Capacity design principles (2) : Logical capacity (for HPEC 4D+2P)

The following assumptions are assumed:

• Each storage node (or drive) in the storage cluster has the same valid physical capacity.

• The cluster does not have faulty storage nodes or faulty drives.

• The configuration, the capacity of each storage node, and the number of drives meet the system requirements.

  For details about storage node requirements, see Storage node requirements in the VSP One SDS Block Setup and Configuration Guide for the model that you are using.

Logical capacity

The logical capacity [MiB] (totalCapacity) is the storage pool capacity available to users. To allow Capacity balance to resolve imbalance among the storage controllers, configure logical capacity [MiB] (totalCapacity) with adequate margin so that planned capacity usage is kept to 80% or below.

You can calculate logical capacity [MiB] (totalCapacity) by using the formula shown further on in this section.

First, calculate physical capacity used as logical capacity [MiB] (usablePhysicalCapacity).

Subtract the capacity reserved for configuration changes (893592 MiB) and the capacity allocated as the rebuild capacity (C_rawDevice×N_rebuild) from the valid physical capacity [MiB] of each storage node. Out of the remaining capacity, capacity for multiples of 893592 MiB is used for physical capacity used as logical capacity [MiB] (usablePhysicalCapacity).

Physical capacity used as logical capacity [MiB/Node] = RoundDown((CrawNode - 893592-CrawDevice × Nrebuild) / 893592) × 893592
  CrawNode: Valid physical capacity [MiB/Node]
  CrawDevice: Valid physical capacity [MiB/Device]
  Nrebuild: Tolerable number of drive failures set for the storage pool
         (0 if the rebuild capacity policy is set to "Variable")
  RoundDown (numerical value): A numerical value is rounded up to an integer.

Out of every 893592MiB of physical capacity [MiB] used as logical capacity, 595728 MiB is used for logical capacity.

In addition, each storage controller needs 4200MiB of managed capacity, which is excluded from the logical capacity.

Logical capacity [MiB] = (((CusableNode / 893592) × 595728) - 4200) × Nnode
  CusableNode:  Physical capacity used as logical capacity [MiB/Node]
  Nnode: Number of storage nodes [number of storage nodes / cluster]

As a result of the calculation, if the logical capacity is 0 or less, the logical capacity will not be configured. However, when the calculated logical capacity is 0 or less while N_rebuild is 1 (default value), if the logical capacity can be formed by setting N_rebuild to 0 and then recalculating, the logical capacity will be formed without securing the rebuild capacity.

The capacity that a storage controller can manage has an upper limit. The capacity exceeding the following values cannot be used as logical capacity.

• Physical capacity used as logical capacity 151017048 [MiB/Node]

• Logical capacity: 100673832 [MiB/Node]

Reserved physical capacity

Physical reserved capacity [MiB] (reservedPhysicalCapacity) is the amount of available physical capacity in the storage pool that is not used for logical capacity. The physical reserved capacity is determined by the following calculation:

Reserved physical capacity [MiB] = (CrawNode-CusableNode) × Nnode
  CrawNode: Valid physical capacity [MiB/Node]
  CusableNode: Physical capacity used as logical capacity [MiB/Node]
  Nnode: Number of storage nodes [number of storage nodes / cluster]