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

10.10
Virtual Copy Snapshots
3PAR InForm OS Concepts Guide InForm OS Version 2.3.1
Each copy of a copy has an additional level added to its name: in this example, the first copy of
S1 is S1_0, and a copy of S1_0 is S1_0_0. Unlike the automatic snapshots created for physical
copies, these snapshots are not assigned names by the system.
The following rules are enforced by the system when you create a snapshot:
The tree grows in alternating layers of read/write and read-only snapshots. You can only
make a read-only copy of a read/write volume, and you can only make a read/write copy of
a read-only volume.
A maximum of 256 read/write virtual copies can be made from one read-only virtual
volume.
A maximum of 500 virtual copies can be made from one base volume.
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.
10.4.1.2 Copy-of and Parent Relationships
In the example in Figure 10-3, there are 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, and 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 administration space. The parent volume
contains information needed to reconstruct the snapshot represented by the child volume. A
parent volume can have a creation date after that of its child if the parent volume was
modified.
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 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.
NOTE: The naming convention used in the example above is recommended, but it
is not enforced by the system. You can name each virtual volume and virtual copy
at the time of creation.