3PAR InForm® OS 2.2.4 Concepts Guide (320-200085 Rev B, March 2009)

10.2
Overview
3PAR InForm OS Concepts Guide InForm OS Version 2.2.4
10.1 Overview
A Common Provisioning Group (CPG) creates a "virtual pool" of logical disks that allows up to
64 volumes to share the CPG's resources and allocate space on demand. Depending on the
products and features licensed for use on a system, you can use CPGs to create virtual volumes
that draw from the CPG's logical disk pool, such as Thinly Provisioned Virtual Volumes (TPVVs).
CPGs enable fine-grained, shared access to pooled logical capacity. Instead of pre-dedicating
logical disks to standard base volumes, the CPG allows multiple volumes to share the buffer
pool of logical disks. For example, when a TPVV is running low on space, the system
automatically assigns logical capacity to the TPVV by mapping new regions from logical disks
in the CPG associated with that TPVV. As a result, any large pockets of unused but allocated
space are eliminated.
By default, a CPG is configured to auto-grow new logical disks when the amount of available
logical disk space falls below a configured threshold. The initial buffer pool of logical disks
starts off at a fraction of the exported virtual capacity of mapped volumes and automatically
grows over time as required by application writes.
10.2 Precautions and Planning
A Common Provisioning Group (CPG) creates a virtual pool of logical disks that allows up to 64
volumes to share the CPG's resources and allocate space on demand. However, CPGs still
require careful planning and monitoring to prevent them from becoming so large that they set
off the system's built-in safety mechanisms. These safety mechanisms are designed to prevent a
CPG from consuming all free space on the system, but they only work properly on systems that
are planned carefully and monitored closely.
While it is possible for a CPG to have up to 64 volumes, it is strongly recommended that no
more than 32 volumes be associated with a single CPG. The reasons for limiting this are as
follows:
■ Virtual volumes in the same CPG can share the same logical disk. In the unlikely event that
the logical disk is damaged (due to multiple simultaneous disk failures, for example), all the
volumes associated with that logical disk will be unavailable.
■ Virtual volume performance may suffer from too much interleaving within the logical disks.