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

8.11
Virtual Volume Types and Variations
InForm OS Version 2.2.4 3PAR InForm OS Concepts Guide
Notice that the creation of snapshots follows certain rules, enforced by the system:
■ The tree grows in alternating layers of read-write and read-only snapshots.
■ There can only be one copy of a read-only virtual volume, but multiple copies of a read-
write virtual volume. (That is, each read-write copy starts a new branch of the tree; each
read-only copy continues an existing branch.)
■ A virtual volume cannot be deleted if a child copy of it exists. (For example, S1 cannot be
removed unless S1_0, S1_0_0, and S1_0_1 are deleted first.)
8.7.3.3 Copy-of and Parent Relationships
Figure 8-4 on page 8.10 shows two different tree structures: the solid arrows show the copy-of
relationships, and the dashed arrows show the parent relationship. For example, S0 is a read-
only copy of baseVV, whereas S1 is the parent of S0. The copy-of relationship simply shows
that the snapshot was created by copying another virtual volume. The parent relationship
refers to the internal organization of the snapshot administration space; the parent volume
contains information needed to reconstruct the snapshot represented by the child volume.
(Note that a parent volume can have a creation date after that of its child, e.g. S1 has a
creation date after that of S0.)
The parent relationship is useful for two reasons:
■ Understanding the performance consequences of virtual copies. The tree representing the
parent relationship shows the look-up paths in the snapshot administration space needed
to reconstruct the earlier state of the virtual volume. The farther away a virtual copy is from
the base volume, the longer it will take to retrieve it. If a snapshot is expected to be kept in
use for a long time, consider making a physical copy instead of a virtual copy.
■ Understanding which virtual copies become stale if the snapshot administration space is full
and the copy-on-write data cannot be written. (A stale snapshot is one that cannot be
completely recreated because the most recent changes will not be included.) The current
snapshot and all its children become stale when a write fails. For example, if there is no
space to write the copy-on-write data when a host writes to S1_0, then S1_0, S1_0_1,
and S1_0_0 become stale.
8.7.4 Thinly Provisioned Virtual Volumes
A Thinly Provisioned Virtual Volume (TPVV) is a base volume that uses logical disks that belong
to a logical disk pool known as a Common Provisioning Group (CPG). Thinly provisioned virtual
volumes associated with the same CPG draw logical disk regions from that pool as needed. As
the TPVVs that draw from the CPG require additional storage, the system automatically creates