3PAR InForm® OS 2.3.1 Concepts Guide (320-200112 Rev B, February 2010)

10.4
Virtual Volume Types
3PAR InForm OS Concepts Guide InForm OS Version 2.3.1
10.2.3 Thinly-Provisioned Virtual Volumes
With a 3PAR Thin Provisioning license, you can also create Thinly-Provisioned Virtual Volumes
(TPVVs). A TPVV uses logical disks that belong to a logical disk pool known as a Common
Provisioning Group (CPG). TPVVs associated with the same CPG draw user space from that pool
as needed, allocating space on demand in small increments beginning with 256 MB per
controller node. As the volumes that draw space from the CPG require additional storage, the
system automatically creates additional logical disks and adds them to the pool until the CPG
reaches the user-defined growth limit that restricts the CPG’s maximum size. The TPVV volume
size limit is 16 TB.
TPVVs are capable of responding to host write requests by allocating space on demand in small
increments, beginning with 256 MB per controller node supporting the TPVV. These allocations
are adaptive since subsequent allocations are based on the rate of consumption for previously
allocated space. With 256 MB per node as the default, allocations increase by increments of
256 MB per node as the system demands. For example, if a TPVV is initially allocated 256 MB
per node but then consumes that space in less than sixty seconds, the next allocation becomes
512 MB per node. However, if the initial 256 MB per node is consumed more slowly, the next
allocation increment remains at 256 MB per node. Under this provisioning scheme, the
maximum allocation increment is 1 GB per controller node supporting the TPVV. In addition, as
the TPVV reaches either its exported size or its user-defined allocation limit, the system allows
allocation of an additional 128 MB per node beyond these limits in order to ensure that the
exported TPVV address space is usable.
CAUTION: Use of allocation limits is recommended to prevent consumption of
physical raw capacity beyond a tolerable limit. However, you should exercise
caution when setting the value of the allocation limit. Upon reaching the
allocation limit, any new writes to TPVVs will fail and/or snapshot volumes
associated with the CPG may become invalid. Under this condition, some host
applications do not handle write failures gracefully and may produce unexpected
failures.
CAUTION: Do not allow the volumes that draw from a CPG to exceed the CPG’s
growth limit. Doing so can invalidate snapshot volumes. Refer to Chapter 9,
Common Provisioning Groups for additional cautions and recommendations.