Volume extent allocation

1. A computer program product comprising a non-transitory computer readable storage medium having program instructions embodied therewith, the program instructions executable by a processor to cause the processor to perform a method comprising:

receiving an extend request, the extend request including a new extent allocation amount for a data set, the data set associated with a number of volumes in a storage facility, wherein the number of volumes includes at least a first volume, a second volume, and a third volume, wherein at least one of the first volume and the second volume is associated with a dynamic volume count (DVC) and is not a candidate volume of the data set, wherein the third volume is not associated with the DVC and is a primary volume or a candidate volume of the data set;

increasing a total percentage of space of each volume which may be used for extent allocations;

apportioning the new extent allocation amount among fractional amounts, the fractional amounts including at least a first fractional amount and at least a second fractional amount;

allocating the first fractional amount to the first volume;

allocating the second fractional amount to the second volume;

determining, after allocating the first and second fractional amounts, that the third volume can accommodate the new extent allocation amount; and

reallocating the first and second fractional amounts to the third volume.

2. The computer program product of claim 1 , wherein the program instructions configured to cause the processor to increase the total percentage of space of each volume which may be used for extent allocations are further configured to cause the processor to perform a method further comprising:

determining that zero of the number of volumes can accommodate the new extent allocation amount; and

increasing, in response to determining that zero of the number of volumes can accommodate the new extent allocation amount, a volume allocation threshold, wherein the volume allocation threshold is associated with the number of volumes and indicates the total percentage of space of each volume which may be used for extent allocations.

3. The computer program product of claim 1 , wherein the program instructions configured to cause the processor to apportion the new extent allocation amount among fractional amounts are further configured to cause the processor to perform a method further comprising:

determining, in response to increasing the total percentage of space of each volume which may be used for extent allocations, that zero of the number of volumes can accommodate the new extent allocation amount; and

apportioning the new extent allocation amount among fractional amounts.

4. The computer program product of claim 1 , wherein the third volume and the first volume are the same volume.

5. The computer program product of claim 1 , wherein the program instructions configured to reallocate the first and second fractional amounts to the third volume are further configured to cause the processor to perform a method further comprising:

allocating the first fractional amount to the third volume;

allocating the second fractional amount to the third volume;

deallocating the first fractional amount from the first volume; and

deallocating the second fractional amount from the second volume.

6. The computer program product of claim 1 , wherein the program instructions configured to cause the processor to apportion the new extent allocation amount among fractional amounts are further configured to cause the processor to perform a method further comprising:

apportioning the new extent allocation amount among at least the first fractional amount, at least the second fractional amount, and at least a third fractional amount; and

allocating the third fractional amount to the third volume.

7. A computer program product comprising a non-transitory computer readable storage medium having program instructions embodied therewith, the program instructions executable by a processor to cause the processor to perform a method comprising:

receiving an extend request, the extend request including a new extent allocation amount for a data set, the data set associated with a number of volumes in a storage facility, wherein the number of volumes includes at least a first volume, a second volume, and a third volume, wherein at least one of the first volume and the second volume is associated with a dynamic volume count (DVC) and is not a candidate volume of the data set, wherein the third volume is not associated with the DVC and is a primary volume or a candidate volume of the data set;

determining that zero of the number of volumes can accommodate the new extent allocation amount;

increasing a volume allocation threshold, wherein the volume allocation threshold indicates a total percentage of space of each volume which may be used for storage of data sets;

determining, after increasing the volume allocation threshold, that no single volume of the number of volumes can accommodate the new extent allocation amount with the increased volume allocation threshold;

in response to determining that no single volume of the number of volumes can accommodate the new extent allocation amount with the increased volume allocation threshold, apportioning the new extent allocation amount among fractional amounts, the fractional amounts including at least a first fractional amount and at least a second fractional amount;

allocating the first fractional amount to the first volume;

allocating the second fractional amount to the second volume;

determining, after allocating the first and second fractional amounts, that the third volume can accommodate the new extent allocation amount; and

reallocating the first and second fractional amounts to the third volume.