Virtual LUN size calculations

When you create custom volumes (CVs), you can specify the capacity of each CV. However, rounding will produce different values for the user-specified CV capacity and the actual entire CV capacity. To calculate the actual capacity of a CV, you must use a mathematical formula. The following topics explain how to calculate the user area capacity and the entire capacity of a CV.

The entire capacity of a CV or an LDEV consists of two types of capacity. One type is the user area capacity that stores the user data. The second type is the capacity of all areas that are necessary for an LDEV implementation including control information. The sum of these two types of capacities is called the entire capacity.

Implemented LDEVs consume the entire capacity from the parity group capacity. Therefore, even if the sum of user areas of multiple CVs is the same as the user area of one CV, the remaining free space generated when multiple CVs are created can be smaller than the free space in the parity group when one CV is created.

When you create an LDEV, the actual capacity of the LDEV depends on whether you select the Capacity Compatibility Mode (Offset boundary) option on the Create LDEVs window. If the Capacity Compatibility Mode (Offset boundary) option is not selected, the LDEV is created with the specified capacity. If the Capacity Compatibility Mode (Offset boundary) option is selected, the specified LDEV capacity is offset by conforming to the prescribed boundary values, and an LDEV is created. The prescribed boundary value depends on the device emulation type and RAID level. For details about boundary values, see Boundary values of volumes.

  • When to select the Capacity Compatibility Mode (Offset boundary) option. In the storage system, data is managed based on a slot, and RAID data protection is performed based on a parity stripe unit. For an LDEV created with capacity offset by a boundary, the efficiency of the drive capacity is improved because the capacity of the LDEV is offset by the unit of the data management. Therefore, if there is an emphasis on the efficiency of the drive capacity, select the Capacity Compatibility Mode (Offset boundary) option when creating LDEVs.
  • When NOT to select the Capacity Compatibility Mode (Offset boundary) option. If you want the LDEV capacity to be a specific size, do not select the Capacity Compatibility Mode (Offset boundary) option when creating LDEVs. For example, if you want to create copy pairs with VSP, HUS VM, or other previous storage system models, you must use exactly the same LDEV capacity for the volumes in a pair.

NoteWhen you use CCI to create a CV, the CV is created with the specified size regardless of the capacity calculation. Therefore, even if you specify the same capacity size (for example, 1 TB), the actual capacity size might be different between CVs created using CCI and CVs created using Device Manager - Storage Navigator.
Parent Topic
  • Configuring custom-sized provisioning

    Configuring custom-sized provisioning involves creating and configuring custom-size volumes (CVs). CVs are created by dividing a fixed-sized volume (FV) into several smaller volumes of arbitrary sizes.

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