure depicts setting the subscription limit of pool capacity.

Monitoring total DP-VOL subscription for a pool

You can configure the Subscription Limit of total DP-VOL capacity to pool capacity. This prevents a new DP-VOL capacity that exceeds the configured subscription limit from being allocated and is associated with the pool. If you specify more than 100% as the Subscription Limit or the subscription limit is not set, you must monitor the free capacity of the pool because it is possible that writes to the DP-VOLs may exceed pool capacity. For details about the Subscription Limit, see Create Pools window.

The used value displayed on the cell for Current in the Subscription (%) is truncated after the decimal point of the calculated value. Therefore, the actual percentage of DP-VOL assigned to the pool may be larger than the value displayed on the window. If you create a new DP-VOL of the same size as the existing DP-VOL, the larger size of capacity which is displayed on the Current cell is necessary.

For example, if 3 GB V-VOL is related to an 11.89 GB pool, the capacity (%) is calculated as follows:

(( ceil(3,072 (MB) / 3,145,548 (MB)) * 4 (Pages) * 42 (MB)) +
3,072 (MB)) / 12,175.36 (MB)) * 100 = 26.61....(%)

In this case, 26 (%) is displayed on the cell for Current in the Subscription (%). If you create a new V-VOL of the same size as the existing V-VOL, 27 (%) or more remaining capacity is necessary.

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