HP Storage Provisioning Manager (SPM) version 2.1 User Guide

The sum of all concrete pools' capacity represents the raw capacity of the entire 3PAR system.
3PAR volumes (a.k.a. virtual volumes) can be allocated directly from concrete pools. In 3PAR terms,
these are called legacy volumes because, from 3PAR InformOS v2.3.1 on, it is recommended to
allocate volumes from CPG (see next section). Concrete pools are not RAID-locked, which means
legacy volumes can be assigned any supported RAID level. As explained later, this means SPM
must report raw capacity for concrete pools.
Common Provisioning Groups (CPG) are thin pools that are created within the scope of a concrete
pool. Intitially, a CPG's capacity is always 0. CPGs are RAID-locked, meaning that volumes
provisioned in a CPG are always assigned the CPG's RAID-level. But this does not mean all volumes
in a CPG have the same RAID level because the RAID level of a CPG can be changed after which
point all newly provisioned volumes will be assigned the new RAID level.
CPGs support both thick provisioning and pure thin provisioning for volumes. CPGs draw capacity
from their concrete pool. The capacity allocated to a CPG grows when:
• A fully provisioned volume is created, in which case necessary disk space is allocated to the
CPG to account for the full size of the volume, RAID overhead and administrative overhead
• A thin volume is allocated, a small amount of disk space is allocated to cover administrative
overhead requirements
• When a host writes to a thinly provisioned volume, increasing its required space
When creating a CPG, a space limit can be specified. This limit indicates the maximum amount
of logical capacity that the system will let the CPG grow to. If the limit is not specified, then the
CPG is allowed to grow unbounded. The CPG space limit is not tied to the actual capacity of the
underlying concrete pool and may represent an unreachable value if there is not enough physical
disk space available as data stored on the array grows.
Calculating capacity for concrete pools is relatively straightforward:
3PAR calculation (concrete pools)SPM capacity value
Raw capacity of the disks associated with the pool.Physical capacity
The raw capacity actually used by legacy volumes and/or
thick and thin volumes allocated on CPGs defined on this
concrete pool.
Committed capacity
Calculated as the sum of all derived CPGs' subscribed
capacities + all legacy volumes' size + the space used for
Subscribed capacity
snapshots. Because SPM volumes capacity is always
reported as logical capacity, subscribed capacity is a
logical capacity.
The raw available capacity equals the physical capacity
minus the committed capacity of the pool. SPM reports the
Available capacity
logical available capacity which depends on the RAID level
chosen for the volume that will be created.
Capacity values for CPG based pools are calculated differently:
3PAR calculation (CPG based pools)SPM capacity value
Reported as logical capacity, it is the min of:
• Committed capacity plus the logical capacity of the
largest thick volume that could possibly be allocated on
the CPG given its RAID level
OR
• Space limit of the CPG
Physical capacity is variable because multiple CPGs can
draw capacity from the same concrete pool (for example,
Physical capacity
Understanding 3PAR capacity reporting 69