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

8.12
Virtual Volume Types and Variations
3PAR InForm OS Concepts Guide InForm OS Version 2.2.4
additional logical disks and adds them to the pool until the CPG reaches a user-defined growth
limit that restricts its maximum size.
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 full
exported TPVV address space is usable.
Unlike standard base volumes, TPVVs only have two components:
■ Snapshot data space, also known as copy space, holds both the volume’s user data and
snapshot copy data.
■ Snapshot administration space, also known as admin space, holds mapping data to track
changes to the volume since a snapshot was created.
For TPVVs, the user space is always set to zero. Therefore, the TPVV always presents the same
size to the host, regardless of how much space it consumes.
CAUTION: Use of allocation limits (see TPVV Warnings and Limits, below) 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.